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perspective w/ 35mm lenses?

 
 
Bart van der Wolf
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Posts: n/a
 
      08-05-2004

"Nostrobino" <(E-Mail Removed)> wrote in message
news:agbQc.1399$(E-Mail Removed) ...
>
> "Bart van der Wolf" <(E-Mail Removed)> wrote in message
> news:41112964$0$34762$(E-Mail Removed)4all.nl...

SNIP
> > I hesitate to take it a step further, because if you read the
> > following whith the wrong mind set, it'll boggle the mind or

(perhaps
> > worse) you'll draw the wrong conclusions. Nevertheless here goes:
> > http://www.trenholm.org/hmmerk/SHBG07.pdf
> > Especially focus (pun intended!) on figure 2. and realise this was
> > shot relatively close-up.

>
> Figure 2 is in fact an excellent example of exactly what I'm talking

about,
> wide-angle perspective. Note that the ping-pong balls appear to be

stretched
> radially, while the two-dimensional spots are not changed in any

way. It is
> impossible to look at that photo and NOT see the wide-angle

perspective.

So you apparently missed the message from that image...

SNIP
> > There you have a demonstration of projection distortion increasing

the
> > size of the discs/dots (more in one dimension) and perspective
> > reducing it at the same time. If you call everything that may
> > contribute to a certain "look" perspective, you'll not be able to
> > unravel what happened to the "dots", and certainly not able to

control
> > it predictably.

>
> But NOTHING happens to the discs or dots. Look again.


Wrong, at least two things happened. When you (or the lens) look(s) at
a disc (that's parallel to the film/sensor surface) at an angle, it
will look like an ellipse from a distance. However, the rectilinear
lens projection on a flat (film/sensor) surface distorts the ellipse
back to a quasi circle. A lot has happened.

> That's precisely the point the author is making: it's only the SOLID
> objects that are affected in this way.


No that's not his point, but an observation that the spheres, unlike
the discs, still look like a sphere from an angle. The author's point
is that the projection on a flat plane, stretches/distorts the
film/sensor image. That understanding can be exploited if one
manipulates the angle of incidence on the film/sensor.

> This is exactly the case with any wide-angle rectilinear lens. (It

is not
> the case with a fisheye lens, which is a whole different ball of

wax.) Only
> SOLID objects have perspective of any kind. Two-dimensional objects

(viewed
> perpendicularly to their plane) have no perspective at all.


Not true, e.g. take two normal CDs or DVDs and hold them at different
distances so that they don't cover each other, you'll be able and tell
which one is closer than the other, aka perspective.

> I think you have misunderstood something about that article. If you

think I
> have missed something, which is always possible, please direct me to

it. I
> saw nothing in it about any "spherical surface" such as you mention,

but for
> the sake of time I only skimmed the article quickly.


The point you are missing is that subjects at the same distance from
the lens, will be imaged with the same magnification. That applies to
all subjects, effectively forming the inside of a sphere. Due to
differences in subject distance the magnification/perspective changes,
flat objects will not look flat (depending on orientation towards the
line-of-vision), and due to the incident angle on the non-spherical
film/sensor there will be projection distortion.

Bart

 
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BillyJoeJimBob
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Posts: n/a
 
      08-05-2004
Nostrobino wrote:
>
> "BillyJoeJimBob" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> >
> > B) changing the direction the camera is pointing (which changes
> > the locations of the vanishing points in the frame).

>
> Obviously the camera DIRECTION has to be changed in order to keep
> the object in the corner of the frame. Neither you nor others on
> your side of the argument have ever said camera direction mattered
> as far as perspective is concerned.


Read the message thread again:

In message <(E-Mail Removed)> ,
http://www.velocityreviews.com/forums/(E-Mail Removed) (brian) wrote:
>>>
>>> The only thing that can change the position of vanishing points is
>>> to change your point of view, i.e., your location, or to point the
>>> camera in a different direction.


It sure sounds like someone on "our side" said exactly that, or
perhaps you consider brian to be on "your side"?

For that matter, read my first post to this thread again:

In message <(E-Mail Removed)>,
(E-Mail Removed) (BillyJoeJimBob) wrote:
>>>
>>> Of _course_ they won't look the same. The 135mm view would not
>>> even contain the object. If the object is in the bottom corner of >>> a 20mm view, then it will be outside the field of view delivered
>>> at 135mm...unless you either change the camera's position or
>>> change the direction the camera is pointing. If you do either of
>>> these, you've changed the perspective by your own definition.


Hmmm... so much for "neither you nor others on your side".

> You've all said it's only camera position, only camera position,
> only camera position that matters.


Since you seem so eager to lump me in with "you've all", please
point out where I've said that camera position is all that matters.
Others may have, but I have not.

Nevertheless, it's not the camera position that matters, but rather
the positions of each sensing point on the image plane relative to
the scene being imaged. We can represent each sensing element
("sensels" in the digital case, "grain sites" in the case of film I
suppose) by a set of three cartesian coordinates (X,Y,Z). If we
change the direction the camera is pointing, the coordinates for
these sensing elements change. It's far easier for folks to say
"if you change the direction you're pointing the camera, you change
perspective", but in reality all you're doing is changing the
positions of the sensing elements relative to the scene being imaged.

When you get right down to it, position actually does seem to be all
that matters. Perhaps I should have said that at the beginning after
all!

> Are you now half or two-thirds of the way to finally understanding
> what I have been saying? You will remember that I said several times
> that perspective is determined by three things: camera position,
> focal length, and the direction the camera is pointing. Now it is
> only focal length that you still have to be convinced about.


The first and third of your three things are actually two parts of
the same thing, namely position of the sensing elements relative to
the scene being imaged.

Let's move on to focal length, which has been and still is the only
one that _I_ dispute.

> The only difference in perspective that results from keeping the
> object in the corner of the frame as focal length is changed is
> BECAUSE of the focal length change.


No, the difference in perspective is caused by a change in position
of the sensing elements relative to the scene being imaged. The
fact that using a different focal length lens caused you to have to
recompose the shot does not mean that the focal length lens caused
the change in perspective. It's the recomposition of the shot that
changed the perspective.

Stating that the lens is what "causes" the change in perspective above
is incorrect. At that point you might as well say that you "caused"
the change in perspective since you put the lens on the camera, or
that the lens manufacturer "caused" the change in perspective since
they made the lens in the first place. After all, if that lens
didn't exist, you wouldn't have had to recompose the shot, thus
changing the perspective, right?

You could go all the way back to "primum mobile" at that point and
blame the whole thing on God if it weren't for those darn bolts of
lightning he tosses about from time to time.

> OF COURSE you cannot demonstrate wide-angle perspective without a
> wide angle! (Duh.)


Change the size of your sensor to encompass a larger portion of the
image circle projected inside the camera. Does this make a normal
lens turn into a wide angle lens? This, incidentally, is why "wide
angle", "normal" and "telephoto" are basically meaningless terms in
the grand scheme of things. For a medium format camera, a 50mm focal
length might be "wide angle". It certainly is not "wide angle" for a
35mm SLR format, nor is it "wide angle" for a DSLR with a 1.6x crop
factor or greater.

For the purposes of our discussion, I am going to interpret "wide
angle" to mean "short focal length" and "long lens","telephoto", and
the like to mean "long focal length".

All you're seeing with what you call a "wide angle perspective" is an
increased field of view. If you put a longer focal length lens on
the camera (without moving the camera in any way), your "wide angle
perspective" is still there; it's just been moved off of the
surface of your sensor/film. Put a bigger sensor or larger format
film in there, and voila, there's the bigger field of view, complete
with your "wide angle perspective". The focal length of the lens has
not changed.

> > Okay, "then do whatever you like at 55mm" is absolutely not any
> > sort of experimental procedure. What, precisely, should I do at
> > 55mm that does not involve moving the camera, the object or
> > changing the direction in which the camera is pointing and yet
> > still keeps the object in the corner of the frame?

>
> What I am saying is that you can do anything you like, and you
> CANNOT duplicate the perspective of a wide-angle lens with a long
> lens. Go ahead and try. I don't care what you do at 55mm, you will
> never get the perspective you did at 18mm.


You're talking field of view at this point, not perspective.

Be careful about not caring what I do, because you may just be
surprised. Since your experiment was originally a thought
experiment, I'm not limited by finances.

So here we go...

I can get an identical field of view at 55mm if I replace the sensor
in the back of the camera with one that is right around three times
as large (55/18 as large) in each dimension, assuming I don't get
vignetting. Since vignetting is a result of lens design and
manufacture and not necessarily of focal length, we will assume my
lenses are vignette free for this gedankenexperiment.

I now have the same field of view at 55mm with the 3x sensor as I did
at 18mm with the original sensor. The CD will be in the bottom corner
of the image in both cases, with the 55mm version being 3x as large in
all dimensions but retaining the same aspect ratio as in the 18mm
version. If the sensor elements on the 3x sensor are also 3x as
large so the total number of "megapixels" is the same in both cases,
the resulting 55mm version of the image will darn near exactly
overlay on top of the 18mm version when I view them in Photoshop
without my having to scale either one.

Wow, that was easy.

> Like it or not, wide-angle perspective exists,
> it's real, anyone with normal eye-brain functioning who has not been
> bamboozled by reading nonsense about perspective can see it, and
> (except by later manipulation of the image of course) you cannot get
> that perspective with a long lens no matter what you do.


Increasing the size of one's sensor/film while keeping the same
focal length seems to work quite nicely... and it's "field of view"
that you're talking about, not "perspective". The two terms are not
interchangeable, though they often appear near each other in text.

> > If you can tell me (in simple terms, please) what I should do to
> > keep an object in the corner of the image at both 18mm and 55mm
> > without moving the camera, the object, or changing the direction
> > the camera is pointing, then I'll do that.

>
> Obviously it's impossible to keep the object in the corner of the
> frame at different focal lengths without changing the direction of
> the camera. Gosh. What to do, what to do?
>
> Okay, here's the solution: You used a CD in your experiment. Leave
> it in the same place on the floor (where it will be in the corner of
> the frame at the longest lens setting), leave the camera pointing in
> the same direction (angled downward of course), and place ANOTHER CD
> on the floor where it will be in the corner of the frame with the
> lens at its widest angle. Small boxes would be better, but CDs
> should do.
>
> Voilą!


Now you're comparing two objects, located at completely different
positions relative to the sensing elements in the image plane. This
falls under "different positions affect perspective" not "focal length
affects perspective".

> The CDs being identical in size and shape, the only difference in
> perspective between the two of them now will be the result of
> difference of field of view,


No, the difference in how the two objects appear is the result of
their respective positions relative to the sensor elements in the
image plane. It's the equivalent of moving the camera, except
CDs weigh less and don't normally require a tripod.

BJJB
 
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Dave Martindale
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Posts: n/a
 
      08-05-2004
Jeremy Nixon <(E-Mail Removed)> writes:

>Please, someone, tell me if I'm wrong.


You're wrong.

The world is not spherical; it's a 3D environment. There is no
spherical image in front of the lens; there's no image at all there,
just the 3D subject. The process of mapping that 3D environment to a
2D image surface is one of projection, but projection does not
necessarily imply distortion.

For lenses, we *define* a distortion-free lens as one that has the same
imaging geometry as a pinhole. Straight lines remain straight, but
they may converge unless the subject is flat and the image plane is
parallel to the subject plane.

Your definition of "distortion" is useless because all lenses and even a
pinhole distort images according to you. If all lenses are classified
as distorted, then this provides no information about the lens.

Dave
 
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DSphotog
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      08-05-2004

"Nostrobino" <(E-Mail Removed)> wrote in message
news:3dcQc.3940$(E-Mail Removed) ...
>
> "DSphotog" <(E-Mail Removed)> wrote in message
> news:1B8Qc.2550$(E-Mail Removed) t...
> > "All the earlier suggestions to "prove" there is no difference in
> > perspective
> > between lenses of different focal length, by enlarging the center of a
> > wide-angle shot to the same size as a long-lens shot, etc., are invalid

> and
> > meaningless. Doing that proves nothing except that perspective is not
> > changed by enlargement. There never was any suggestion that it would be.

> OF
> > COURSE you cannot demonstrate wide-angle perspective without a wide

angle!
> > (Duh.)"
> >
> > Since you now agree that enlarging and image doesn't change perspective,

>
> There isn't any "now" about it, Dave. I have never said, hinted, suggested
> or implied that enlarging an image changes its perspective. Quite the
> contrary.
>
> What I have said is that enlarging the center of a wide-angle image to
> "prove" its perspective is the same as that of a long lens doesn't prove

any
> such thing.
>
> Perspective (and again, I have said this many times) is a quality of the
> picture as a whole.
>
> This, by the way, is what's wrong with your earlier example of using many
> pictures to make one. You can do that, all right, and it's been done many
> times, but it never really duplicates the perspective of a wide-angle

shot.
>
>
>
> > what exactly do you think happens when you replace a wide angle lens

with
> a
> > telephoto?

>
> The field of view is reduced and the image that remains is enlarged.
>
>
> > ENLAGEMENT I believe.

>
> Necessarily, yes. But not of the whole original image, of course.
>
>
> > And without changing perspective. Wadda ya know.

>
> Ah, but it DOES change the perspective, as I have repeatedly point out.
>
> No shot you can take with a long lens, or any number of shots pasted
> together, can accurately reproduce the perspective of, say, a 17mm or 20mm
> lens. You can paste together a lot of shots that will give you the same
> angular coverage, but the perspective won't be right. Try it and see. This
> is really the proof that perspective is necessarily a quality of the

picture
> IN ITS ENTIRETY.
>
> I've already suggested a method much easier than doing all that pasting.
> Just take an ultra-wide-angle shot, then enlarge a corner of it and see if
> you can duplicate that perspective with a much longer lens. I can tell you
> right now you won't be able to, at least not with a conventional camera.

To
> establish perspective, both shots have to contain two or more solid

objects
> in the same placement.
>
>
>
> > Thanks for your help with this.

>
> Any time. Let me know how you make out with the paste-ups, and/or when
> you've given up and finally admitted I'm correct in this.
>
>

My conclusion is that what you really are is a very obtuse TROLL. All be it,
a fairly well educated troll with a decent command of the language. And a
really successful one at that.

I do find it interesting that you will tell others that something is "It's a
well-known fact, Jeremy" or "This is well known", but have so much trouble
when anyone tells you the same thing.

I'm done playing. Have a nice life.

Best Regards,
Dave


 
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Nostrobino
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Posts: n/a
 
      08-05-2004

"DSphotog" <(E-Mail Removed)> wrote in message
news:4ygQc.3172$(E-Mail Removed) t...
>
> "Nostrobino" <(E-Mail Removed)> wrote in message
> news:3dcQc.3940$(E-Mail Removed) ...
> >
> > "DSphotog" <(E-Mail Removed)> wrote in message
> > news:1B8Qc.2550$(E-Mail Removed) t...
> > > "All the earlier suggestions to "prove" there is no difference in
> > > perspective
> > > between lenses of different focal length, by enlarging the center of a
> > > wide-angle shot to the same size as a long-lens shot, etc., are

invalid
> > and
> > > meaningless. Doing that proves nothing except that perspective is not
> > > changed by enlargement. There never was any suggestion that it would

be.
> > OF
> > > COURSE you cannot demonstrate wide-angle perspective without a wide

> angle!
> > > (Duh.)"
> > >
> > > Since you now agree that enlarging and image doesn't change

perspective,
> >
> > There isn't any "now" about it, Dave. I have never said, hinted,

suggested
> > or implied that enlarging an image changes its perspective. Quite the
> > contrary.
> >
> > What I have said is that enlarging the center of a wide-angle image to
> > "prove" its perspective is the same as that of a long lens doesn't prove

> any
> > such thing.
> >
> > Perspective (and again, I have said this many times) is a quality of the
> > picture as a whole.
> >
> > This, by the way, is what's wrong with your earlier example of using

many
> > pictures to make one. You can do that, all right, and it's been done

many
> > times, but it never really duplicates the perspective of a wide-angle

> shot.
> >
> >
> >
> > > what exactly do you think happens when you replace a wide angle lens

> with
> > a
> > > telephoto?

> >
> > The field of view is reduced and the image that remains is enlarged.
> >
> >
> > > ENLAGEMENT I believe.

> >
> > Necessarily, yes. But not of the whole original image, of course.
> >
> >
> > > And without changing perspective. Wadda ya know.

> >
> > Ah, but it DOES change the perspective, as I have repeatedly point out.
> >
> > No shot you can take with a long lens, or any number of shots pasted
> > together, can accurately reproduce the perspective of, say, a 17mm or

20mm
> > lens. You can paste together a lot of shots that will give you the same
> > angular coverage, but the perspective won't be right. Try it and see.

This
> > is really the proof that perspective is necessarily a quality of the

> picture
> > IN ITS ENTIRETY.
> >
> > I've already suggested a method much easier than doing all that pasting.
> > Just take an ultra-wide-angle shot, then enlarge a corner of it and see

if
> > you can duplicate that perspective with a much longer lens. I can tell

you
> > right now you won't be able to, at least not with a conventional camera.

> To
> > establish perspective, both shots have to contain two or more solid

> objects
> > in the same placement.
> >
> >
> >
> > > Thanks for your help with this.

> >
> > Any time. Let me know how you make out with the paste-ups, and/or when
> > you've given up and finally admitted I'm correct in this.
> >
> >

> My conclusion is that what you really are is a very obtuse TROLL. All be

it,
> a fairly well educated troll with a decent command of the language. And a
> really successful one at that.


No, trolls don't do what I've done.


>
> I do find it interesting that you will tell others that something is "It's

a
> well-known fact, Jeremy"


Are you implying that what I told Jeremy is a well-known fact is NOT a
well-known fact?



> or "This is well known", but have so much trouble
> when anyone tells you the same thing.


Aye, there's the rub. There are well-known facts, and then there are bits of
nonsense accepted by many people as fact, which latter are still nonsense no
matter how many people believe them. For example, there are probably even
more people who believe all the Bermuda Triangle silliness than there are
who believe that "perspective is determined by camera position alone" and
"focal length has nothing to do with it."

The man who can think for himself can often sort these things out and tell
the difference between them. The man who cannot, will go on thinking
whatever he thinks most other people think.


>
> I'm done playing. Have a nice life.


I'm disappointed you're not going to even try to show me how the perspective
of a wide-angle lens can be duplicated with a long lens, which (if as you
claim perspective is determined by camera position alone) should be easy for
you. I have not the foggiest idea myself how you would do it, and was
looking forward to this instructive explanation.

But I understand. As the saying goes, "You can talk the talk, but can you
walk the walk?"


>
> Best Regards,
> Dave


Best regards to you.


 
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DSphotog
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Posts: n/a
 
      08-05-2004

"Nostrobino" <(E-Mail Removed)> wrote in message
news:zWgQc.1482$(E-Mail Removed) ...
>
> "DSphotog" <(E-Mail Removed)> wrote in message
> news:4ygQc.3172$(E-Mail Removed) t...
> >
> > "Nostrobino" <(E-Mail Removed)> wrote in message
> > news:3dcQc.3940$(E-Mail Removed) ...
> > >
> > > "DSphotog" <(E-Mail Removed)> wrote in message
> > > news:1B8Qc.2550$(E-Mail Removed) t...
> > > > "All the earlier suggestions to "prove" there is no difference in
> > > > perspective
> > > > between lenses of different focal length, by enlarging the center of

a
> > > > wide-angle shot to the same size as a long-lens shot, etc., are

> invalid
> > > and
> > > > meaningless. Doing that proves nothing except that perspective is

not
> > > > changed by enlargement. There never was any suggestion that it would

> be.
> > > OF
> > > > COURSE you cannot demonstrate wide-angle perspective without a wide

> > angle!
> > > > (Duh.)"
> > > >
> > > > Since you now agree that enlarging and image doesn't change

> perspective,
> > >
> > > There isn't any "now" about it, Dave. I have never said, hinted,

> suggested
> > > or implied that enlarging an image changes its perspective. Quite the
> > > contrary.
> > >
> > > What I have said is that enlarging the center of a wide-angle image to
> > > "prove" its perspective is the same as that of a long lens doesn't

prove
> > any
> > > such thing.
> > >
> > > Perspective (and again, I have said this many times) is a quality of

the
> > > picture as a whole.
> > >
> > > This, by the way, is what's wrong with your earlier example of using

> many
> > > pictures to make one. You can do that, all right, and it's been done

> many
> > > times, but it never really duplicates the perspective of a wide-angle

> > shot.
> > >
> > >
> > >
> > > > what exactly do you think happens when you replace a wide angle lens

> > with
> > > a
> > > > telephoto?
> > >
> > > The field of view is reduced and the image that remains is enlarged.
> > >
> > >
> > > > ENLAGEMENT I believe.
> > >
> > > Necessarily, yes. But not of the whole original image, of course.
> > >
> > >
> > > > And without changing perspective. Wadda ya know.
> > >
> > > Ah, but it DOES change the perspective, as I have repeatedly point

out.
> > >
> > > No shot you can take with a long lens, or any number of shots pasted
> > > together, can accurately reproduce the perspective of, say, a 17mm or

> 20mm
> > > lens. You can paste together a lot of shots that will give you the

same
> > > angular coverage, but the perspective won't be right. Try it and see.

> This
> > > is really the proof that perspective is necessarily a quality of the

> > picture
> > > IN ITS ENTIRETY.
> > >
> > > I've already suggested a method much easier than doing all that

pasting.
> > > Just take an ultra-wide-angle shot, then enlarge a corner of it and

see
> if
> > > you can duplicate that perspective with a much longer lens. I can tell

> you
> > > right now you won't be able to, at least not with a conventional

camera.
> > To
> > > establish perspective, both shots have to contain two or more solid

> > objects
> > > in the same placement.
> > >
> > >
> > >
> > > > Thanks for your help with this.
> > >
> > > Any time. Let me know how you make out with the paste-ups, and/or when
> > > you've given up and finally admitted I'm correct in this.
> > >
> > >

> > My conclusion is that what you really are is a very obtuse TROLL. All be

> it,
> > a fairly well educated troll with a decent command of the language. And

a
> > really successful one at that.

>
> No, trolls don't do what I've done.
>
>
> >
> > I do find it interesting that you will tell others that something is

"It's
> a
> > well-known fact, Jeremy"

>
> Are you implying that what I told Jeremy is a well-known fact is NOT a
> well-known fact?
>
>
>
> > or "This is well known", but have so much trouble
> > when anyone tells you the same thing.

>
> Aye, there's the rub. There are well-known facts, and then there are bits

of
> nonsense accepted by many people as fact, which latter are still nonsense

no
> matter how many people believe them. For example, there are probably even
> more people who believe all the Bermuda Triangle silliness than there are
> who believe that "perspective is determined by camera position alone" and
> "focal length has nothing to do with it."
>
> The man who can think for himself can often sort these things out and tell
> the difference between them. The man who cannot, will go on thinking
> whatever he thinks most other people think.
>
>
> >
> > I'm done playing. Have a nice life.

>
> I'm disappointed you're not going to even try to show me how the

perspective
> of a wide-angle lens can be duplicated with a long lens, which (if as you
> claim perspective is determined by camera position alone) should be easy

for
> you. I have not the foggiest idea myself how you would do it, and was
> looking forward to this instructive explanation.
>
> But I understand. As the saying goes, "You can talk the talk, but can you
> walk the walk?"
>
>
> >
> > Best Regards,
> > Dave

>
> Best regards to you.
>
>

Care to post your home address?


 
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nospam
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Posts: n/a
 
      08-05-2004
In article <zWgQc.1482$(E-Mail Removed)>, Nostrobino
<(E-Mail Removed)> wrote:

> I'm disappointed you're not going to even try to show me how the perspective
> of a wide-angle lens can be duplicated with a long lens, which (if as you
> claim perspective is determined by camera position alone) should be easy for
> you. I have not the foggiest idea myself how you would do it, and was
> looking forward to this instructive explanation.


it has been explained several times in this thread.
 
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Nostrobino
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      08-05-2004

"BillyJoeJimBob" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Nostrobino wrote:
> >
> > "BillyJoeJimBob" <(E-Mail Removed)> wrote in message
> > news:(E-Mail Removed)...
> > >
> > > B) changing the direction the camera is pointing (which changes
> > > the locations of the vanishing points in the frame).

> >
> > Obviously the camera DIRECTION has to be changed in order to keep
> > the object in the corner of the frame. Neither you nor others on
> > your side of the argument have ever said camera direction mattered
> > as far as perspective is concerned.

>
> Read the message thread again:
>
> In message <(E-Mail Removed)> ,
> (E-Mail Removed) (brian) wrote:
> >>>
> >>> The only thing that can change the position of vanishing points is
> >>> to change your point of view, i.e., your location, or to point the
> >>> camera in a different direction.

>
> It sure sounds like someone on "our side" said exactly that, or
> perhaps you consider brian to be on "your side"?
>
> For that matter, read my first post to this thread again:
>
> In message <(E-Mail Removed)>,
> (E-Mail Removed) (BillyJoeJimBob) wrote:
> >>>
> >>> Of _course_ they won't look the same. The 135mm view would not
> >>> even contain the object. If the object is in the bottom corner of >>>

a 20mm view, then it will be outside the field of view delivered
> >>> at 135mm...unless you either change the camera's position or
> >>> change the direction the camera is pointing. If you do either of
> >>> these, you've changed the perspective by your own definition.

>
> Hmmm... so much for "neither you nor others on your side".
>
> > You've all said it's only camera position, only camera position,
> > only camera position that matters.

>
> Since you seem so eager to lump me in with "you've all", please
> point out where I've said that camera position is all that matters.
> Others may have, but I have not.


Then I stand corrected, and thank you for the correction. My only excuse is
the sheer number of posts in this topic, the number of posters, and the fact
that in general you all seem to have been taking the same position. I
recognize that there have been differences between you, but they tend to all
run together after a while.



>
> Nevertheless, it's not the camera position that matters, but rather
> the positions of each sensing point on the image plane relative to
> the scene being imaged. We can represent each sensing element
> ("sensels" in the digital case, "grain sites" in the case of film I
> suppose) by a set of three cartesian coordinates (X,Y,Z). If we
> change the direction the camera is pointing, the coordinates for
> these sensing elements change. It's far easier for folks to say
> "if you change the direction you're pointing the camera, you change
> perspective", but in reality all you're doing is changing the
> positions of the sensing elements relative to the scene being imaged.
>
> When you get right down to it, position actually does seem to be all
> that matters. Perhaps I should have said that at the beginning after
> all!


Well, in a sense everything related to this has to do with position. The
position of the camera, the direction it is pointing, and the focal length
( = field of view) all have to do with the positions of things.


>
> > Are you now half or two-thirds of the way to finally understanding
> > what I have been saying? You will remember that I said several times
> > that perspective is determined by three things: camera position,
> > focal length, and the direction the camera is pointing. Now it is
> > only focal length that you still have to be convinced about.

>
> The first and third of your three things are actually two parts of
> the same thing, namely position of the sensing elements relative to
> the scene being imaged.


But of course that isn't absolutely correct either. If, for example, one of
those right-angle first-surface mirror devices were mounted on the lens, the
film plane (or corresponding sensor) would be rotated 90 degrees
horizontally relative to the field of view. Sure, this is a quibble and
perhaps an unfair one, but after all we are getting pretty fussy about
details here.


>
> Let's move on to focal length, which has been and still is the only
> one that _I_ dispute.
>
> > The only difference in perspective that results from keeping the
> > object in the corner of the frame as focal length is changed is
> > BECAUSE of the focal length change.

>
> No, the difference in perspective is caused by a change in position
> of the sensing elements relative to the scene being imaged.


But that is only enabled by the focal-length change.


> The
> fact that using a different focal length lens caused you to have to
> recompose the shot does not mean that the focal length lens caused
> the change in perspective. It's the recomposition of the shot that
> changed the perspective.


The point has been and remains that you CANNOT duplicate the perspective of
a wide-angle lens with a long lens.


>
> Stating that the lens is what "causes" the change in perspective above
> is incorrect. At that point you might as well say that you "caused"
> the change in perspective since you put the lens on the camera, or
> that the lens manufacturer "caused" the change in perspective since
> they made the lens in the first place. After all, if that lens
> didn't exist, you wouldn't have had to recompose the shot, thus
> changing the perspective, right?


Your objection then is with the verb "cause"?

I don't think I used it. I said "perspective is determined by," not
"caused." I don't think I'd have used "cause" in that way.


>
> You could go all the way back to "primum mobile" at that point and
> blame the whole thing on God if it weren't for those darn bolts of
> lightning he tosses about from time to time.
>
> > OF COURSE you cannot demonstrate wide-angle perspective without a
> > wide angle! (Duh.)

>
> Change the size of your sensor to encompass a larger portion of the
> image circle projected inside the camera.


Not very much, you couldn't, except of course with a dSLR using 35mm SLR
lenses. That may be what you have in mind, I understand that, but still you
could only increase the sensor size to an effective diagonal of 43 mm or so.
Lens makers (for 35mm cameras) don't give you much extra in the way of
coverage.


> Does this make a normal
> lens turn into a wide angle lens? This, incidentally, is why "wide
> angle", "normal" and "telephoto" are basically meaningless terms in


Well, "telephoto" is actually very specific in its meaning, though it is now
commonly misused to mean any long-focus lens.


> the grand scheme of things. For a medium format camera, a 50mm focal
> length might be "wide angle".


In fact a 50mm lens is an extreme wide-angle on a 6x6cm camera. I used
precisely this example myself, somewhere way back in this thread.


> It certainly is not "wide angle" for a
> 35mm SLR format, nor is it "wide angle" for a DSLR with a 1.6x crop
> factor or greater.
>
> For the purposes of our discussion, I am going to interpret "wide
> angle" to mean "short focal length" and "long lens","telephoto", and
> the like to mean "long focal length".
>
> All you're seeing with what you call a "wide angle perspective" is an
> increased field of view. If you put a longer focal length lens on
> the camera (without moving the camera in any way), your "wide angle
> perspective" is still there; it's just been moved off of the
> surface of your sensor/film.


Then it's not "still there."


> Put a bigger sensor or larger format
> film in there, and voila, there's the bigger field of view, complete


Not really, no. Put a "larger format in there" and what you get is a
circular image on it that doesn't fill the format, unless of course the lens
you're using was actually designed for the larger format.


> with your "wide angle perspective". The focal length of the lens has
> not changed.


These things are all necessarily relative. With most digital cameras today,
there are so many sensor sizes that actual focal lengths are essentially
meaningless and are generally not even mentioned. If e.g. someone says his
Minolta S414 has a 35-140mm lens, what he really means is that that's what
it is in 35mm camera equivalence. I have one and I don't have know offhand
what its actual focal length range is. I'd have to look. Similarly, I know
my X-series Minoltas have "37-111mm" lenses and my F300s have "38-114mm"
lenses. Again, what the actual focal length ranges are I don't remember and
don't really have any reason to remember. My 7i and 7Hi cameras have
identical lenses, and in this case I do happen to remember that the lens is
7.2-50.8mm, but the important thing is that its 35mm equivalence is 28-200mm
(and the zoom control ring is so marked, actually). The same lens exactly on
my Dimage 5 is a "35-250mm" lens because of that camera's slightly smaller
CCD.

All this makes sense with digital cameras since most people are familiar
with 35mm camera focal lengths, and would be hopelessly confused by true
f.l. numbers if those were all they were given.

However, all this has no direct bearing on wide-angle perspective, which is
really a function of field of view. It's convenient to use focal lengths
instead of angular measurement, simply because f.l. is the measure that
people are most familiar with. More people will understand what you mean by
"28mm lens" than would if you said "75-degree lens," and so on.


>
> > > Okay, "then do whatever you like at 55mm" is absolutely not any
> > > sort of experimental procedure. What, precisely, should I do at
> > > 55mm that does not involve moving the camera, the object or
> > > changing the direction in which the camera is pointing and yet
> > > still keeps the object in the corner of the frame?

> >
> > What I am saying is that you can do anything you like, and you
> > CANNOT duplicate the perspective of a wide-angle lens with a long
> > lens. Go ahead and try. I don't care what you do at 55mm, you will
> > never get the perspective you did at 18mm.

>
> You're talking field of view at this point, not perspective.


They are largely inseparable.


>
> Be careful about not caring what I do, because you may just be
> surprised. Since your experiment was originally a thought
> experiment, I'm not limited by finances.
>
> So here we go...
>
> I can get an identical field of view at 55mm if I replace the sensor
> in the back of the camera with one that is right around three times
> as large (55/18 as large) in each dimension, assuming I don't get
> vignetting. Since vignetting is a result of lens design and
> manufacture and not necessarily of focal length, we will assume my
> lenses are vignette free for this gedankenexperiment.


Well, okay. We both know that wouldn't work in the real world, but I'll go
along with you.


>
> I now have the same field of view at 55mm with the 3x sensor as I did
> at 18mm with the original sensor.


Or close enough. Yes.



> The CD will be in the bottom corner
> of the image in both cases, with the 55mm version being 3x as large in
> all dimensions but retaining the same aspect ratio as in the 18mm
> version. If the sensor elements on the 3x sensor are also 3x as
> large so the total number of "megapixels" is the same in both cases,
> the resulting 55mm version of the image will darn near exactly
> overlay on top of the 18mm version when I view them in Photoshop
> without my having to scale either one.


Fair enough.


>
> Wow, that was easy.


Uh . . . what was easy? Did I miss something?


>
> > Like it or not, wide-angle perspective exists,
> > it's real, anyone with normal eye-brain functioning who has not been
> > bamboozled by reading nonsense about perspective can see it, and
> > (except by later manipulation of the image of course) you cannot get
> > that perspective with a long lens no matter what you do.

>
> Increasing the size of one's sensor/film while keeping the same
> focal length seems to work quite nicely... and it's "field of view"
> that you're talking about, not "perspective".


Both. They are largely inseparable (at least insofar as three-dimensionality
is concerned), but when I'm speaking of perspective it's perspective I mean,
not field of view.


> The two terms are not
> interchangeable, though they often appear near each other in text.


I agree, they are not interchangeable. Field of view means the included
angle "seen" by the lens and film/sensor, nothing more. It has definite
implications for perspective but is not necessarily a determinant of
perspective; e.g., it is possible to take a photograph that has no
perspective whatever, but it still has a field of view.

Perspective is the visible relationship between solid objects within the
field of view, i.e. the apparent size, shapes, angles, convergences etc. of
those objects. If there is no three-dimensionality (e.g. a photo of blank
sky), there can be no perspective, though there is still a field of view.

We are agreed on that I think, yes?


>
> > > If you can tell me (in simple terms, please) what I should do to
> > > keep an object in the corner of the image at both 18mm and 55mm
> > > without moving the camera, the object, or changing the direction
> > > the camera is pointing, then I'll do that.

> >
> > Obviously it's impossible to keep the object in the corner of the
> > frame at different focal lengths without changing the direction of
> > the camera. Gosh. What to do, what to do?
> >
> > Okay, here's the solution: You used a CD in your experiment. Leave
> > it in the same place on the floor (where it will be in the corner of
> > the frame at the longest lens setting), leave the camera pointing in
> > the same direction (angled downward of course), and place ANOTHER CD
> > on the floor where it will be in the corner of the frame with the
> > lens at its widest angle. Small boxes would be better, but CDs
> > should do.
> >
> > Voilą!

>
> Now you're comparing two objects, located at completely different
> positions relative to the sensing elements in the image plane. This
> falls under "different positions affect perspective" not "focal length
> affects perspective".


The different positions that are VISIBLE only become so because of the
change of focal length.

Now if you say "the difference in perspective was there all along, you just
couldn't see it," I agree with you. But we are, after all, speaking of
photographs. It is only the content of the photograph that matters as long
as we are speaking of the perspective of that photograph.


>
> > The CDs being identical in size and shape, the only difference in
> > perspective between the two of them now will be the result of
> > difference of field of view,

>
> No, the difference in how the two objects appear is the result of
> their respective positions relative to the sensor elements in the
> image plane. It's the equivalent of moving the camera, except
> CDs weigh less and don't normally require a tripod.


<chuckle>

Well, BillyJoeJimBob, I will say this for you in all sincerity: You appear
to be the only one here who has a mind worth engaging, and I very much
appreciate it.

N.


 
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Nostrobino
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      08-05-2004

"Bart van der Wolf" <(E-Mail Removed)> wrote in message
news:41118094$0$10528$(E-Mail Removed)4all.nl...
>
> "Nostrobino" <(E-Mail Removed)> wrote in message
> news:agbQc.1399$(E-Mail Removed) ...
> >
> > "Bart van der Wolf" <(E-Mail Removed)> wrote in message
> > news:41112964$0$34762$(E-Mail Removed)4all.nl...

> SNIP
> > > I hesitate to take it a step further, because if you read the
> > > following whith the wrong mind set, it'll boggle the mind or

> (perhaps
> > > worse) you'll draw the wrong conclusions. Nevertheless here goes:
> > > http://www.trenholm.org/hmmerk/SHBG07.pdf
> > > Especially focus (pun intended!) on figure 2. and realise this was
> > > shot relatively close-up.

> >
> > Figure 2 is in fact an excellent example of exactly what I'm talking

> about,
> > wide-angle perspective. Note that the ping-pong balls appear to be

> stretched
> > radially, while the two-dimensional spots are not changed in any

> way. It is
> > impossible to look at that photo and NOT see the wide-angle

> perspective.
>
> So you apparently missed the message from that image...
>
> SNIP
> > > There you have a demonstration of projection distortion increasing

> the
> > > size of the discs/dots (more in one dimension) and perspective
> > > reducing it at the same time. If you call everything that may
> > > contribute to a certain "look" perspective, you'll not be able to
> > > unravel what happened to the "dots", and certainly not able to

> control
> > > it predictably.

> >
> > But NOTHING happens to the discs or dots. Look again.

>
> Wrong, at least two things happened. When you (or the lens) look(s) at
> a disc (that's parallel to the film/sensor surface) at an angle, it
> will look like an ellipse from a distance. However, the rectilinear
> lens projection on a flat (film/sensor) surface distorts the ellipse
> back to a quasi circle. A lot has happened.


Well, I wouldn't call that distortion. And it's a real circle, not a quasi
circle.

I agree that if you look at a disc from an angle it will appear as an
ellipse. It will do so with a camera lens also, except in the special case
(such as in that figure) where the lens axis is perpendicular to the
subject. As long as that is the case, the lens is rectilinear, and the film
plane is also perpendicular to the lens axis, a disc will necessarily be
represented on the film as a circle.


>
> > That's precisely the point the author is making: it's only the SOLID
> > objects that are affected in this way.

>
> No that's not his point, but an observation that the spheres, unlike
> the discs, still look like a sphere from an angle. The author's point
> is that the projection on a flat plane, stretches/distorts the
> film/sensor image. That understanding can be exploited if one
> manipulates the angle of incidence on the film/sensor.


But geometry REQUIRES that the circle in the subject be represented as a
circle on the film, as long as those conditions are met (lens axis
perpendicular to the subject). There isn't any optical distortion involved;
quite the contrary.



>
> > This is exactly the case with any wide-angle rectilinear lens. (It

> is not
> > the case with a fisheye lens, which is a whole different ball of

> wax.) Only
> > SOLID objects have perspective of any kind. Two-dimensional objects

> (viewed
> > perpendicularly to their plane) have no perspective at all.

>
> Not true, e.g. take two normal CDs or DVDs and hold them at different
> distances so that they don't cover each other, you'll be able and tell
> which one is closer than the other, aka perspective.


I agree. (It's understood that they're on the same plane and not at eye
level.)



>
> > I think you have misunderstood something about that article. If you

> think I
> > have missed something, which is always possible, please direct me to

> it. I
> > saw nothing in it about any "spherical surface" such as you mention,

> but for
> > the sake of time I only skimmed the article quickly.

>
> The point you are missing is that subjects at the same distance from
> the lens, will be imaged with the same magnification. That applies to
> all subjects, effectively forming the inside of a sphere.


I still don't know where your "sphere" is coming from. In the example shown,
all the discs are as you say "imaged with the same magnification." They're
on a flat surface. There isn't any sphere.


> Due to
> differences in subject distance the magnification/perspective changes,


In the case of that figure, the differences in distance from disc to lens
are exactly canceled out by relatively equal differences from lens to image
plane. There isn't any perspective at all as far as the discs are concerned
because they're (effectively) not solid objects, they're two-dimensional.
The ping-pong balls show perspective because they ARE solid objects.


> flat objects will not look flat (depending on orientation towards the
> line-of-vision), and due to the incident angle on the non-spherical
> film/sensor there will be projection distortion.


Sorry, you've lost me there again. I just don't understand why you think
there's anything spherical involved.

N.


 
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Nostrobino
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      08-05-2004

"Nostrobino" <(E-Mail Removed)> wrote in message
news:Y0iQc.438$(E-Mail Removed). ..
>

[ . . . ]
>
> <chuckle>
>
> Well, BillyJoeJimBob, I will say this for you in all sincerity: You appear
> to be the only one here who has a mind worth engaging, and I very much
> appreciate it.


You and Bart, I should have said. He has some interesting ideas also, though
I have great difficulty following them.


 
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