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 dow