Hi George [and others]:
> I'm not quite sure how to do tests which will measure contrast. . .
> If there are any suggestions for better tests, I'm open to input.
A real world subject for me is a US Geological Survey topographic map with
mountains on it. (I "microfilm" these.) The brown contour lines are a great
test for medium contrast and fine black printing for high contrast. Be sure
to pick one with some steep mountains in the center as well as in one corner.
Use Post It notes on the map to show the focal length and shooting aperture.
Spot meter with a OM-4 and use the Highlight bias, or expose off a grey card
and give -1 stop compensation to make the map a bit darker.
When you evaluate, check for detail in the same part of the map for each
*slide.* You'll no doubt be able to discern differences in detail and will
also get an idea of the contrast of the images IF you were careful to exposure
each frame the same. You can also determine vignetting at wide apertures and
which aperture it essentially disappears at.
To have better matched frames for edge contrast evaluation, you might want to
bracket on the plus exposure side the wider aperture shots to compensate for
light fall off (vignetting) in the corners.
Other tips: be sure to balance your two photofloods or other light sources,
which should be at 45 degree angles to your copy. Minimize hot spots on the
copy and check for balance by looking at the shadows either side of an object
placed in the center of the copy. Finally, use a bubble level on the table
and on the back of the eyepiece to insure both the copy and camera are level.
Measure both horizontally and vertically. If the table is not flat,
compensate in an equally off center amount with the bubble level on the
camera.
Note that this also provides an outline of technique for good result from copy
work. But then you probably already know most or all or more of that
technique!
Gary Reese
Las Vegas, NV
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