Absolute Beginner's Guide to Digital Photography

causes parallel lines to tilt towards each other if the camera is tilted. Before

Photoshop, these lenses were the only way 35mm and medium-format architectural

photographers fixed the distortion of buildings. These lenses are now essentially

obsolete because you can fix the problem in Photoshop.

Catadioptric (Mirror) Lens

A catadioptric lens (also called a mirror lens) is similar

in design to a telescope. It incorporates curved mirrors

as well as glass elements within the lens. The result is

a very long focal length that is smaller and lighter

than a typical lens of equivalent focal length.

These mirrors come into vogue and then fall back

out again every few years. These lenses are the

smallest telephoto lenses you can buy, but they

have some issues. The front mirror in the lens has

the unusual effect of causing out-of-focus high-

lights to take on a donut shape, rather than the

usual disk shape produced by an ordinary lens.

Mirror lenses are also slow because of a fixed aper-

ture. The aperture is usually f/8 or f/11.

Automatic Focus

Automatic focus (AF) does the focusing for you. In the simplest designs, you push

the shutter-release button and the lens snaps the image into focus. The camera

adjusts the lens to focus sharply on whatever object is at the center of the viewfinder

within the focusing brackets. This type of autofocus works well in situations where

the main subject is—and stays—in the middle of the picture. The camera will dis-

play a confirmation light when it has focused, but the light does not assure that the

picture will be sharp overall.

If your subject is not within the focusing brackets, you can use autofocus lock to

make it sharp:

1. Frame the subject within the focusing brackets.


2. To temporarily lock in the focus, press the shutter release button halfway down.


3. Keeping the shutter partially pressed, reframe the scene (move the subject out
   of the center), then press the shutter release button all the way down.

Wide-area focus systems provide more elaborate electronics. The viewfinder displays

several focusing brackets. By rotating a dial or thumbwheel on the camera, you select

54 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY

Catadioptrics aren’t as

sharp as high-quality tele-

photos. Most professionals use a

telephoto and a tele-extender

when they need extreme tele-

photo capabilities.