Lenses
Introduction
Lens
interchangabil-
ity
Selection of the appropriate lens for the job ensures optimum resolution in
reproduction. If you duplicate slides or photographs without available
negatives, your selection of lenses could mean the difference in doing a good
job or not being able to do the job at all. Review Illustrator Draftsman
Volume 1, chapter 1, for elementary information on lenses.
Lens interchangability is one of the best features of SLR cameras. When you
cannot change your viewpoint or the position of the camera, or when
different focal-length lenses will enhance your subject, change lenses. Most
SLRs are designed with an manufacturer-exclusive method of lens
attachment. Some SLR manufacturers use screw-mounts; others use bayonet
mounts. Lenses from different manufacturers are either incompatible or
require special adapters to fit the camera body or align electronic contacts.
Do not force lenses onto camera bodies. Forcing lens mounts may
irrepairably damage the electric contacts that feed light meter, shutter speed,
and aperature information into the camera.
Lens
characteristics
Lens characteristics define lenses. Focal length, aperture and lens speed, and
depth of field are characteristics that combine to influence the images you
create.
Focal length
Focal length is the distance between the optical center of a lens and the focal
plane (film plane) of the camera when the lens is focused at infinity. To
understand this definition, you must fully understand the terms focal plane,
optical center, and infinity.
Focal plane -The surface or area at the back of the camera occupied by
film on which images transmitted by a lens are brought into sharp focus.
Optical center -The optical center of a lens is a point, usually (although
not always) within a lens, where light rays from two different sources
entering the lens are assumed to cross.
Infinity -Infinity is a distance so far removed from the camera that the
light rays may be regarded as parallel. Infinity is expressed by the
symbol 60 and is a setting on a camera focusing scale.
Continued on next page
7-28
