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OK, first, i'll assume that the filters are not Parfocal. You need to get parfocal filters, as they all will focus at the same point after the filter. They do this by either layers or thickness of the glass. This is why you buy them in sets.
Secondly, i'll assume again that by "clear filter (no filter)" means there is no filter and just the ccd in line with the light path. Again, the light focuses at different points, and because there is air in the path, it will focus at a different point as it will when there is glass in its light path. So by placing any glass, clear, colored etc, dont make a hoot what type, even plastic, it will refract (Bend) light differently with each wavelength focusing at different points thru different glass. I could go on and on, oh wait i did. thats enough.. !
One last thing, make sure your filters have I.R block, as this will also give you blurred images.
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