Re: Effects of lens length

[P3D Don Chaps <dchaps@xxxxxxx>]

On Wed, 4 Sep 1996, P3D Marvin Jones wrote:

> > 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. There is a famous shot in the movie Jaws (which has been copied by many
> 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.
> 
> 
If you take both pictures from the same place, the perspective will be 
the same and you could crop. If you move so the subject is the same size 
on the film (move closer with the wide angle), then no go.
Perspective is the relationship of distances between objects, the camera 
only records 
it. In your Roy Scheider example, it is moving the camera which causes 
the change in perspective.
Every print has a viewing distance at which the perspective will seem 
correct. multiply the focal length of the taking lens by the enlargement 
factor. for example, 50 mm lens, 8x10 enlargement, 50 times 8 = 400 mm 
viewing distance. A picture taken with a 25mm and enlarged to 8x10 (we're 
assuming 35mm film, of course) would have a viewing distance of 200mm. If 
you make a 16x20 from that neg made with a 25mm and cut out an 8x10 
section to match your 8x10 made from the 50mm, the perspective will be 
the same (16 x 25 = 8 x 50) and both will have a viewing distance of 
400mm for the appearance of "normal" perspective.
I think that there is a series of pictures in Ansel Adams "Camera and 
Lens" which illustrates this, if you want to see it without taking the 
pix yourself.


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