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11-18-2015, 04:23 PM   #16
Pentaxian
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QuoteOriginally posted by Paul Ewins Quote
downside is that half of a plasmat is less corrected than a whole plasmat which probably wipes out the advantage of looking for a more modern APO version.
My thoughts exactly.

11-19-2015, 11:13 AM   #17
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Even though Plasmats are designed in halves, there is still a consideration for the performance as a whole and optimization is done to that end. This is especially true regarding spherical aberration. The use of a symmetrical Plasmat automatically corrects coma, lateral color and distortion, So using only the rear section, one loses that symmetry and it advantages. Yes, it would probably be better to use an APO triplet that is corrected for all aberrations in itself. The 100mm macro lens for the Pentax 67 has such a front attachment. But it could still be an expensive trial and error project.
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