P3D Re: Hypo - Ortho - Hyper OR convergence angle?

[Brian Reynolds <reynolds@xxxxxxxxx>]

Dr T wrote:
> 
> How about the terms "wide", "normal" and "telephoto" used to classify 
> lenses?  These seem arbitrary to me too.  If you view a picture taken 
> with a wide angle lens close enough or one taken with a telephoto from 
> a distance you get a "normal" perceptive.  But this is a way to classify
> a lens and also you get an idea of the effect if the images are viewed
> from a constant distance (or FL lens, etc.)
> 

Actually these terms are even worse than (and more abused than) Hypo,
Ortho and Hyper.  First off the terms should be short, normal and
long.  Wide and telephoto both describe the lens design, not it's
focal length.  Secondly in order for short, normal and long to mean
anything you have to know what film format you are discussing.

A wide lens describes the angle of coverage for that lens.  A 210mm
lens for 8x10 would have to be wide angle (at least 75 degrees) in
order to cover the entire film surface.  A 210mm lens for 4x5 could
use a somewhat normal angle of coverage (42 degrees).  A 210mm lens
for 35mm can have a narrow coverage angle (23 degrees).

A telephoto lens is a lens design where the nodal point (the point
from which you measure the focal length) is in front of the lens.
This means that the distance from the mounting point (a flange for
large format lenses) to the film surface is less than the focal length
of the lens.

Short, normal and long refer to the focal length of the lens as
compared to the diagonal of the image on the film.  There is
considerable debate as to what a "normal" lens is.  50mm is considered
normal for 35mm cameras, but is actually a bit long for 8p, and
considerablly long for the stero formats (except for 2x2x2).  162mm is
considered normal for 4x5, but the available lenses tend to be 150mm,
and most people use 210mm for their "normal" lens.

By the way, focal length has nothing to do with perspective.
Perspective is controlled by the subject to lens distance (and lens
orientation if you have a camera with movements).  If you take a
series of pictures with your camera on a tripod and use a a set of
different lenses (or different focal length settings on a zoom) the
perspective will be the same for all the pictures.  For an example of
this see chapter 7 of Ansel Adams' "The Camera".


-- 
Brian Reynolds                  | "Humans explore the Universe with five
reynolds@xxxxxxxxx              |   senses and call the adventure science."
http://www.panix.com/~reynolds/ | - Edwin P. Hubble


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