This has been posted in the past on this list but did not include the reference
to digital cameras and their image sensors. So here it is again for what it is
worth.
A Brief Comment on Perspective
In order to view a picture with the proper perspective, we wish to preserve the
angular relationships between the original scene and the picture. As a result,
we should view a contact print from a film camera, or a print equal in size to
the dimensions of the sensor in a digital camera, at a distance equal to the
image distance in the camera.
This distance is equal to the focal length of the lens unless the lens is focused
on a close-up subject.
For accurate perspective an enlargement (a print larger than the contact print or
sensor size) requires a viewing distance equal to the product of the focal length
and the enlarging magnification. So, a magnified print from a 35mm film frame to
5x7 inches and originally photographed with a 50mm lens should be viewed from a
distance of about 5 inches / 1 inch where M then is 5x and so the proper viewing
distance for perspective is 250 mm or about 10 inches. In the case of a digital
camera with a 16x24 mm sensor from which a 5 x 7 inch print is made the situation
is 5 inches / .66 inches and the M is then 7.5x and the viewing distance for
proper perspective is 50 x 7.5 or 375mm or about 15 inches.
In both of the above cases the taking lens is assumed to be of 50mm focal length.
And M stands for MAGNIFICATION or enlargement factor.
Quoted from "Introduction to Photographic Principles" by Lewis Larmore, Director,
Advanced Research Laboratory, Douglas Aircraft Company. Dover Publications, Second
Edition, May 1965. Updated to include digital cameras on 06-07-09.
Andy's extra comment: The consequence of this is that if we view a print from the
"wrong" distance (as related to seeing a print in normal perspective) a print will
exhibit "strong" perspective if viewed from too far a distance (usually the case
when wide angle lenses are used) and "weak" perspective (usually the case when
photographs with long focus or telephoto lenses are made) if viewed from too near
a distance.
==== if you notice any errors in the stuff above please bring to my attention ====
andy
