perspective w/ 35mm lenses?

Discussion in 'Digital Photography' started by PrincePete01, Jul 16, 2004.

  1. PrincePete01

    Jeremy Nixon Guest

    No, it's not. You've changed the relative angle of the image plane to the
    subject, which changes the perspective. If you didn't move the camera, but
    used a shift lens to keep the subject in the frame of both pictures without
    changing the angle of the image plane, the perspective would be the same.

    See, your examples involve moving the camera. You can't move the camera if
    you want to demonstrate that focal length changes perspective.
     
    Jeremy Nixon, Aug 4, 2004
    #81
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  2. PrincePete01

    Nostrobino Guest

    Correct. The camera position remains the same. This, remember, is what your
    side insists is the SOLE determinant of perspective--the camera position,
    nothing else.

    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. You've all said it's only camera position, only camera position,
    only camera position that matters. If you are now at last admitting that
    other factors come into it, your whole argument begins to collapse.

    Ah-HA!

    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 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.

    Objects in the corner of a wide-angle shot appear radially stretched (under
    normal viewing circumstances). This is by definition a difference in
    perspective. It is what gives the wide-angle shot its "wide-angle look."

    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.)


    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. 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.

    Once again (and this, too, I have repeated several times), the perspective
    of any picture is a characteristic of that picture IN ITS ENTIRETY. You
    cannot take a little piece of a picture (which is what you're trying to do)
    and say that conveys the perspective of the original whole.

    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.

    Voilà!

    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, the first one illustrating long-lens perspective and the
    second one wide-angle perspective. Enlarge the farther one to suit if you
    feel it necessary, but I doubt it will be.

    Of course if this gets out, the pundits who write nonsense for
    "authoritative texts" will probably hate you and may snarl at you if they
    pass you in the street, but look at what you have accomplished!
     
    Nostrobino, Aug 4, 2004
    #82
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  3. PrincePete01

    Nostrobino Guest

    Sure it is.

    Well, duh.

    But you've been saying it's only the CAMERA POSITION that determines
    perspective.

    Changing "the relative angle of the image plane to the subject" is what
    MAKES a wide-angle a wide-angle. Look, the geometry is simple: A 50mm lens
    on a full-frame 35 covers about 47 degrees corner to corner. That means that
    an object in the corner of the frame would be about 23.5 degrees off a line
    drawn perpendicular to the film plane. A 24mm lens on the other hand covers
    about 84 degrees, so an object in the corner would then be about 42 degrees
    off the perpendicular. That's what wide-angle lenses DO.

    This is the basic error that you people on the it's-only-the-camera-position
    side of the argument have been making all along: you want to first take away
    everything that makes a wide-angle lens what it is, and THEN say there is no
    such thing as wide-angle perspective.

    Not exactly, but anyway we're not talking about shift lenses. We're talking
    about conventional wide-angle lenses, or the zoom equivalents thereof.

    Then take a look at my reply earlier today to BillyJoeJimBob, which has
    already answered your complaint.
     
    Nostrobino, Aug 4, 2004
    #83
  4. SNIP
    Sigh, and now you are confusing projection distortion with
    perspective. Projection on a flat surface means the corners are at a
    larger distance than the center normal.

    Bart
     
    Bart van der Wolf, Aug 4, 2004
    #84
  5. PrincePete01

    Jeremy Nixon Guest

    No, it's not. You've moved it.
    It is. The image plane *is* the camera position. Move it, and, well, you
    have moved it. Don't move it, and you haven't moved it.
    No, we're talking about perspective, and how the only thing that changes it
    is moving the camera. Every example you give of changing the perspective
    by changing the lens also moves the camera. If you want to demonstrate that
    focal length changes perspective you need to give an example where you
    *don't* move the camera.
    No, it hasn't. You're moving the camera, every time, and then claiming that
    you don't have to move the camera to change perspective.
     
    Jeremy Nixon, Aug 4, 2004
    #85
  6. PrincePete01

    Nostrobino Guest

    What "projection distortion"? The lenses being discussed are (within
    reasonable tolerances) rectilinear and distortion-free.

    You may have something in mind there, but you have concealed it artfully.
     
    Nostrobino, Aug 4, 2004
    #86
  7. PrincePete01

    Jeremy Nixon Guest

    You're still not understanding how the image is created. You are projecting
    a spherical image onto a flat plane -- this results in distortion of some
    kind, depending on what type of projection you use. When you use a
    projection that keeps straight lines straight (your "normal rectilinear
    lens") then parallel lines will not remain parallel.

    A flat image from a camera is *always* "distorted" in some way in order to
    make it flat.
     
    Jeremy Nixon, Aug 4, 2004
    #87
  8. PrincePete01

    Nostrobino Guest

    No, the camera is in the same position.

    No, the camera position is the camera position. Where the camera is located.
    The place at which the camera is set up or held. The spatial and temporal
    situation of the camera. The camera's exact latitude and longitude. That's
    the camera position.


    I haven't moved it.

    I already have, earlier today. See my reply to BillyJoeJimBobZeke.

    You haven't read the reply. Read it. The camera is not moved a millimeter
    and is left pointing in exactly the same direction, not altered by a degree,
    a minute or even a second of arc.

    Time for you to start working on your retreat strategy.
     
    Nostrobino, Aug 4, 2004
    #88
  9. PrincePete01

    Jeremy Nixon Guest

    Look, when you move it, it's not in the same position. You've pointed
    it in a different direction! That requires "movement".
    Then how did it end up pointing in a different direction? The image plane
    has moved. If you move it, perspective changes. If you don't, it doesn't.
    You mean the one where you're photographing a totally different object?
    That one was too silly to even talk about, wasn't it? I mean, it's at a
    different angle to the camera, so obviously the perspective is going to
    be different; the example doesn't even make sense, and doesn't apply to
    the subject of perspective changes at all.
    Sure, and in order to compensate for that, you're looking at a different
    object in a different position! Okay, so since this obviously wasn't
    implied, I'll spell it out: you need to not move the camera *or* the
    subject. Okay? Because changing the relative position of those things
    changes the perspective. Zooming in, however, does not.
     
    Jeremy Nixon, Aug 4, 2004
    #89
  10. PrincePete01

    Nostrobino Guest

    You are still hung up on that "spherical image" misconception. There isn't
    any spherical image where rectilinear lenses are concerned.

    What is projected onto the flat plane is a two-dimensional representation of
    a three-dimensional world.

    Sure they will, if the subject is essentially two-dimensional (i.e., any
    flat surface that's perpendicular to the lens axis). If the subject is NOT
    flat and perpendicular then of course parallel lines in it will converge.
    This is NOT DISTORTION. It is an accurate two-dimensional representation of
    what is in front of the lens.

    Nonsense. If the subject is flat and perpendicular, you can take
    measurements from the image and they will (within reasonable tolerances)
    correspond to measurements taken from the subject itself. There's
    substantially NO DISTORTION.

    That's all any single rectilinear lens can do. If you want three-dimensional
    representation, use a stereo camera and you will get it, still without
    visible distortion and still using rectilinear lenses.
     
    Nostrobino, Aug 4, 2004
    #90
  11. PrincePete01

    Nostrobino Guest

    No, the camera is still in the same position relative to the subject.

    doesn't.

    This is a new version of the there's-no-such-thing-as-wide-angle-perspective
    argument? You're now saying that it's IMAGE PLANE and its relationship to
    the subject that matters, not camera position?

    Is this the final version, or will there be more changes as we move along?

    <GUFFAW!>

    THAT'S WHAT WIDE-ANGLE LENSES DO!

    I have already pointed this out at least a couple of times, haven't I? Come
    on now, how about a little intellectual honesty here? All of your stunts to
    "prove there's no such thing as wide-angle perspective" have relied entirely
    on making comparisons which carefully remove or ignore the very aspects of
    the image that clearly DEMONSTRATE wide-angle perspective.

    The objects are identical; what's the difference? But if it really bothers
    you, go ahead and swap them back and forth. It won't change the fact that
    the one in the wide-angle shot still shows wide-angle perspective. And by
    this time I think you know it as well as I do.

    As I have already said: go ahead and take a long-lens shot of the same
    object any way you like, any way at all, position camera, object, film plane
    however you want them--and you still will not be able to duplicate the
    wide-angle perspective with a long lens.


    Which is exactly what a wide-angle lens does, hence the wide-angle
    perspective.

    Have we covered this enough, or do you want to go over it all another 40 or
    50 times?

    Because then you are only magnifying something that never did, and never
    could, illustrate the wide-angle perspective in the first place. This, too,
    we have gone over and over.

    Time to start working on your retreat strategy, Jeremy.
     
    Nostrobino, Aug 4, 2004
    #91
  12. PrincePete01

    Jeremy Nixon Guest

    Of course there is. If you take every point that is a certain distance away
    from the lens, they will create a sphere. You are projecting that sphere onto
    a flat surface. You get distortion when you do this.
    Imagine that you are photographing a building. In the lower left corner of
    the image, part of the structure forms a right angle. You've used a wide-angle
    lens to get your "wide-angle look". Is the right angle in the world still a
    right angle in the picture? No, it's not. It's not whether you used a wide-
    angle lens or not, but I had you using one just to factor out that part of
    the discussion.

    Why is a right angle in the corner of the image not a right angle in the
    picture?

    Because you've flattened the spherical image onto a flat plane, and the
    resulting distortion is greatest in the corners of the image.

    Why is there such a thing as a "wide-angle look"? Why do wide-angle lenses
    show greater distortion at the edges than telephoto lenses?

    Because they are capturing a greater area of the spherical image, so there
    is more curvature in the original that is projected onto the flat plane,
    and the relative angle between image plane and subject is greater at the
    edges with the wider field of view, so there is more distortion at the edge
    of a wide-angle image. This distortion is not absent in a telephoto image,
    you just can't see it as obviously because you're using a smaller portion
    of the sphere.

    In the above example, if you use a fisheye lens, the right angle *will* be
    a right angle in the picture. You've used a different projection, one that
    maintains right angles. The perspective is the same, but the projection is
    different, so the distortion is different -- straight lines will not remain
    straight.

    So, in one image, a right angle in the world is not a right angle in the
    picture. In the other, a right angle is a right angle, but straight lines
    are not straight. Which one is an "accurate" representation of a three-
    dimensional world?
    The converging parallel lines are not distortion, but are not necessarily an
    accurate representation of what is in front of the lens. Why do architectural
    photographers use shift lenses, if converging parallel lines are accurate?

    The distortion is a product of flattening the image. The converging parallel
    lines are a product of the relative angle of the image plane to the subject.
    The flattening distorts everything based in part on this angle, which is why
    things at the corner of your wide-angle shots are "different" from things in
    the center of them. It's why right angles can't stay as right angles unless
    the object is perpendicular to the lens axis, as long as you're using a
    "normal" lens.

    It's *all* distorted. It's just a matter of which distortion you want. It
    is not possible to create a flat image as a photograph without distorting
    *something*, because the lens is looking at a sphere, not a plane.

    If you focus your lens to 10 feet, what are you photographing? It's not a
    flat plane 10 feet in front of the camera, parallel to the image plane. It
    is the inside surface of a sphere 10 feet from the lens in all directions,
    and you're photographing a portion of that surface by flattening it, which
    distorts the image in much the same way that creating a map on a flat piece
    of paper makes Greenland look as big as Africa. They use different types
    of projection in map-making, but it's the same idea -- representing a
    sphere on a flat surface.
    Sure there is, it is just canceled out by the (unnatural) flat subject. If
    you want to photograph a two-dimensional surface in order to prove that there
    is no distortion when photographing a three-dimensional world, I guess that's
    about as valid as moving the camera to prove that you can change perspective
    without moving the camera, though.
     
    Jeremy Nixon, Aug 4, 2004
    #92
  13. PrincePete01

    DSphotog Guest

    YOO-HOO !!! I can.

     
    DSphotog, Aug 4, 2004
    #93
  14. PrincePete01

    DSphotog Guest

    One of the many things that you're missing her is that camera POSITION
    involves THREE planes, not just the two that you have choosen to use. 1)
    Forward and backward (closer or farther from subject) 2) left and right
    horizontally and 3) up and down vertically.

    FYI - The film plane is the effective camera position. All the other parts
    of the camera simply support what one is trying to accomplish at the film
    (or image if you wish) plane.

    When you turn the camera and point it down you have, in fact, changed the
    distance from the object in question of the left and right part of the plane
    and the top and bottom as well. Thus, you have "moved" the camera.

    Regarding the above "Yoo-Hoo", ever played with a view camera??

    Regards,
    Dave
     
    DSphotog, Aug 4, 2004
    #94
  15. PrincePete01

    DSphotog Guest

    "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,
    what exactly do you think happens when you replace a wide angle lens with a
    telephoto?
    ENLAGEMENT I believe. And without changing perspective. Wadda ya know.
    Thanks for your help with this.

    Regards Again,
    Dave
     
    DSphotog, Aug 4, 2004
    #95
  16. PrincePete01

    Jeremy Nixon Guest

    No, it's not. You've moved it.
    The image plane is the camera.
    All of your stunts to prove that you can change perspective without moving the
    camera or the subject have moved either the camera or the subject, which
    everyone agrees changes perspective.
    Their relative position to the camera is the difference. Since that is
    different, the perspective will be different.

    I thought you were only arguing that focal length changes perspective, but
    now it appears that you're saying that relative position of camera and
    subject does *not* change perspective? That no matter what angle you have
    on an object, the perspective will be the same? You're not really making
    much sense here.
    You're really trying to say that using a wide-angle lens moves the subject
    or the camera to a different position? How does it accomplish this amazing
    feat?
     
    Jeremy Nixon, Aug 4, 2004
    #96
  17. artfully.

    I'll try and keep it simple; shine a flashlight beam straight onto a
    surface from short distance. You'll see a circular spot. Now without
    changing the distance to the surface, shine it at a slight angle
    towards an imaginary rectangle's corner. You'll see a sort of an
    ellipse, even egg shape if you look carefully, but the shape depends
    on distance.

    Due to the projection angle the circle is distorted because it hits a
    flat plane and not a spherical surface. This is the projection
    distortion caused by conversion from a spherical to a rectangular
    projection.

    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.

    Does it boggle the mind already? Told you it would.
    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.

    Bart
     
    Bart van der Wolf, Aug 4, 2004
    #97
  18. PrincePete01

    Nostrobino Guest

    Well, if I am at the position of the flashlight (which would be assumed),
    I'd still see a circle. Someone more perpendicular to the wall where the
    beam hit it would see an ellipse, that's true. But the person in that
    position would see an ellipse where I shone the flashlight beam
    perpendicularly to the wall, also (the ellipse then going in the other
    direction, i.e. vertically instead of horizontally). In fact there's only
    one angle from which he would see a perfect circle, and that's the same
    angle at which I shone the beam on the wall, but on the other side.

    Okay, now I see what you mean.

    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.

    No, it's mostly an article about the Scheimpflug principle, and I'm
    basically familiar with that.

    But NOTHING happens to the discs or dots. Look again. That's precisely the
    point the author is making: it's only the SOLID objects that are affected in
    this way.

    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.

    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.

    With ordinary rectilinear lenses there is no "spherical surface" to be
    concerned with. The plane of front focus (i.e., the area of sharpest focus
    in front of the lens) is in fact just that, a PLANE, or as close to a plane
    as the lens designers and makers have been able to make it.

    A fisheye lens on the other hand evidently does "see" the world as the
    inside of a hemisphere. When doing lens testing with a 16mm fisheye and the
    familiar 1951 USAF test targets, with the central target a few feet in front
    of the lens I had to arrange the others in a semicircle around it in order
    for them to be in focus. That is emphatically not the case with any
    well-made rectilinear lens, no matter how wide an angle.
     
    Nostrobino, Aug 4, 2004
    #98
  19. PrincePete01

    Jeremy Nixon Guest

    A plane is accomplished through (intentional) distortion. You're still
    photographing a sphere.
     
    Jeremy Nixon, Aug 4, 2004
    #99
  20. PrincePete01

    Nostrobino Guest

    You if that's what was happening, but it isn't (except with a fisheye lens).

    As I just mentioned to Bart, the surface of front focus is a plane, not a
    sphere.

    It depends. If I'm photographing the building face on, i.e. perpendicularly
    to the front of the building, then any right angle in the lower left corner
    (or elsewhere) is still a right angle.

    If I'm photographing obliquely it is not, because horizontal lines converge
    into the distance. That's perspective. A full frontal photo (if the subject
    were absolutely two-dimensional, no projections etc.) would have no
    perspective at all.
    Again, whether it is or isn't depends on how the photo was taken.

    WHAT "spherical image"? There isn't any spherical image.

    Actually they don't. That is to say, there really isn't any distortion
    (assuming a good-quality wide-angle lens). The apparent radial stretching of
    objects that increases as they approach the corners results from a
    difference in point of view. If viewed from the "proper" position (a
    position analogous to that of the lens relative to the subject when the
    picture was taken), the apparent distortion would disappear.

    Try it. Look at a photo with that wide-angle "distortion" and move your eye
    closer to the center of the picture. At some point the corners will look
    perfectly normal.

    There is no curvature.

    Objects exist in three-dimensional space. A lens produces a two-dimensional
    representation of that space on the film. The size, shape, angles and
    distances of the objects on that two-dimensional representation are
    collectively what is meant by perspective. That's all there is to it.

    No, I assure you it will not. It will be even farther from a right angle
    than it was in the oblique photo taken with the rectilinear lens.

    Your premise being wrong, your conclusion is wrong. A fisheye lens will not
    reproduce your right angle in the lower corner of the image. Far from it.

    Sure they are. They are just what you see with your eyes.

    To remove the unwanted perspective effect.

    Yes, that's PRECISELY what it is, with a rectilinear lens.

    Emphatically NOT! That is true with a fisheye lens (as I've mentioned
    elsewhere), but definitely not the case with a rectilinear lens.

    Jeremy, this appears to be the root of your misunderstanding, or at least
    part of it. Get a book on photographic optics and theory. I don't know where
    on earth you got these misconceptions, but you are WILDLY in error.

    Don't take my word for it. Start another thread in this newsgroup and pose
    the question, "Is the field of best focus in front of a rectilinear lens a
    FLAT PLANE, or SPHERICAL?"

    There is no point in continuing with this until you've gotten yourself
    straightened out on this.
     
    Nostrobino, Aug 4, 2004
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