What is an "internal focusing" lens, and what are its advantages?
An internal focusing (IF) lens focuses by shifting select elements (usually elements in front of the lens diaphragm) within and independent of the outer lens barrel. With IF lenses, the outer barrel doesn't change in length or turn as the lens is focused. This makes IF lenses well-suited for use with lens-mounted accessories such as ring lights and filter systems.
Lens shades can also be larger and thus more effective, because the autofocus motors in IF lenses don't have to turn them. Other benefits of IF lenses: They're generally smaller and closer-focusing than comparable non-IF lenses.
I know that digital-only lenses cause corner vignetting when used on 35mm cameras, but I thought 35mm lenses worked on DSLRs without a hitch. So I was surprised, when I put Sigma and Tamron lenses on my new [Canon] EOS Digital Rebel XT, and the camera's LCD flashed "Error Code 99." What are my options?
Most often, Canon's Error Code 99 indicates incompatibility between a camera and lens. (Canon 35mm film cameras similarly blink the battery symbol.) A temporary fix is to shoot only at maximum aperture. The error code may disappear and normal readouts will return to the LCD.
Long-term, the issue can be solved inexpensively by returning the lens to its U.S. distributor to have its flexboard (or microprocessor) updated. Most independent lensmakers offer this for current lenses at no cost; you pay only the return postage. Call the customer service department at Sigma (800-896-6858), Tamron (800-827-8880), or Tokina (THK Photo, 800-421-1141) for more info.
What are aspheric lens elements, and what do they do?
Put simply, they're lens elements that are not sphere-shaped. Most camera lens elements have arc-shaped concave or convex front or back surfaces (see diagram); they're spheric. For many lens types, spheric elements can be combined to transmit light without introducing areas of fuzziness or bent lines (a.k.a. aberration and distortion).
However, with some lens types (usually ultrawide and large-aperture lenses), optical engineers can't easily control aberration or distortion with spherical elements alone. These lens types require one or more non-spherical ("aspheric") elements. Most aspherics are hybrids of a molded aspheric element made of plastic cemented onto a spheric element of glass. All-glass aspherics are less common because of the time and expense involved in grinding and polishing glass to the correct shape.