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Old Apr 13, 2004, 12:45 AM   #1
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Default *ist crop factor/perspective

I lay awake at night thinking about these things (yes I know I need to get a life).

The *ist has a 1.5 crop factor. That is to say that compared to a 35mm film camera the *ist only captures 2/3 of the projected image (somebody correct me if I am wrong here). Thus we relate a 50mm focal length lens on the *ist as being equivalent to a 75mm focal length lens on a 35mm SLR film camera. That begs the question: What about the perspective ??

On a 35mm film SLR a 50mm lens is considered “normal”. That is to say a 50mm lens produces an image that is close to the same perspective as the human eye. A long lens such a 200mm tends to “compress” the perspective and a short lens such as 30mm tends to expand the perspective. Please seethe following URL for a good demonstration:

http://www.photozone.de/3Technology/...alCompress.htm


Mounting a 50mm lens on a *ist neither changes the focal length or focal point. In actuality less of the projected image is captured. The perspective remains the same. Thus saying that a 50mm lens on an *ist is equivalent to a 75mm lens on a film camera is very inaccurate because it ignores the compression factor. A 50mm lens is still a “normal” lens . . . it just captures a smaller image. Note: to capture the full image at the same resolution would require approximately twice the number of pixels.

Now to really confuse the issue:
When one compares a digital camera with fixed zoom lens (example: Panasonic DMC-FZ10, 25mm to 420mm equivalent) is the 50mm equivalent a “normal” perspective? I am thinking that because these cameras have specifically designed lens they more closely match the perspectives that we would expect for film camera. Everyone please feel free to correct my ignorance on this matter.


New Lens:
What about the perspective of lens designed with smaller image circles specifically for digital SLRs such as sigma's new lens ???


Another question to ponder:

If a given lens on a 35mm film SLR camera is known to have “edge” or “corner” problems, will these be less apparent on the *ist because it is blind to the normal edges and corners.



I do not seem to remember these points in any reviews or forum threads but then I am beginning to become a bit senile. Or perhaps they are so obvious that everyone else recognized them and I am a bit slow on the uptake. Or lastly I am completely out to lunch (as my boss has stated on a number of occasions). I appologize if I am covering old ground.


Thoughts? Comments? Remarks? Flames?
Perhaps your "perspective"???


Lewis Balentine
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Old Apr 13, 2004, 11:42 AM   #2
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Quote:
New Lens:
What about the perspective of lens designed with smaller image circles specifically for digital SLRs such as sigma's new lens ???


Another question to ponder:

If a given lens on a 35mm film SLR camera is known to have “edge” or “corner” problems, will these be less apparent on the *ist because it is blind to the normal edges and corners.
New lens: these new (DX) digital lenses will prodouce round photos on black background used on a analog SLR.


Another question to ponder:
Yes - this is why it's called a crop-factor (or mulitplier).
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Old May 19, 2004, 3:39 AM   #3
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I think you mix two different terms: perspective, and field-of-view. Remember, light travels as STAIGHT LINES. If you ignore lenses with distortions, everything a lens "sees" is what *you* see. The differense is that it magnifies only a part of it. A wide angle lens captures a large field-of-view, the extreme fish-eye lenses covers 180 degrees! A telephoto lens covers a small field-of-view, perhaps only a few degrees.

A telephoto lens seems to compress reality, but it does not. It only produces what-is-there, inside the small field-of-view. What you look at through a telephoto lens is often far away. That is why the perspective seems "compressed", because two objects you look at all are far away. Relative to each other it is almost the same distance. Think about the moon pictures, and some wolf or tree in front. The moon is the same size as the wolf!! Hey, did the lens do that? No, it is because you are so far away from the wolf that it relative size, seen from your perspective, is compareable to the size of the moon (seen from earth!) Telephoto lenses also have a small depth-of-field, which increases the feeling of "compression".

With a wide-angle lens, you are often close to some objects, and much farther away from others. Because of the large field-of-view, the objects farther away seems very small. This gives you a total other perspective than in the telephoto case.

Frame the world with your hands and fingers, and you'll see what I meen.

This meens that perspective is a function of distance to objects. Attaching a 50mm on *istD only changes its field-of-view, nothing else. But you will notice that because it has been smaller, you must move farther away from your objects in order to capture them inside the image frame. Thus, you change your perpsective...

Hope this helps...
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