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Colin, how thick is that filter? As you would know, adding glass to the light path alters the focal length and can make sharp focus impossible unless you allow for it in the setup of spacing. Most DSLRs are very sensitive to sensor spacing, and mods, or adding filters, may require changing the shims that set sensor position. Just as we do for our astro mono cameras, spacing and filter thickness have to be considered. Remember the Rule of 3Rds: for every 3mm of addition glass in the light path you will need 1mm more spacing. So a typical 2mm filter on front will lengthen the focal length by about 2/3s mm. Autofocus will usually not work.
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