Why do some zoom lenses have a single, constant maximum aperture and others have variable apertures? Which is better?
Both offer advantages. As you zoom out, variable aperture zooms admit less light through to the film or sensor. As a result, the f-number changes. Example: A 28-300mm f/4.5-5.6 has a maximum aperture of f/4.5 at 28mm, and as you zoom out, the f-number changes to f/5.6 at 300mm.
Variable apertures let engineers incorporate longer tele settings in smaller, more affordable packages. In the example just cited, a constant-aperture 28-300mm f/4.5 would need to maintain the aperture as you zoom out, and so would require significantly larger glass elements to deliver the 300mm focal length at f/4.5; the resulting lens would be large and expensive. Designing the lens to dim to f/5.6 when zoomed out, however, contains both size and cost.
Because most metering is TTL, the exposure system automatically compensates for the shrinking aperture. From an exposure standpoint, most photographers will find no downside to variable-aperture zooms. However, photographers who set exposure with handheld, external meters typically bracket exposures more when shooting with variable-aperture zooms.
Another potential downside to variable aperture? Depth of field. The smaller apertures at longer focal lengths make it harder to throw a background out of focus as you zoom out.
How does image stabilization in SLR/DSLR lenses work, and should I spend the extra money for it?
For average SLR shooters, camera shake introduces blur in photos made at shutter speeds slower than the reciprocal of the lens focal length. A 300mm lens used without a tripod or flash will usually show blur at shutter speeds below 1/300 sec. Image stabilization lets you shoot at 1/250 sec, 1/125 sec, even 1/60 sec, and still enjoy sharp pictures.
It does this thanks to a floating element (see diagram, next page) whose movements are controlled by a gyroscope-like sensor that recognizes up/down (pitch); and another, sensitive to left/right (yaw) movement. Processors calculate the direction and degree of off-axis camera movement, and the device automatically shifts the floating element to compensate for that movement.
If the lens goes up, the floating element rises as well, bending the incoming light rays to strike the imaging plane from an angle that's relatively uninfluenced by the (slight) camera movement.
As for the second part of the question: We've never met a photographer who regretted investing in IS.