looking for EF-S lens with decent auto-focus speed/accuracy

[theregoesjb]

I have a Canon T2i with the kit lens and 50mm 1.8, I love that 50mm but it is a bit slow for autofocus. I dont find it reliable to try and capture a moving subject at just the right moment.

I more often than not manually set the focus with the live view zoomed in and then hope to capture something. However, with the wide aperture (and the intent being to have a shallow depth of field) it is very easy to miss the focus when people/animals move.

I had been looking between the Tamron and Sigma 17-50 2.8 lenses, mostly for the constant 2.8 and general upgrade from the kit lens. But today I was putting more thought into what I really need and realized auto-focus speed is a big one.

Any thoughts?

 

[Studio7Four]

The one recommendation I have at the moment is to not limit yourself to EF-S lenses. You won't be taking full advantage of an EF lens, but if it's in your budget you might have options for better performance (and be set if you ever upgrade to a full frame camera).

 

[spiralout462]

Your camera will focus much faster through the viewfinder than it will in "live view".

 

[TCampbell]

You will get noticeably faster focus if you use the viewfinder. This has to do with how the focus system works.

With "live view" the camera uses something called "contrast detection" auto-focus and works just like a point & shoot camera. It's easiest to imagine trying to photograph a bar-code (UPC code) because this is typically a VERY high contrast (black stripes on a white background) subject (boring yes... but it's easier to make the point.)

If you look at a bar-code and it's focused, you have "black" areas and "white" areas. There are no "gray" areas. If the camera is focused, each pixel on the sensor should be either "black" or "white". At the edge of a stripe, you might have a "black" pixel right next to a "white" pixel. But that's because it's focused. If you deliberately de-focus the lens, you get a black-ish pixel, followed by a charcoal gray pixel, followed by a medium gray pixel, followed by a lighter gray pixel... followed eventually by a "white" pixel. That's an example of "low" contrast because each pixel has just a slightly different level of tonality than the previous pixel. So this focus system works by trying to detect the difference in contrast between adjacent pixels and adjusting focus until it can "maximize contrast" between adjacent pixels. Hence it's called "contrast detection".

The problem with "contrast detection" is that it has to sample the live-view image, then change focus (randomly), then re-sample the image while trying to determine if the contrast got better or worse. It does this a few times until it figures out which direction is improving focus and keeps going that way until it detects that it's gone too far ... and has to back up. It basically "hunts" for focus and this takes it a bit of time.

The OTHER system is called "phase detect" auto-focus. This system can only be used when you use the viewfinder because the focus sensors are NOT on the camera sensor.... they're special sensors on the floor of the camera just below the reflex mirror. The mirror has a semi-silvered section (think of "one way glass") which allows the sensors to see out even though the mirror is down. A tiny secondary mirror hides behind the primary reflex mirror to bounce light into the AF sensors. Both primary and secondary mirrors fold flat and swing up when the camera takes a shot.

The "phase detect" sensors have a beam-splitter (a prism) which splits light into two "phases". The prism is designed so that the light on the left and right halves (or top & bottom half depending on the orientation of the sensor) will converge IF (and only if) the lens is focused on the subject. It's sort of like taking a photographic print, and using scissors to cut it into two halves and then mis-aligning the halves. When the image is "focused" the two halves will match up. But when out of focus, one half will be higher than the other.

But due to the way phase-detect AF works (and this is brilliant), the camera can IMMEDIATELY tell (a) which of your 9 AF points can achieve focus at the shortest focus distance, and (b) whether the focus is too close for that subject or too far (based on the direction of phase shift), and (c) it can also tell PRECISELY how far out of focus the lens is (based on the amount of phase shift.) That means that with JUST ONE sample, it instantly knows which direction and how far to rotate the lens focus to perfectly nail focus. This is a very fast and efficient focus system.

Whenever possible... use the viewfinder to focus because that method uses the faster "phase detect" AF system -- whereas live-view uses the slower "contrast detection" AF system which has to guess and hunt for focus.

It's not all roses for the phase-detect system. The sensors have directionality and it is possible to fool them. If your sensors are oriented vertically and you're taking a photo of a vertically oriented subject (e.g. think "picket fence" where the pickets are running vertically) then the sensor might not notice that the image is out of phase. But if the camera were rotated 90º it would be very obvious (because the top half of the picket would be shifted left or right of the bottom half.) To solve that problem camera companies often orient some AF points horizontally and others vertically. But the even better solution is to use "cross type" sensors which have prisms to split light both horizontally and vertically at the same time.

The speed of the focus motor is also a factor. Canon's "USM" focus motors are the fastest. The "STM" motors come in 2nd. The auto-focus lenses that are neither "USM" nor "STM" are the slowest.

 

[theregoesjb]

Thanks so much for the in-depth reply, this is extremely helpful. My next question then (assuming I focus through the viewfinder), is if the focus speed/accuracy will probably not vary greatly between lenses like the kit and 50mm 1.8, and lenses like the tamron/sigma 17-50 2.8 (or even the canon 2.8) ?

I think I will need to spend some more time with what I have regardless, but I'm still curious for the future.

 

[KmH]

AF is done with the lens wide open.
So lens maximum aperture is a consideration for auto focus.

A f/1.8 lens lets more light in than a f/3.5 lens. Don;t forget that variable aperture lenses zoomed out to the longest focal length often have a smaller lens aperture of about f/5.6.

F/3.5 is 2 stops smaller a lens aperture than f/1.8 is. F/3.5 lets in 4x less light than f/1.8 does.
F/5.6 is 3 stops smaller a lens aperture and lets in 6x less light than f/1.8 does.
F/5.6 is 1.3 stops smaller a lens aperture than f/3.5 is.

Canon DSLRs do not have an auto focus motor in the camera.
The AF motor has to be in the lens.
Canon makes a couple of different grades of AF motor and 3rd party lens makers AF motors are different from Canon AF motors.
In general, the more expensive AF lenses for Canon DSLRs have a higher grade, and faster, AF motor in them.