"Derek Fountain" wrote in message
...
I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon
DSLR
and have been pondering lenses. I find a lot of lenses which have a fixed
aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought
that fixed aperture would be a bad thing...

One of the things about a compact camera is that changing aperture from
one
end of the range (f2.8 for the S40) to the other (f8 for the S40) really
doesn't do a great deal in many cases. I was rather looking forward to
working, experimenting and learning with a system that allows a good range
of aperture adjustment. But now, in my quest for a small number (like 1 or
2) of quality lenses as a starting point, I find myself homing in on
lenses
with fixed aperture.

Am I right to be concerned about this, or is having a single wide lens
fixed
at f4 a good thing for reasons I don't understand?

For a lens that is (for example) a 24-70mm 2.8 L, the "2.8" simply indicates
that the maximum aperture of 2.8 is available throughout the zoom
range...which is highly preferable to most consumer zoom lenses that end up
giving you a reduced max aperture as you zoom toward the tele end.
Remember, this ONLY refers to maximum aperture, and does NOT indicate a
limitation of adjusting to smaller apertures. On lesser lenses, you'll see
a range of apertures given, but that only indicates how much the **maximum**
aperture changes throughout the zoom range. Most zooms allow larger
apertures at the wide end of the zoom than at the telephoto end. You can
always reduce the aperture to whatever the limitations of the lens is
(usually anywhere from f22-f32, or perhaps f64, up to whatever it's max ap
is).

The only modern lenses (that I'm aware of) that have a TRULY "fixed"
apertures (CANNOT be changed) are mirror lenses--which are NOT a good choice
for most people, due to their strange rendition of background out-of-focus
elements, and highlights.