"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?

The higher priced lenses like the Canon L 17~40 is not a fixed aperture
lens, it's a constant aperture lens. This means it stays the same through
the entire zoom range. Example when wide open at f:4 it stays f:4 all the
way from 17 to 40mm. If I set it at f:8 it stays a constant f:8 through the
entire zoom range. A less expensive lens can be f:3.5-5.6 so at 17 it's
f:3.5 but drops to f:5.6 at the 40mm end. The TTL light meter will
compensate for this, but if I switch to manual, like when I use a studio
flash I would have to correct for the offset as I zoom.

I hope I have explained this clearly, if not ask and I can try to simplify
it more.

Darrell Larose
Ottawa, Ontario