Lens Characteristics and Specifi cations 235
When focusing a zoom lens manually, focus with the lens set to the longest focal length
regardless of the focal length setting that you plan to use. At the longer focal length, the image
on the focusing screen has the highest magnifi cation and the minimum depth-of-fi eld, and
focusing is likely more accurate. This suggestion does not have to be considered with auto-
matic focusing.
Optical and Digital Zoom
In the specifi cations for digital amateur cameras, you fi nd terms like optical zoom and digital
zoom. An optical zoom is the type described in the previous paragraph where the image on
the sensor is magnifi ed more or less by moving some of the lens elements in the lens. You
always use the entire image that is recorded on the sensor. Image quality is the same at all
focal length settings, except perhaps for minor, usually unrecognizable differences in the lens
quality at different settings.
In a digital zoom, nothing changes in the lens. The lens records one specifi c area like a
fi xed focal length lens. The “zoom” is done by magnifying a part of the image recorded on the
sensor. This is the same approach that we have used for years in fi lm photography by printing
just a segment of the entire negative. Enlarging just a part of the negative results in a loss of
sharpness, and exactly the same is true for a digital zoom where the image sharpness in the
smaller image area is greatly reduced. That is the main reason why digital zooms are never
used in a camera for serious photography.
Angle of View
The angle of view is the angle of the area that the lens covers on a specifi c image format (see
Figure 14-2). The angle of view can be related either to the diagonal or to the horizontal or ver-
tical dimension of the picture. If you care to study such fi gures, make certain you know what
they represent (Figure 14-2). A lens that has a diagonal angle of approximately 50 degrees is
considered standard on any image format in fi lm or digital photography. The angle of view and
thus the area coverage is directly proportional to the focal length. The angle of view of a lens
that has a focal length twice as long is half as large, and the lens covers a horizontal and verti-
cal area that is half as wide and high at the same distance and on the same size sensor or fi lm
area. Vice versa, a 35 mm wide angle has an angle of view twice as large as a 70 mm lens and
covers an area twice as large horizontally or vertically on the same image format.
Area Coverage
The angle of view of a lens determines the area coverage and the magnifi cation. The area
coverage for any lens is directly proportional to the subject distance. At twice the distance, a
lens covers an area twice as wide. If a lens covers a larger area, subjects are recorded smaller
because the magnifi cation is lower.
Lens Factor
In literature and specifi cation sheets for digital cameras, especially the DSLR types, you come
across a relatively new term — “lens factor.” It has to do with the fact that digital sensors in
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