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P3D Re: formats & focal lengths


  • From: Brian Reynolds <reynolds@xxxxxxxxx>
  • Subject: P3D Re: formats & focal lengths
  • Date: Tue, 5 Oct 1999 05:41:24 -0600

George Themelis wrote:
> --- Rod Sage <rsage@xxxxxxxxxxxxxxxxxxxx> wrote:
> > The Realist shows the same perspective vertically as the full
> > frame image, but the full frame image will still show a wider
> > perspective. In effect, the Realist will project a normal 
> > image and the full frame will project a wide angle image. 
> 
> Great!  You did not understand my point and you are propagating
> the same confusion regarding perspective and size of the image. 
> Let me try again:  Perspective is tied to viewing.  You can go
> ahead and crop an image as much as you like.  If you do not
> change your viewing method (i.e. magnification) then you have
> not affected the perspective or "effective focal length" of the
> taking lens.
> 

Perspective is the relationship (location and size) of different
subjects within a scene.  Perspective is only effected by the
positioning of the taking lens with respect to the subjects in the
scene.  The focal length of the taking lens does not effect
perspective (assuming that you are far enough away from your subject
that the difference in the physical size of the lenses is not
significant as compared to the lens to subject distance).  If you take
a picture from the same location with both 50mm and 100mm lenses and
enlarge the 50mm picture so that its subjects' size match that of the
100mm picture you will see that the relationships between the various
subjects is the same in both pictures.  See Ansel Adams' "The Camera"
(page 106).

Actually Mark and Rod mean angle of view.  The angle of view is
dependent on the lens focal length and the film format.  A 50mm lens
on a (full frame) 35mm camera is considered "normal".  An 80mm lens is
considered mildly long, and a 150mm lens is long.  (Telephoto is a
lens design, not related to the angle of view or the focal length.)
For a Medium Format camera a 50mm lens is wide angle, a 80mm is
"normal" and a 150mm is long.  For a 4x5 camera a 50mm is extremely
wide angle, a 80mm is wide and a 150mm is "normal".

If you take pictures from the same location with all three cameras and
lenses you would find that the perspective is the same for all nine
pictures.  Within a focal length (e.g., 50mm on all three cameras),
the image size would also be the same.  What would differ is the
amount of the scene visible on the film (the coverage of the lens).
The 35mm frame would be the same as a 24x36mm cropping of the center
of the MF or LF frames.  The MF frame would be a 56x56mm cropping of
the center of the LF frame.  If you use a 50mm lens from a 35mm camera
on a LF camera you get a circular image approximately 50mm in
diameter.  That's because the 35mm lens is not designed to cover more
than a 35mm frame of film.

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx              | "Oooh!"
http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
NAR# 54438                      |       "Dexter's Laboratory"