Lens sharpness wide open

[John_McLeod@designlink.com (John McLeod)]

I just received a mint 50/1.4 R lens from Don Chatterton.  After shooting a
couple of rolls of slide file under controlled conditions (on a tripod, at
sequential aperture and shutter speed combinations), it appears that the lens
is an excellent performer with one minor exception.  I am trying to decide
whether to keep the lens or return it to Don and will appreciate any input
from the group.

While in no way an expert in optical design or performance, I do know from
years of experience with various Nikon, Canon, Leica, and Zeiss lenses that
(a) "identical" lenses vary some from one to the next and (b) virtually all
lenses perform less well wide open than they do stopped down a few stops. 
Most all lenses that I have owned, for example, exhibit some degree of light
fall-off at the edges.  The 50/1.4 from Don Chatterton is very good in this
respect -- light fall-off is minimal at 1.4 and virtually undetectable by 2.8
or 4.0.  I just sold a Canon EOS-1n and two very sharp lenses (an 85/1.8 and
a 50/1.4) that showed somewhat more fall-off when shot wide open.

My "concern" is that the Chatterton 50mm lens is a little soft in the center
at 1.4, 1.8, and possibly 2.0.  The two Canon lenses that I just sold, on the
other hand, were "very sharp" in the center when shot wide-open.  When I say
"very sharp" I mean as sharp at the point of focus in the center of the frame
at 1.4 or 1.8 as at 4.0 when viewed with an 8x loupe.  This was a nice
feature of the Canon lenses.  Yes, the light fall-off was significant, and
yes, there were probably other problems that I could not detect (e.g. edge
sharpness, contrast, etc.), but at least I could get a "tack sharp" (in my
mind) photo wide open in the center of frame when necessary.

Apart from the question of whether I need to get a life and stop worrying
about these arguably unimportant issues, is it unusual to see a degree of
softness in a lens when shot wide open?  Will most all 50/1.4 R lenses
exhibit this characteristic?  Is there any relevance here to Don's point that
lenses that are optimized to be shot wide open (e.g. the Noctilux) tend not
to perform that well at f8.0 or f11 and that a lens that is a touch soft at
1.4 is normal and may even be a "good thing" from a design perspective (i.e.
such a lens is optimized to be used, say, between 4.0 and 8.0 and if it had
been designed to be "right on" at 1.4 it probably would be worse at moderate
apertures)?

Thanks for bearing with me on this.  All comments and input are welcome.

John McLeod


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