</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 "Optical Bench" 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 "Understanding Lens Contrast And the Basics of MTF", by Mike Johnstone, available at The Luminous Landscape web site, http://www..luminous-landscape.com under "Tutorials", "Understanding Series"...
Luminous Landscape is a good site, but strongly tilted towated "digital" imaging.