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From: marcus@pyuxt.UUCP (M. G. Hand)
Newsgroups: net.rec.photo
Subject: Re: sharpness
Message-ID: <113@pyuxt.UUCP>
Date: Thu, 24-May-84 22:26:49 EDT
Article-I.D.: pyuxt.113
Posted: Thu May 24 22:26:49 1984
Date-Received: Thu, 31-May-84 19:18:56 EDT
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Organization: Bell Communications Research, Piscataway N.J.
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<one fo you and the rest for me...>

Every manufacturer uses a variety of different glasses in the construction
of each lens - the different refractive indices help to control the
various aberrations of the lens design. The control of internal reflections
is a secondary consideration and anyway have most impact on contrast.
Some of the Canon lenses, incidentally, - mostly
the longer ones - use quartz elements which transmit blue light better.
Some of the cheap lenses have noticeable colour fringing - a rainbow effect
around the edges of objects caused by the glass transmitting light of
different wavelengths at different speeds, more commonly recognised as
the dependence of refractive index on wavelength.

Another important factor in image sharpness that i don't think has yet been 
mentioned is the physical alignnment of elements inside the lens.  This
becomes more difficult as the focal length of the lens shortens (actually
it is more *critical*) - hence the high price of good super-wide angle
lenses. Allignment at longer focal lengths is less critical - transmission
(lens speed) is ususally more important.

There is also some confusion between resolution and contrast - a high contrast
low resolution lens often appears to give better sharpness than a lower
contrast high resolution lens.

Finally, the design of any lens is to some degree a compromise between
these factors, others which have been mentioned in other follow-up
arguments, and cost. Obviously the price you pay will reflect the care
with which the lens has been designed and assembled.  I would expect
Leica lenses to be close to perfect by this criterion!  This is also
illustrated by Canon again - they market 4 (yes four) different 300mm
lenses which vary speed, quality, glass (or quartz) used and, of course,
price.

			marcus hand (pyuxt!marcus)