On Sat, 11 Jul 2009 12:23:51 -0400, Alan Browne
wrote:

wrote:
On Fri, 10 Jul 2009 08:20:27 -0700 (PDT), RichA wrote:

People buy fast lenses for 2 reasons:
-The want the speed to capture action.
-They want the shallow DOF afforded by such lenses.

For for the first reason, speed is not always the best choice.
Shooting at a higher ISO sometimes works out better than using a lens
wide open and once you determine that it does for a specific lens,
then there is generally no reason to shoot a specific subject at the
wider aperture. The benefits are better image quality (fewer image
aberrations), possibly more accurate focus and the potential to use a
much cheaper lens. The downside is noise and giving up shallow DOF.

Here is an example of a shot at 800 ISO with a lens wide open and 1600
ISO with the lens stopped down. Shutter speeds were the same, so you
aren't giving that up.

The trick is to determine which lenses fall into this category.

http://www.pbase.com/andersonrm/imag...08342/original

I thought all lenses worked better if stopped down.

Generally _most_ lenses are sharpset stopped down 2 - 3 stops from wide
open. Most diffract to a softer image beyond f/11, noticeably so beyond
f/16 or so - though in most common print sizes this not usually noticeable.

Likewise, the softer wide open shots only show as soft at 8x12" or so
(for a 35mm cropped sensor camera)

This is only true for all poorly figured DSLR glass. The same cannot be
said for true diffraction-limited glass (the most precise lens polishing
that money can buy), as is found in nearly all higher-quality P&S cameras.

Simple test: If you open up your aperture wider at any point and the image
gets worse, that is NOT diffraction-limited optics. Meaning, it's just
poorly figured glass. DSLR buyers are so easily fooled. They still live by
that oft-disproved saying "you get what you pay for". The lens and camera
makers more than happy to take advantage of their naïveté and ignorance.