Effects of lens length

[P3D Bob Aldridge <bob.aldridge@xxxxxx>]

->Date: 04 Sep 96 16:11:23 EDT
->From: P3D Marvin Jones  <72657.3276@xxxxxxxxxxxxxx>
->To: "INTERNET:photo-3d@xxxxxxxxxxxxxxx" <photo-3d@xxxxxxxxxxxxxxx>
->Subject: Re: Effects of lens length
->Message-ID: <960904201123_72657.3276_EHM64-1@xxxxxxxxxxxxxx>
-> 
->> question - if you did this, would you have the same stereo image
->> as if you had shot both sides with lenses of the same FL? That is-
->> if you crop an image taken with a 50 mm lens to the same part as
->> an image taken with a 80 mm lens, and make them the same size -
->would > you have the same thing as two pictures both taken with 80 mm
->lenses? > 
->No. The length of a lens impacts the relationship of foreground and
->background objects. 

Actually, the length of the lens DOES NOT change the relationship of
foreground and background - it's the VIEWPOINT that does that. From the
same viewpoint (which is what we are talking about, here) all that the
longer lens does is to magnify part of the scene.

This, of course, is why different film formats have different "standard"
lenses. A 50mm lens on a medium format camera will give precisely the
same image ON THE FILM as a 50mm lens on a 35mm camera, but the 35mm
camera will have cropped out part of the scene.

Conversely, an 80mm lens on a medium format camera will take in around
the same angle of view as a 50mm lens on a 35mm camera...

->There is a famous shot in the movie Jaws (which has been copied by 
->others since) in which the camera started on a shot of Roy Scheider
->at a fairly wide-angle setting of a zoom lens. As he realized that
->the shark was attacking, the camera was pulled away from him quickly,
->and the lens zoomed in toward him at the same rate. The result was
->that the actor appeared not to change, but the background seemed to
->rush up toward him. As the lens became longer, in effect, the
->relationship between the foreground object (the actor) and the
->background appeared to compress, and the effect was heightened by the
->camera movement and the zoom counteracting each other to give the
->appearance that the actor did not move. I'd guess this effect would
->seriously harm the stereo effect in the hypothetical situation you
->suggest. 

Aahh! But in this example, the VIEWPOINT changes! The focal length is
changed simultaneously the control the magnification of the image of Roy
Scheider, but it's the viewpoint change that does the damage!

Bob Aldridge
Stereoscopic Society Projectionist
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