The Canon EOS Digital Rebel XSi/450D Companion

CUSTOMIZING THE XSI

LENSES

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Shoot at wide open; then shoot again at f16 or f22. Is there a sharpness drop-

off? If there is, that’s not a deal breaker. Most cameras have an sharpness sweet

spot that sits within a specific range of apertures. Ideally, you want this range

to be as wide as possible.

Chromatic aberrations Are there weird purple, blue, or magenta fringes

around high-contrast areas in your scene? is can be hard to test in a cam-

era store, but if they’ll let you step outside, try shooting through some trees

against a bright sky or along telephone wires. You’ll most likely find these is-

sues on the edges of the frame. Also, try a range of apertures. Sometimes,

these colored fringes show up only at the extremes of the aperture range.

Distortion Does the image bow inward or outward at the edges of the frame?

Wide angles will almost always have a little distortion, and you can usually

correct it in a good image editor. Still, it’s good to know whether the lens has

these issues.

e upper-left image suffers from barrel distortion, which causes the lines to bow

outward. e upper-right image is fairly distortion free, while the lower image has

a problem with pincushion distortion, which causes its lines to bow inward.

Vignetting is is a darkening of the corners that usually appears in wide-an-

gle lenses and usually only with wider apertures. While it can ...