Ultra-quick evaluation of scanned photo with Mak lens

[Stupendous Man <richmond@p674p06.isc.rit.edu>]

  Looking at the scan of the blue-sky image Cameron sent, it appears to me
that the difference in brightness between the center and corners is 
something like:

                  (brightness at center) - (brightness at corner)
                 ------------------------------------------------
                           (brightness at center)

                           117 units - 71 units
                =         ----------------------
                                  117 units

                
                = 40 percent, approximately

  
  This assumes that the film response was linear, and that the SCANNER
respose was linear, too.  And that the JPEG-compression didn't make
things wierd.  And that I understood how to interpret the little numbers
ImageMagik displayed as I moved the cursor around :-)  That's a lot of assumptions.  

  Let's see: the Mark IV chip will be 2048 pixels * (15 microns per pixel)
wide -- that's about 3 cm side to side.  Your photograph was a little 
more than 3 cm in the long direction ... so, if the lens really does
produce the variations in your scanned photo, it will produce roughly
the same variations with the Mark IV CCD.

  Anyone want to speculate on the maximum amount of vignetting we may
permit in a candidate for the Mark IV lens?  Remember that we can compensate
for a known illumination pattern, but that the noise in the corners will
always be higher than the noise near the center.  Arne, what would _you_
consider acceptable?

  Oh, Cameron (or other people with big lenses): please do go ahead and
use PPF film, if you've got it handy.  For the current purposes, I would
say it's perfectly adequate.  Good luck with unguided pictures ... a few
of mine have come out okay.


  
-- 
-----                                      
Michael Richmond                   "This is the heart that broke my finger."
mwrsps@rit.edu                     http://p674p06.isc.rit.edu/richmond/