Originally Posted by
Doremus Scudder
This has been discussed in another thread recently. I learned quite a bit from the discussion.
The upshot seems to be that focusing with the filter in place is not usually necessary when the filter is in front of the lens (i.e., between lens and subject) at "normal" working distances since the focus shift is usually tiny compared to the lens-to-subject distance. Depth of field and depth of focus are usually more than enough to handle the slight shift (which is usually less than normal focusing tolerance when focusing by eye).
However, when working close up, the focus shift introduced by a filter might be significant (depending on magnification), so focusing with the filter in place is a good idea in that situation.
Of course, focusing through the filter if possible, would be the wisest option and reduce the possibility of compounded errors. Using a weaker filter as ersatz for a denser one, even if not quite the same thickness/refractive index, would certainly reduce the error some (if not mostly).
It's all a question of tolerances anyway, especially when we're maximizing the depth of field.
Best,
Doremus
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