Fabulous Ditigal Photography Tutorials

I stumbled upon the following Digital Photography Tutorials after signing up for StumbleUpon.com:

http://www.cambridgeincolour.com/tutorials.htm

This one on Diffraction & Photography caught my eye:
Diffraction Limited Photography: Pixel Size, Aperature and Airy Disks

Bob Johhnson has a great artical on Diffraction issues at EarthBoundLight.com.
Diffraction: When Smaller Apertures No Longer Mean Sharper Pictures. He mentions it on a this Nikonian post