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07-14-2012, 08:36 PM   #16
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No actually the focus is shifted by a fraction of the filter thickness--1/4 as I recall--but anyway it is. That is why Kodak Wratten filters are sometimes better (much thinner). Also for critical work clear filters of the same thickness as the other filters are used--so the point of focus is consistent as one changes filters.

07-14-2012, 08:51 PM   #17
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lytrytyr, my apologies, you were correct about the filter not having an effect for light coming from ** infinity **, DMS
07-15-2012, 05:34 AM   #18

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DMS, I can certainly see how oblique rays would get kicked aside,
so the effects you describe would be relevant for closer objects.
07-15-2012, 06:37 AM   #19
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QuoteOriginally posted by lytrytyr Quote
A well-made filter should be plane,
so shouldn't make any geometric change
to rays coming in from infinity.
A filter is another element,and as such it will have some effect (may be negligible) to the entire system. Everything has manufacturing tolerances and nothing is perfectly plane. That's why for lenses that accept drop-in filters, the manufacturer provides a clear glass filter to be used always when a real filter is not needed.

Allowing focusing past infinity on long telephotos will accommodate (and correct) any deviations from all variables in the system.


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