Idiot question alert - 52 focus points

Started Jan 2, 2014 | Questions thread
Grevture Veteran Member • Posts: 4,188
Well, to be more precise ;)

SergioSpain wrote:

Grevture wrote:

Well, in most cases you want to be able to do both. That is why we have options like matrix exposure where the camera makes lots of measurements and analyze the result in various ways, and also simple spot metering where you measure at one point only.

Focusing works pretty much the same. Sometimes you just want to measure in one spot and completely ignore what is going on around that spot. Sometimes you want to measure over a smaller area, and ignore things outside that area. And sometimes you want to measure over a larger area and figure things out from all that information.

That's a bit misleading though. You can take an average exposure reading and shoot with that, but there's no such thing as an average focus reading.

Well, for the most part I agree. Which is why I only mentioned averaging very briefly as a example of calculations in general

And if there were, it wouldn't work in most cases! If you have all 51 points active, it doesn't mean that the camera is using focusing information from all the points. It means that if the subject is moving, the camera can try to track it and keep it in focus using one of the 51 points. The various points can also be used for predictive tracking to predict where the subject will be and where the focus will be when the shutter is triggered.

Which is part of what I was thinking about. I just tried to keep it simple and general since the OP did not seem to be asking for a complete and precise engineering perspective on AF

But again, there is no such thing as average focus.

Actually, to delve down to details far beyond what the OP probably is interested in: AF systems do indeed calculate averages. Keep in mind a typical mirror based AF system can make many hundreds of measurements every second, there will be a lot of variation which need to be averaged out since the refocusing of the lens is orders of magnitudes slower then the data collection.

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