Danny wrote:
everyone seems to want 'fast glass' - f/2.8, 1.7, 1.4, etc, but I'm left
wondering 'why' As most people know, lenses are not at their sharpest wide
open - usually requiring f/7 - 9 (ish) before they attain their maximum
potential.

So why the fuss about 'fast' lenses?, and why pay the huge premiums to own
them? Most of my own lenses are 2.8, with one at 1.7, and one at 1.4 - yet
I invariably have to stop down in order to get a nicely sharp image, which
sort of defeats the object of the 'fast glass', doesn't it?

I mean, why pay three times the price for a 'fast' lens, when you only end
up stopping it down? I've got a theory that it's the mania for bright lenses
that's behind a lot of the 'focus fuss' that clogs up the various forums -
perhaps people don't realise that you're just not going to get the same
image quality out of wide open lens and are mistaking 'softness' for focus
issues. Just a thought.


Danny,
I agree with you in the short focal length range. But even then there
are situations where the aberrations wide open are acceptable in order
to get the shot. For example, a low lit room with no flash.
I generally image landscapes at about f/8 to f/11 for maximum depth
of field and image sharpness, when I do small formats (35mm, DSLRs).

But in the super telephoto range, the lenses are designed to
deliver very high quality images without stopping down.
Look at the 300 mm f/2.8, 400 mm f/2.8, 500 mm f/4 and 600 mm f/4
lenses. They have spectacular performance wide open, and you
need it for action shots.

Roger
Example on my web site, see bid and bear galleries:
http://www.clarkvision.com