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From: Bill (no email)
Date: Wed Apr 06 2005 - 17:49:15 EDT
> Perceived image sharpness is made up of resolution and contrast. The
> European tradition of fine lens making, from firms like Leica and Rodenstock
> and Schneider, has emphasized resolving power over contrast. There is a good
> reason for this; a low contrast image retains more information (tonal range)
> which is lost as the contrast is increased. Increasing the contrast is
> usually possible later on, eg, when reproducing the image on paper or in
> print, although transparency film is an exception here (a compensating
> factor is the higher contrast of transparency film over negative stock).
Peter
Film gets a way off topic, and there are many ways to control the tonal
range and curve of B&W. Not so easy with transparency film, but doable
within narrow limits. Film adds adjacency effects and many other goodies
into the equation.
Resolving power and contrast are quite relevant. Like the ridiculous and
the sublime, they are often so close (in perception, and when plugged into
the entire system) a little nudge pushes the sublime to the ridiculous and
vice versa. Photogs are taught the old saw that a lens is at its best
several stops from its maximum aperature (by opening, not f-number). Good
evidence claims is not true from an optical standpoint, but true when film
is added into the equation. In my early photo days, the "poop sheet" packed
in with professional film and curve data from the "Great Yellow Father" were
arrived at sans lens--pinhole.
Apologies to all, I sneaked in a lot there after the "off topic" comment.
<G> Would be happy to kick it around off list.
As to sextant scopes, my experience is limited, but they seem as a group to
fall well below my level of expectations for a prime 35mm, 2 1/4" or view
camera lens, especially around the edges. Not knocking any particular
manufacturer, just a general comparison.
Off topic again, but I had often wondered about the quality of Cannon and
Nikon $400 wide-ratio auto-focus zoom lenses designed for "35mm" digital
camera. (Note: the the chip sized of Nikon and most Cannon's are smaller
than 35mm film.) I felt they fell below standards for a prime or zoom lens.
With the 10+ megapixel digitals, it seems the lenses are indeed falling
apart.
Related to the above, a question that kicks about the back of my mind, and I
hope the historians and sextant digerati can field: Has any optical
manufacturer of scopes ever offered a scope with an adjustable aperature?
It seems this could be handy, especially for fine tuning star/planet
brightness. Or a zoom scope? My guess would be no, as it would be to
fragile for a wet and hostile environment, but willing to be surprised.
Along that line, quality camera lenses have for quite a while now been
multicoated (exterior and interior elements) to reduce flare, ghosting etc.
I don't see that on sextant scopes (again, I admit to limited experience).
If true, is this again possibly due to exposure in a harsh environment?
Bill
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