Quote:
Originally Posted by Atmos
Adding filters ADDS to the optical back focus but SUBTRACTS from the mechanical.
A flattener that has a 55mm back focus, when adding a 1mm filter you calculate it from a 55.33mm optical back focus.
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Yes this is known as the "Rule of 1/3rds". Each 1mm of glass lengthens the focal length by 0.33mm. You don't actually add the filter thickness as it sits in the light path, only the optical effect of it being there. Don't forget that any camera sensor compartment cover glass will also enter into calculation.
Also note thatsome flattener backspacing is measuredfrom the shoulder of the flattener, not the end of the threaded section, better check on that.