[meteorite-list] Meteorite Photography (Must read!)

From: Chauncey Walden <clwaldeniii_at_meteoritecentral.com>
Date: Wed, 27 Jan 2010 22:07:00 -0700
Message-ID: <4B611B74.1020009_at_comcast.net>

To restate what Chris pointed out as the optical law: diffraction
effects begin as soon as you stop the lens down. A perfect lens would be
sharpest across the field of view and in the plane of focus at its
widest aperture. There are no consumer grade perfect lenses so some
diffraction worsening of center sharpness must be tolerated to bring the
off axis image into general conformance. To those who are using a single
image for the final image, depth of field may be more critical than
optimum sharpness as the widely out of focus areas may be more damaging
to the overall image than the loss of maximum sharpness due to
diffraction. Only the plane of focus has the sharpest image; depth of
field can be defined as the total distance in front of and behind the
plane of focus in which the circle of confusion does not exceed some
user defined limit. In other words, one plane only is perfect and the
farther you get from that plane fore and aft, the fuzzier the image gets
until it is unacceptable to the user. The really critical point in the
original post was the "stacking" of multiple images - each image being
the sharpest obtainable of that plane of focus until you had multiple
planes of focus that covered the entire depth of the subject. In the
old days this was only obtainable by using a mechanical focus table and
a slit light. The object to be photographed would be placed on the table
with the camera vertically above it. The lens would be focused at the
plane of illumination and set to its sharpest aperture. The illumination
would come from the side/sides in the form of light projected through a
narrow horizontal slit (say between two razor blades). The lens of the
camera would be opened and the table would rise through the light at a
speed sufficient to give proper exposure. Since each plane of the object
would be illuminated only when it was at the plane of focus of the lens,
all planes of the object would be recorded at maximum sharpness. Tricky,
but now more easily done in the digital world by just taking multiple
images at different distances at optimum aperture and using stacking
software to combine them into one all sharp image. Sort of the ultimate
unsharp mask.
Received on Thu 28 Jan 2010 12:07:00 AM PST