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Almost every single review of an STM lens mentions the following:

The front end doesn't rotate while focusing, which is great for using a polar filter.

Does this mean that I can't use a polarizing filter on non-STM lenses?

And if I can on both, then what benefits am I getting when using an STM lens?

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You can use a polarising filter on any lens. However, using a "directional" filter like a polarising filter or a graduated neutral density filter on a lens where the front element rotates when it focuses is a bit of a pain: you get the scene lined up, rotate the filter so that it's where you want it, focus... and then the filter rotates, so you have to adjust it again. Doable for static scenes like landscapes, not really workable if you're often changing your focus distance.

What does this have to do with STM lenses? Fundamentally, nothing - but it is the case that Canon's older kits lenses (the 18-55 and 55-250 non-STM lenses) had front elements which rotated when focusing, while the newer STM variants of those lenses have non-rotating front elements, so it's often noted in reviews that this is an advantage of the STM lenses over their older counterparts.

  • And of course, on the even newer USM lenses, it's a non-issue (well, at least all the ones I've looked at - there might be some that have rotational issues; haven't surveyed them all). – twalberg Sep 27 '18 at 12:38
  • @twalberg wasn't USM around for a decade or so before STM were developed? – flolilo Sep 27 '18 at 12:42
  • @flolilolilo Yeah, that wasn't worded quite right - meant to imply that many of the newer lenses are USM, not that the USM technology itself is newer. – twalberg Sep 27 '18 at 12:43
  • Canon introduced the first USM lens in 1987 with the EF 300mm f/2.8 L USM. The first STM lens was introduced 25 years later in June 2012. – Michael C Sep 28 '18 at 1:54
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Does this mean that I can't use a polarizing filter on non-STM lenses?

You can use a polarizing filter on any lens. See Philip Kendall's answer for details.

STM refers to the type of motor used in the lens. The lack of front-element rotation is a consequence of lens design, not motor choice.

And if I can on both, then what benefits am I getting when using an STM lens?

STM motors are quieter and smoother than previous motor technologies.

  • Regular motors are loud and slow.
  • Motors in third-party lenses tend to also be loud and slow.
  • USM motors make "chipmunk" sounds and jumpy focus movements.

However, newer "nano USM" motors are smoother, quieter, and faster than STM.

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