ND filter installation with RF STM Lenses

Question

I’d like to know if there is a chance of throwing a lens out of focus when mounting either screw in or 100mm square nd filter systems.
I recently switched to Canon RF with 35/50/85 prime lenses, which do not focus internally, and I use 16 stop filters so I cannot focus with the filter on in general.

Answer 1

Adding a flat filter in front of a lens has imperceptible (i.e., essentially zero) effect in the the focusing of a lens focused lower than 1:1 magnification. That is, any non-macro lens that is focused within its normal focusing range, and not using front-added macro diopters, won't be affected by adding front filters such as NDs or ND-grads, color filters, polarizers, or the like.

Such front-added filters aren't usually useful in the macro-focus regime, and even if used, their effect on focusing is fairly small (depending largely on the system magnification). Of course, at high magnification, the lens usually isn't focused, as much as the camera+lens (and possible front-added filters) system collectively is moved forward/back to focus.

Answer 2

Usually, a camera lens will tolerate the mounting of a screw-in filter without changing the focus. That being said, attaching a filter has some effect. The filter should be optically flat and making a flat can be challenging. The flat glass forward of the lens does microscopely change the focal length however this should not be of any concern, too small to have much impact.