Re: Lens alignment etc, (TECH-3D digest 155)

[T3D <bobh@xxxxxxxxxxxxxxxxxx>]

RE: SAM wants to know some simple tests. For matching a pocket 
microscope with a reticle and about 50x could be used to view a 
target at a fixed distance with say an exact one inch or 25mm line. 
Measuring how each lens imaged the size would give a clue as to how 
close they were in focal length or % magnification (actually 
reduction) of the line.
A lens that is sharp but has curvature of field will on one exposure 
of a test chart look to have perhaps sharp center and fuzzy corners, 
BUT if the target is moved in increments on either side and new 
frames exposed, then if the corners come in sharp, the lens is good 
and inability of depth of field to cover the curvatuer is the only 
fault. If no position brings in the corners then the lens doesn't 
cover the field or is poor otherwise. Some years ago Patterson had 
two editions of test charts that were quite nice. Most of the others 
I had seen were not very good except the ancient Bureau of Standards 
test cards. There is a Canadian test scheme using circles with a gap 
that the Japanese are very fond of.  I forget the name, Watson 
Circles??? BobH


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End of TECH-3D Digest 156
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