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What make a lens normal ?

 
 
Dave Martindale
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      10-29-2006
Clark Martin <(E-Mail Removed)> writes:

>I understood a normal lens to be one that produces a perspective
>equivalent to the human eye. For example, you have two people, of equal
>height, one 100 feet away and the second 200 feet away. If you look at
>them through a normal lens or with the Mark I eyeball the farther person
>will look half as tall. If you look at them through a telephoto they
>will look closer in height. And if you look at them through a wide
>angle the far person will look even less than half.


That's definitely *not* true, as you can verify for yourself by simply
trying different lenses in the scenario you describe.

Under the conditions you describe, the near person will *always* be
twice as tall in the image as the far person, no matter what lens you
use. Changing focal length will make both of them smaller or larger at
the same time, but the 2:1 ratio between them always remains.

Dave
 
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x@x.com
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      10-30-2006
Dave Martindale <(E-Mail Removed)> wrote:
> http://www.velocityreviews.com/forums/(E-Mail Removed) writes:


> With a pinhole, you can have any "focal length" you want. With a lens,
> you find the point of best focus, and then its focal length *is defined
> by the image magnification* near the lens axis. In a sense, that's why
> the simple geometrical lens formulas work for complex lenses.


Right, a pinhole is always in or out of focus (or always out of focus by the
same amount). So in a lens, what happens physically when one turns the
focus ring (I've asked this in a different thread, but not getting any
answer) ?

>>Hmmm, interresting. Reading this you could think the four thirds system is
>>the way to go, but their lenses at equivalent speed (I guess quality is
>>somewhat subjective) aren't cheaper than the competition.


> Don't they say that they designed the system so that the rays of light
> reaching the corner of the sensor arrive at a shallower angle than they
> would with film? This means a large lens mount diameter and probably
> not reducing the flange depth, then using retrofocus designs like the
> other DSLR manufacturers would have to. There goes small, light, and
> inexpensive normal lenses.


Yes, but I had not realised that they had to make the lens larger in order to
achieve this. I did find that the lenses were pretty large, but thought that
this was due to the engine for the zoom and focus inside the lenses. That and
the fact that they decided to snob north-America with their newer better
model made me look at a different brand !



Yves.

 
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lemel_man
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      10-30-2006
(E-Mail Removed) wrote:

>
> Right, a pinhole is always in or out of focus (or always out of focus by the
> same amount). So in a lens, what happens physically when one turns the
> focus ring (I've asked this in a different thread, but not getting any
> answer) ?
>

The following link shows how a pinhole creates an image, and why it's
dim and not very sharp.
http://i106.photobucket.com/albums/m..._man/Lens1.jpg
A perfect pinhole would allow only one ray through for each point on the
object; the image would be perfectly sharp everywhere, but very dim.
If you enlarge the pinhole to let more light through, you get a little
blurred circle of light for each object point, so the image is brighter,
but blurred. The link also shows how a perfect convex lens bends rays
of light.
When a ray of light passes from one transparent medium to another (air
to glass, for example), it will bend unless it is perpendicular to the
boundary. A convex lens (or magnifying glass)is designed so that rays
parallel to its axis are bent so as to pass through a single point. This
single point is called the Focal Point, and the distance of this point
from the centre of the lens is called the Focal Length of the lens.

This link,
http://i106.photobucket.com/albums/m..._man/Lens2.jpg
shows how a convex lens produces an image. Of the rays from the top of
the image, one is parallel to the lens axis and gets bent to pass
through the focal point as shown, another goes straight through the
centre of the lens. A sharp image is formed where they meet. This effect
happens for all the rays from every object point that passes through the
lens.
Think of it as a cone of light coming from each object point to the
lens, then getting bent to form another cone on the other side of the
lens. The tip of this cone is then an image of the original object
point. If you place the image plane at this point, the image will be
sharp, but if you move the plane nearer to, or further from, the lens,
the rays no longer meet and instead of getting a sharp point you get a
little blurred circle and the image is out of focus.
When you turn the focus ring on a camera lens you simply move the entire
lens closer to or further from the film plane.

The amount of blurring is determined by the size of the image circle
that is meant to be a point; the bigger the circle the more the image is
blurred. The size of the circle is determined by the thickness of the
cone and the distance from its tip: the thicker the cone, the more
critical the focus. In camera terms, the F-number of a lens is the ratio
of the focal length (FL) divided by the diameter of its aperture (D): a
50mm FL lens with a D of 20mm would have an F-number of 2, and a D of
10mm would give an F-number of 4. Big F-numbers mean thinner light cones
which imply less critical focussing.

--
I hope this helps,
Gary Wooding
(To reply by email, change feet to foot in my address)

--
Posted via a free Usenet account from http://www.teranews.com

 
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Dave Martindale
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      10-30-2006
(E-Mail Removed) writes:

>Right, a pinhole is always in or out of focus (or always out of focus by the
>same amount). So in a lens, what happens physically when one turns the
>focus ring (I've asked this in a different thread, but not getting any
>answer) ?


It depends on the lens design. The "classical" way of doing this is
mounting all of the lens elements in a rigid barrel with fixed spacing
between them, and then use a focusing mount (which just uses a helical
thread) to move the whole optical barrel forward and back at the same
time. So the lens focal length remains exactly the same; you just
change the distance to the film/sensor as the subject gets closer.

On the other hand, some lenses have "internal focusing". In these, the
optics are arranged in two or more groups that can move independently,
and turning the focusing ring operates a cam that moves one or more
groups, changing the internal spacing of the lens elements. This might
change the actual focal length, or just move the position of the rear
principal plane without actually moving the rear element any. It's up
to the lens designer. This allows more design flexibility, but requires
some of the complexity of manufacture of a zoom lens.

Dave
 
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x@x.com
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      10-31-2006
Dave Martindale <(E-Mail Removed)> wrote:

> It depends on the lens design. The "classical" way of doing this is
> mounting all of the lens elements in a rigid barrel with fixed spacing
> between them, and then use a focusing mount (which just uses a helical
> thread) to move the whole optical barrel forward and back at the same

.../..
> On the other hand, some lenses have "internal focusing". In these, the
> optics are arranged in two or more groups that can move independently,


So on a zoom, assuming a very simple lens design, the focussing ring moves
the lens very slightly, but the zoom ring moves it by a lot more ?

On zooms, the size of the image does not change as you move the focusing ring.



Yves.

 
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Dave Martindale
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      10-31-2006
(E-Mail Removed) writes:

>So on a zoom, assuming a very simple lens design, the focussing ring moves
>the lens very slightly, but the zoom ring moves it by a lot more ?


All you can say with certainty is that the focusing ring moves one or
more components of the zoom in such a way that the plane of best focus
moves towards or away from the camera. While the zoom ring moves
one or more components of the zoom in such a way that the in-focus plane
does not move but magnification does change.

But the two movements will be different internally, since they have
very different effects.

>On zooms, the size of the image does not change as you move the focusing ring.


Not quite true, since the size of the image does change as you adjust
the focusing ring for *any* standard design lens, fixed focal length or
zoom. The exception to this is lenses that a telecentric in image
space, which are pretty unusual in photography.

Dave

 
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