</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Ed Sukach @ Dec 14 2002, 05:54 AM)</td></tr><tr><td id='QUOTE'></span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (tschmid @ Dec 14 2002, 02:26 AM)</td></tr><tr><td id='QUOTE'>
This is not a real focus shift, it is an apparent focus shift. Some aberrations can actually be diminished by a slight defocus (i.e. an offset to the extension computed by the exact focal length and the exact magnification ratio/subject distance). ...</td></tr></table><span id='postcolor'>
Again, I agree.

You may have noticed that I wrote about &quot;Optical Bench&quot; testing - that dates me... I think the last lens testing I did was around 1990. Now, most lens quality is defined in terms of Modulation Transfer Function (MTF).

An excellent article is &quot;Understanding Lens Contrast And the Basics of MTF&quot;, by Mike Johnstone, available at The Luminous Landscape web site, http://www..luminous-landscape.com under &quot;Tutorials&quot;, &quot;Understanding Series&quot;...

Luminous Landscape is a good site, but strongly tilted towated &quot;digital&quot; imaging.</td></tr></table><span id='postcolor'>
Correction: One too many periods in the address - Should be:
http://www.luminous-landscape.com.