Is STM more accurate than USM

frankchn wrote in post #17596182
As others have said in this thread, it depends on the lens and the USM implementation. I don't doubt that the f/1.8 STM might be more accurate than the f/1.4 USM lens which used a rather poor micromotor USM implementation.

However, the fact that Canon still uses USM AF in its new super-telephoto lenses should mean that the USM variants those lenses use are still better than STM at pure autofocusing (video excluded).

I think that's a speed factor. USM is near instant, STM is pretty sluggish.

In a super tele focus is easy to get right because the differences are so pronounced. USM is clearly the right choice there.

Tom

Speed is definitely a big factor, but I think USM lenses does have more distinct steps (or focus positions) than STM lenses (out of necessity perhaps, especially in the supertelephotos).

Since Canon specifically advertises STM as good for video, so I think the main advantage STM lenses has over USM lenses is that STM lenses can be commanded move at a certain speed (i.e. it can simulate a focus pull) much more smoothly than USM lenses.

kachadurian wrote in post #17595193
My answer, Maybe.

Because I love a 50mm, when the new 50mm f1.8 STM came out I had to buy one.

First, as alway I did the lens calibration on all three of my bodies. All on my lenses get varying calibrations on different bodies. The STM required no adjustment on any on the bodies. It was right on.

I used it a wedding last weekend.

This is a situation I shoot in all the time. People moving, in very dim light. I do a wedding just about every weekend June thru September. At the end of the ceremony it always comes to this. No light, weird color from LED DJ lights. It's an AF nightmare, AF longshot. You shoot 5 frames to get 1 that's just right. MY assistants have never even been able to get even that, maybe 1 in 10.

I have used a 50mm f1.4 USM, Sigma 50mm f1.4 hypersonic (not the Art) and the Canon 50mm f1.2L. I always shoot at f1.8, even with a 1.4 or 1.2 lens both to get a bit better odds and to have DOF that isn't razor thin. Last Saturday I used the new 50 STM. We were shooting with the cameras we always use. The only thing different was the lens.

It is way slower to focus, not snappy at all, but much more accurate. When it hits, it is exact. I got most in focus. I could see it happening right in the viewfinder. It was slow, but then got it without hunting. To see what would happen in someone else's hands I gave it to my assistant and she hit at least as well as I have ever done with other lenses.

Has anyone observed this or is it a fluke?

Tom

I just sent my 50mm STM back to B&H for a replacement . It missed AF 40-50% of the time at various apertures, at various shutter speeds and in various lighting conditions on three different bodies. I hope the next one does better. My 10-18 mm STM on the other hand always nails AF regardless of aperture, shutter speed or lighting conditions. I love using that inexpensive little lens.

From an electrical engineering standpoint, stepper motors (in STM implementation) are quite different from regular DC motors (as used in USM).
Basically, when you apply voltage to a DC motor it spins at a certain speed. This requires a control circuit to get the DC motor to a certain position.
A stepper motor is far more suited for precision. When you apply voltage to a stepper motor, it makes a step (a certain amount of mm's).
So I think it is potentially more accurate.

You'd have to dig into the camera and lens autofocus system to know for sure though... but Canon wont exactly give that away :P.

I just heard this weekend that STM lenses are specially developed for video and USM are better for photos. The STM is slower because the focussing for video has to be smoother. (that's what I'm told)

frankchn wrote in post #17597218
Speed is definitely a big factor, but I think USM lenses does have more distinct steps (or focus positions) than STM lenses (out of necessity perhaps, especially in the supertelephotos).

Since Canon specifically advertises STM as good for video, so I think the main advantage STM lenses has over USM lenses is that STM lenses can be commanded move at a certain speed (i.e. it can simulate a focus pull) much more smoothly than USM lenses.

Lyn2011 wrote in post #17597460
I just heard this weekend that STM lenses are specially developed for video and USM are better for photos. The STM is slower because the focussing for video has to be smoother. (that's what I'm told)

There are 2 types of STM AF motors.

The 18-55 STM, 18-135 STM and 55-250 STM are all VERY fast focusing(much faster than the non-STM versions). They are also just as fast as some USM lenses.

The EF-M 22 STM, EF-S 24 STM, EF 40 STM, and EF 50 STM all use Gear-Type STM which is not all that silent or fast.

The fact that STM lenses are good for video is probably secondary for most users.

My newly acquired 24-105 STM has the stepper motor. Going from one end of the focus range to the other is painfully slow. I would never try to deploy this lens at a soccer or football game, where the location of the action can change very quickly. In normal AI-Servo stuff like chasing a grandkid around the park, or the dogs around the backyard, it is not noticeably different than my 70-200 IS.

GeoKras1989 wrote in post #17597548
My newly acquired 24-105 STM has the stepper motor. Going from one end of the focus range to the other is painfully slow. I would never try to deploy this lens at a soccer or football game, where the location of the action can change very quickly. In normal AI-Servo stuff like chasing a grandkid around the park, or the dogs around the backyard, it is not noticeably different than my 70-200 IS.

The 24-105 STM is the only STM lens I don't own.

I don't understand how your 24-105 STM can have slow AF sometimes, but as fast as the 70-200 other times.

Can you explain further?

msowsun wrote in post #17597563
The 24-105 STM is the only STM lens I don't own.

I don't understand how your 24-105 STM can have slow AF sometimes, but as fast as the 70-200 other times.

Can you explain further?

I should have explained more carefully. If I focus on an object really close, say a flower, and my next shot is a kids playing 100' away, the focus takes a bit of time to get there. That is slower than the 70-200, certainly. If I am tracking a dog running around the park, the focus distance only changes incrementally as the dog runs. It is not that the 24-105 changes focus speed. A running dog does not move 100' instantly. The 24-105 does not become as fast as the 70-200. Tracking the running dog is just a lot less demanding than jumping 100' between shots. I consider my 24-105 STM the equal of the 70-200 when it comes to tracking a moving object (I don't do birds!), but not when switching from one end of the field to the other. Is that as clear as mud?

I get it now. The Digital Picture review seems to agree with you as well.... http://www.the-digital-picture.com …-3.5-5.6-IS-STM-Lens.aspx

msowsun wrote in post #17597581
I get it now. The Digital Picture review seems to agree with you as well.... http://www.the-digital-picture.com …-3.5-5.6-IS-STM-Lens.aspx

I wholeheartedly agree with Bryan on that one. Focus speed was my biggest worry. I am happy with my cheap zoom.

STM generally will not over shoot the correct focus for an object. Whereas the USM is fast, when tracking (or otherwise) it may go just past correct focus and the reverse direction to get it right. It is this difference IMO that keeps the two at about the same tracking performance.

The fact that it doesn't overshoot correct focus is another important video AF feature.

LonelyBoy wrote in post #17595565
I thought that was a matter of newer bodies and newer lenses, including some USM lenses (like the 35/2 IS), not just STM? That was how Roger at LR described it in a series of articles.

Is the article you are refering to,
http://www.lensrentals​.com …-ii-1-vs-2-and-old-vs-new

Left Handed Brisket wrote in post #17597589
STM generally will not over shoot the correct focus for an object. Whereas the USM is fast, when tracking (or otherwise) it may go just past correct focus and the reverse direction to get it right. It is this difference IMO that keeps the two at about the same tracking performance.

The fact that it doesn't overshoot correct focus is another important video AF feature.

I think that depends on how you define 'overshoot'. I just put my 24-105 on a 60D and did some video. It overshot focus everytime I focused while the video was running. The focus adjustment was better paced for video, the overshoot was slight, the correction was near immediate, and most of all, no focusing noise in the video. Much better than any other non-STM lens I've ever tried video with. I could actually use this lens for video.

FULL DISCLOSURE: I am not a videographer. I've done perhaps 10 hours of DSLR video, ever.

I'm not a video guy either, but I'm working on it. I believe the STM is optimized for use with DPAF, which is why they came out at the same time.

Pretty sure.

I think.