asst. notes, not worth archiving

[T3D Peter Abrahams <telscope@xxxxxxxxxx>]

Modulation transfer function is a quantitative measure of contrast.
Modulation is contrast, and transfer is from one side of the optical system
to the other (although you can find MTF of fog or other unexpected
measurements).  I believe Zeiss used the term contrast instead of
modulation, without significant intentions.  It is a very complex subject,
and my opinions of it are not valuable, but I'm not sure this is entirely
correct:

>>with MTF ... calculating the system mtf from the
>> components MTFs is straight forward.

Wavefront errors are not generally additive; if you have an aberration that
causes one wavelength of error in the front lens, and the same in the back
lens, it doesn't add up to two wavelengths of error, except coincidentally.
 This is not equivalent to modulation, but is related. It might seem that
contrast can only be worsened by an optical element, and the successive
elements could only add to modulation losses, not correct an 'earlier'
loss.  But I believe that aberrations can cause MTF losses, and they can be
corrected by a 'later' optical element.  A clarification of this quote
would be helpful to me.

And.....thanks to whoever scanned or entered the Zeiss paper.  It's not
immortal, but it's worth having.  I agree with Zeiss that the figures for
resolution alone are deceiving, that you need many figures & explanatory
text, that it depends on the film, and much else they say.   But when they
launch into, resolving power isn't as important as you'd think; it sounds
like some Japanese came up with some lenses that were better at that
criteria.  

And....I have a medium-fat dictionary, and it doesn't list a secondary
meaning for 'obtuse' that is 'abstruse'.  Likewise, Roget's lists only
synonyms for 'stupid'.   Now, 'abstruse' can mean 'unintelligible', but it
also means 'wise'.  If some readers are unaware what this is about, good;
your lives are better for it.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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