09-04-2017 07:07 PM
09-04-2017 07:30 PM - edited 09-04-2017 07:33 PM
So I'm hoping the information you'd re missing is that you have to "wake up" the focus system when using a Canon STM lens.
STM (Stepper Motor) lenses are special in that they are a "focus by wire" lens. To focus manually, you have to "wake up" the focus system (half-press the shutter will usually do the trick) and then you can rotate the focus ring and the camera will respond by following your adjustments. But there's no physical connection between the focus ring and the focus elements... you're just giving the lens electronic input. Anything that actives the camera's focus system will do the trick (even back-button focus.)
Any Canon lens with a "USM" focus system (Ultra Sonic Motors) does have a mechanical connection between the focus ring and focusing elements within the lens. But USM lenses have a kind a clutch in that the ring is allowed to slip -- this protects the lens from damage if the camera is trying to auto-focus while you are turning the focus ring (it'll just slip - not a defect... it's intentional and meant to protect you.)
Some very basic (old) entry-level consumer lenses had very basic focus motors with gears, no clutch, not electronic... and for those lenses you should switch the lens to manual focus (disable auto-focus using the switch on the barrel) if you want to manually focus. The basic kit lenses used to be this type of focus system, but Canon has replaced them all with STM lenses. Some entry-level DSLRs still come with these lenses (my "guess" is that Canon doesn't actually manufacture them anymore and is just waiting to run out of stock). The lenses all were placed with "STM" versions which have better optics, better aperture blades, better focus, etc.
One other note about the STM lenses..... they are extremely quiet. If you are recording video/audio and using the internal mic, the focus motors are so quiet that it's unlikely you'll be able to hear them on the recording.
Canon does make a EF 50mm f/1.4 USM which allows full-time manual focus (and is not "focus by wire")
Canon did make an EF 50mm f/1.8 II which had the old very basic focus motor... but I do not recommend that lens. It had a pentagonal aperture (not well-rounded) and created a jittery characteristic to out-of-focus blur (not smooth).