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I am considering purchasing a Canon sl2. I notice that it has the same 9 point 1 cross type auto focus system that has been used for many many years.

My current Canon t3 uses this auto-focus system.

I have read statements on forums that suggest because the canon sl2 has a much more powerful processor in the Digic 7, that its autofocus will actually perform better than older cameras that also utilise, seemingly, the same 9 point autofocus system.

Is this true? If so, in what way could the same autofocus system benefit from a faster processor?

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Canon's autofocus algorithms and implementation details aren't public (just ask Sigma and other third party lens manufacturers) so I doubt there's a definitive answer, but I'd be surprised if the DSP in a DSLR contributed to AF performance in a meaningful way if you talking about a Rebel's One-Shot AF mode.

Here's my thinking:

First, Lensrentals has a quick primer on autofocus that's worth a read: https://wordpress.lensrentals.com/blog/2010/07/how-autofocus-often-works/

The lesson here is that phase detect AF (like the SL2's) is based on a relatively small number of inputs. The lens tells the camera's AF its focal length, aperture and current focus point. The phase detect chip tells the camera's AF system how out of phase the auto focus points are. The camera then tells the lens, "I think you need to move focus x distance towards infinity" and the lens drives focus towards that point.

In one-shot mode, focus isn't going to be checked until the lens is at the desired location. In the world of electronics, it takes a long time for that movement to happen--far longer than the original calculation. And that calculation—"how far do I tell the lens move"—isn't nearly as complicated as, say, de-bayering 24 million pixels and then applying JPEG compression, which is something that an SL2 can do multiple times a second.

In short, the processor's speed is likely a pretty small contributor.

So, where does that all go out the window? I'd say the DSP is going to have a more meaningful contribution when: - Using live view, since the 24mp imaging sensor is used for AF - On a camera that uses the metering sensor to select an auto focus point - Evaluating which of a large number (e.g. 50+) auto focus points to select - Potentially, in your canon's "AI servo" mode, which continually tracks focus. On the 7,5, and 1 series canons the AF menu is blisteringly complex but those cameras also have a lot more feeding into the AF system such as iTR.

Does a 1DX blow a rebel out of the water because it's got a better DIGIC in it, or because it has a more expensive AF sensor, more AF points and an RGB metering sensor? Hard to tease out. For instance, one reason that the 1D-series focuses so well is simply because it has larger batteries that give the lens' motors more voltage so they can move the glass in the lens faster.

Anyways, the bottom line is the SL2 (like any DSLR's) AF system is a well-calibrated modern marvel of complexity but I wouldn't expect the camera to perform any differently in terms of AF performance vs your T3. Take a trip down to your local camera shop to try it out for yourself.

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Yes, a faster processor could improve autofocus.

Focusing is done in real-time. The processor is (more or less) constantly sampling data from the chosen AF sensor(s), and deciding whether it's in focus yet. If it's not, it decides how fast to drive the focus motor, and in which direction.

The AF sensors are basically small, low-resolution versions of the same sort of sensor that the camera uses to take the picture (assuming a digital camera, anyway). Given the low resolution and small amount of data, involved, it would probably be fairly easy to design the sensor to support being read hundreds of times a second or so. The limit on how often we can sample the data and determine focus is likely to be the processor. So, with a faster processor, we can sample the focus more often.

As it stands now, most AF cameras slow down the focus motor as they get close to optimal focus. They also often scan past optimal focus a bit, and have to seek back to optimal. With a faster processor that can sample focus more frequently, we can prevent (or at least substantially reduce) both of these.

Whether this actually affects precision depends on how the software is designed. On one hand, it could be designed to always keep seeking better focus until it reaches a specified level of precision. In this case, a faster processor might not give more precise focus, just the same focusing precision a lot faster.

Alternatively, the camera could be designed to limit the time it spends searching for better focus (especially if it's already found at least reasonably good focus). In this case, a faster processor could easily lead to achieving focus that's actually more precise.

That said, I wouldn't expect to see a large practical difference under most circumstances. In a typical case, a modern camera already focuses very quickly and dependably. I'm not sure there's really a huge amount of improvement that can be made in this regard.

  • I agree with the assessment—probably contributes, probably not in a measurable way—but when you're talking about an SLR the AF system is more predictive than this. It attempts to determine where the focus "should" be (and tells the lens to drive to that point). It doesn't generally continuously sample the entire sensor. That's more how contrast detection AF works, and is one of the reasons contrast detection AF is so pokey compared to an SLR. – davidhfe Jan 17 '18 at 7:15

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