Re: Effects of lens length

[P3D <CJMCE@xxxxxxxxxxxxxxxx>]

>>>> question -
>>>> 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.
>>
>>Sorry Marvin, but I think the answer is "yes".  The FL will only affect
>>the magnification. If you are standing at the same spot then you can
>>magnify or reduce pictures taken with different FLs for perfect match.
>>BUT, if you move to match the field of view of the two lenses, then
>>you have altered the perspective or, as you say, the relationship of
>>foreground and background objects.
>>
>>It's a simple concept and yet not many people get it right.
>>
>>George Themelis
>
>Now that I think about it, I believe George is correct. The effect
>in "Vertigo" was created by moving the camera position, and adjusting
>the zoom factor to keep the actor's image the same size. The actor/
>background relationship change was due to the change in camera position
>rather than the change of "zoom".

>The effect I'm thinking of would be holding the camera position 
>essentially the same (separated by stereo spacing) and only changing
>the "zoom" factor. In considering what I see when I do a zoom with
>my 25 - 80 SLR lens, the images gets larger or smaller, but
>it does not change the spatial relationships within the frame itself.
>
>  -------- Bob Wier ----- wier@xxxxxxxxxxxxxxx -----
 
A photo is a perspective view that is created by a "lens" which can be 
considered as a "perspective point of view".  The manifestation of this is
the presence of vanishing points.  Depending on the closeness of parallelism of
the film plane (or CCD array plane) to the X-Y-Z coordinate system of the
object(s) in the photo, one or two or three of the vanishing points may even
fall within the photo itself.  Oftentimes one, sometimes two vanishing points
may be at infinity, but never all three.  The orientation and position of the
vanishing point(s) are a function of the scale of the photo which is determined
by the camera station position and attitude and the focal length of the lens.
The only way to recapture the same view is to stand at exactly the same point
and take another photo with exactly the same focal length lens IF using exactly
the same film format.

In forensic photogrammetry, there's a technique called "reverse projection" that
is favored by a few specialists at the sub-professional level.  (Curiously, the
FBI has favored this empirical approach for a number of years ...)  Anyway, what
they do is try to recapture the perspective in an existing photograph in order
to determine the unknown dimensions of some object(s) in that existing photo.
This is attempted by returning to the crime scene with a camera with a zoom 
lens.  The photographer then empirically (by guess and by golly) determines the
position of the original photographer's stand point by walking around and around
until he obtains a view similar to the original photo.  This is accompanied by
iterative (over and over) trials of zooming different focal lengths and moving
about until the photographer decides that he is EXACTLY where the original photo
was taken and he has EXACTLY the same perspective view.  Supposedly, the 
culmination of this guessing game walk-about yields the exact same perspective,
and THEN they have assistants enter the field of view with tape measures, 
surveyor's range poles or leveling rods and take a number of photos to 
demonstrate the size of things.

In Forensic photogrammetry, the analytical determination to the above is
superior in that a rigorous mathematical solution can give a determination
not only of unknown dimensions but of the ERRORS of the computed unknowns.
The method of reverse projection cannot do the latter; a critical point in
the Courtroom.

Clifford J. Mugnier  (cjmce@xxxxxxx)
Topographic Engineering Laboratory
Department of Civil & Environmental Engineering
University of New Orleans
New Orleans, Louisiana  70148
Voice: (504) 280-7095
FAX:         280-5586      


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