Lens Characteristics and Specifi cations
THE TECHNICAL PART THAT YOU MUST KNOW ABOUT LENSES
Lenses create the image on the fi lm or the sensor of the digital camera or digital back, there-
fore, they are a major element that determines how the image is recorded in the camera.
Understanding the characteristics of lenses and how they can be used to create the desired
results and make the images visually effective must be part of the photographer’s photo-
graphic education regardless of whether you work with fi lm or digitally. This knowledge
helps you to determine which lens should be used for a specifi c job and how to use it to pro-
duce the desired image. That is the reason this chapter covers the technical aspects and the
use of lenses in great detail.
Lenses produce the image on a sensor in a digital camera in exactly the same way they
do on fi lm except that the fi lter in front of the digital sensor needs some technical considera-
tion. For photographic purposes, everything in this chapter applies to fi lm and digital photog-
raphy except the fi gures for angle of view and covering power vary somewhat because of the
size difference between the fi lm format and the sensors sizes in the digital backs or digital
cameras.
FOCAL LENGTH
Lenses are distinguished mainly by their focal length, which is an optical characteristic built
into the lens that can be changed only by adding or removing optical components, such as
a teleconverter or by moving some of the lens elements as done on zoom and some macro
lenses. Focal length is unchanged when you add extension tubes, switch a lens from one cam-
era to another, or use a lens for different fi lm formats or for different sensor sizes.
To classify lenses into standard, wide angle, and telephoto, consider the focal length in
relation to the format. A lens is considered standard when its focal length is about equal to
the diagonal of the picture format for which it is used. That comes out to about 62 mm for the
36.7 49.0 sensor, 56 mm for the 33.1 44.2 size, 52 mm for the 36.7 36.7 size, 70 mm
for the 6 4.5 fi lm format, and 78 mm for the 2¼ square making 80 mm the standard focal
length for this format. An 80 mm focal length can also be considered a good standard lens for
the 36.7 49.0 sensor size and the 6 4.5 fi lm format. The term “standard” is somewhat
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