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p.3 #9 · The perfect CA target? | |
note: forget the knife target; the edge produces CA the lens can see, adds to lens-produced CA.
Next test setup only requires two identical objects, one red and one blue. Place objects together at same focusing distance, focus on either red or blue object, but don't try to get both in focus at once. How much the other color is out of focus on the image is the measurement of CA.
Can use ruler with Red marks and one with Blue marks. Focus on red or blue measurement mark; see how far out of focus is the other color mark, what other color different mark is in focus. Difference is CA for that lens focused at that distance with that f/# setting.
I haven't tried that yet; mm markings may be too big to show difference, might need finer rulers. I may try this this weekend by printing colored rulers on piece of paper for angled target.
Can try three objects, Red, Yellow, Blue -- some lens' image may show Red and Yellow in focus, but Blue out-of-focus; some lens image may show Red and Blue in focus but yellow out-of-focus, etc., depends on lens CA correction formula(e) used. Maybe even try side-by-side rulers each color from color chart!
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