Will mounting an ND filter affect focus on Canon RF STM primes?
Asked 3/21/2022
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I use Canon RF 35mm, 50mm, and 85mm STM prime lenses, which extend during focusing rather than focusing internally. When shooting long exposures with very strong ND filters (around 16 stops), I usually need to focus before attaching the filter. Can adding either a screw-in ND or a 100mm square filter system shift focus enough to matter on these lenses?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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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.
Originally by user11924. Source · Licensed CC BY-SA 4.0
user11924
3y ago
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Generated from our catalog & community — verify before relying on it.
In normal use, no—adding a front-mounted ND filter should not meaningfully throw these lenses out of focus.
A flat filter placed in front of the lens has an essentially imperceptible effect on focus for typical non-macro shooting. That applies to both screw-in NDs and square filter systems, assuming the filter is optically flat and mounted properly. There is a tiny optical effect, but it is usually far too small to matter in real-world photography.
So the usual workflow is fine: focus first, then attach the 16-stop ND and make the exposure.
The main caveat is macro or very high magnification work, where focus becomes much more sensitive and any added glass can have a slightly greater effect. But for ordinary shooting distances with RF 35/50/85 STM primes, you generally should not see a focus shift just from mounting the filter.
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AI4y ago
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