From: Joe Smigiel (jsmigiel@kvcc.edu)
Date: 12/18/02-08:20:04 AM Z
>>> zphoto@montana.net - 12/18/02 12:30 AM >>>
<I thought, Joe, that if you stood in the same spot and switched lenses it
would increase and decrease apparent perspective.>
I'm talking about physical perspective, not "apparent perspective" which I take to mean psychological perception.
<In other words, to
accomplish the same perspective with different lenses, you would have to
move your body with camera further back or further forward to get the same
exact size of scene in your viewfinder and only then would perspective be
the same. It is the distance from camera to subject that creates
perspective, not the lens, but if you are standing at the same point as you
say, above, perspective will change.>
Absolutely not. Physical location determines perspective.
<Hence, in practice (not theory) focal
length, standing at the same point in space, does change perspective. >
Nope.
<Maybe
it just changes the relative size between objects, but our eyes still
perceive that as perspectival change, in practicality. Please correct me if
I have this wrong. >
Yes. You are talking about object size, not perspective.
<I used to, also, believe that DOF for the same reasons
did not change; in other words, if you had a 28mm lens on your camera, and
then switched it to a 100mm lens and stepped back far enough so your subject
in the viewfinder was exactly the same size as when you were using the 28mm
lens, that DOF would be exactly the same. >
Hmmm? I've never thought about that. I would suspect that the dof would be the same (at some specific focal distance) if you moved back in space to match the object size. If you stay at the same spot, the longer lens will have less depth of field at the same aperture due to the sizes of the circles of confusion. The f/number as a ratio between focal length and aperture area would require 2 stops down to achieve the same depth if you doubled the focal length (f/4 @ 50mm would equal f/8 @ 100mm if memory serves me correctly.) Of course, you would probably have to be focussed at the same distance from the film (not the object) if this were to be true. Of course, this would mean you'd have a different picture, different perspective, etc. In my book, the way to work is to first find the spot you need to be in for the proper perspective, and then haul out the equipment and make adjustments for things like focal length, dof, etc. Otherwise, you are putting the cart before the horse.
<Then I find out that this is not
true, according to this website:
http://dfleming.ameranet.com/dof_imagesize.html which states that "wide
angle lenses do provide more DOF than telephoto lenses, when subject
distance is greater than approximately 1/4 of the hyperfocal distance of the
shorter lens."
Chris>
Could be, but I really don't care to even think about that one. Seems a weird way to think about such things. Wide-angle lenses (read short focal length) would have more dof than longer lenses at any distance, not just "1/4 of the hyperfocal distance" other things being equal. I know depth of field depends on focal length, aperture, and subject distance. Long lenses have less dof, near focal distances produce less dof, and large apertures give less dof, other things being equal.
Joe
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