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From: kar@ritcv.UUCP (kar)
Newsgroups: net.rec.photo
Subject: Re: wide-angle lens distortion
Message-ID: <968@ritcv.UUCP>
Date: Thu, 29-Mar-84 09:16:35 EST
Article-I.D.: ritcv.968
Posted: Thu Mar 29 09:16:35 1984
Date-Received: Fri, 30-Mar-84 03:21:26 EST
References: <2587@rabbit.UUCP>, <961@ritcv.UUCP> <135@ur-laser.UUCP>
Organization: Rochester Institute of Technology, Rochester, NY
Lines: 38

In response to Bill Lampeter's statements:
> Wide angle lenses certainly may distort straight lines.  These distortions are
> known as barrel and pin cushion distortions.  In fact, such distortions are 
> not limited to only wide-angle lenses but are often most noticeable in them.
> Since wide angle lenses may indeed distort, you can see it in prints.  

These distortions are present (to some degree) in lenses of all focal lengths.
The claim I was replying to, "wide angle lenses distort straight lines",
clearly implies that this distortion is a result of something that is unique
to wide angle lenses, and not barrel or pincushion distortion.

The most apparent "distortions" one sees when viewing a picture made with a
wide angle lens are the exaggeration of depth and non-parallel lines.  The
depth effect can produce geometric "distortions" as well, for example, circles
may appear to be elliptical, but all of these are eliminated when the print is
viewed from the proper distance.  This is almost always too close for comfor-
table viewing of prints made with lenses shorter than 35mm, but if you are as
nearsighted as I am you will be able to get it close enough for the
"distortions" to vanish.

The other "distortion", non-parallel lines, is typified by photos of towering
buildings in which the walls of the building seem to be falling toward each
other.  This only occurs when the film plane is not parallel with the object
being photographed, in which case the camera is faithfully recording what it
sees.  With 35mm cameras, you must keep the camera perfectly level when taking
pictures of buildings and so forth in order to avoid the "distortion."  With a
view camera, you would simply raise the lens board so as to include the top
of the building in the photograph without having to tilt the camera.  I
recall reading an article about a camera with a permanently mounted super-wide
angle lens for making panoramic photographs.  The camera had a built-in level
to simplify the vitally important task of levelling it.

These two "distortions" can happen with lenses of any focal length, but are
most apparent with very wide angle lenses.  They are NOT caused by your lens,
however.

	Ken Reek, Rochester Institute of Technology
	seismo!rochester!ritcv!kar