Wayne S. asks:
<< I have noted that in Gary's lens tests that sometimes the contrast is
down a bit at wide open and at f/16 and f/22. Is the lower contrast at
high f stops caused by diffraction or the reflecting aperture blades and
film/first curtain? >>
It helps to think of contrast in an abstract sense: the ability of white
to be white and black to be black. Of course that is if you are
photographing a high contrast image in black and white, but you can get
contrast beween clor tones, too. Low contrast images can be high
resolution, but tend towards two shades of grey rather than black vs.
white. That can make details very hard to discern without close study.
It is due to diffraction effect when a lens is stopped down. When
shooting at wide apertures, you have the cumulative effects of
aberrations, astigmatism, coma, etc. Some of those aberrations get
tamed with stopping down. The optimum aperture is where there is a
trade-off between control of aberrations and increasing diffraction.
The film surface is one of the brightest objects of reflectance once a
light ray passes the first element. Nothing we can do about it, although
a multicoated rear element helps. That is the element which would pick
up a reflection.
Gary Reese
Las Vegas, NV
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