Lens advice USM vs STM

[bevvet]

Hey, so I just recently upgraded to a Rebel T8i after using a T3i for years, and I’ve been using the same lenses (EFS 18-55mm and EF 70-300) for the new one. I’m currently in school majoring in photography and to make money on the side I do professional shoots here and there, and I’ve found that having to shift between the two lenses has been extremely cumbersome, so I’ve made the decision to buy a used 18-135mm EF lens.

However, i can’t decide if I should get the USM or the STM. From what I understand, it’s just the auto-focus system, but I can’t see any real difference between the two from what I’ve tried to find. Does it even matter? I primarily shoot portraits and cinematography on occasion, if that helps.

[Tronhard]

Hi and welcome!

I have had both versions and I would agree that optically-speaking, there is little difference between the two.  It's more about the focusing system.  Both are fly-by-wire and so one can leave the lens on AF and still manually focus, which is excellent.  The USM one is faster to focus (a bit), and is particularly suitable for video work - it even has contacts for its own optional focusing motor.  I prefer the cosmetic feel of the USM version and it is quite a few years newer technology than the STM version, but probably more significant for you will be how much you have to spend for either.  

A good indicator of the quality of an optic is how it takes a crop, and the following pair was shot with the USM version:  All images shot hand-held, available light
80D, EF-S 18-135 USM@135mm, f/5.6, 1/200sec, ISO-2000  Cropped version of previous shot

This one was taken with the older STM version of a Māori dance group in brutal midday sun.60D, EF-S 18-135STM@79mm, f/7.1, 1/400sec, ISO-200