How Does Autofocus Actually Work?

It's the unsung hero of your photos, and it's always been improving. It's that little switch on the lens. We've become accustomed to AF, and for good reason. It works, and it works very well. 

This is a basic explanation, and it's not going to show you how the camera can see eyes and keep its focus while the subject is moving around. Just think for a moment where where we are today with all the various types of focusing you can use. You can even set your camera to detect animal eyes. We consider autofocusing to just be something that's always been there, just like many can't imagine a world without a fiber internet line, a smartphone, or Google Maps. 

When comparing these two methods of automatic focusing, the advantage of phase detection is the speed. Contrast detection has to search for the contrast in the image to detect what needs to be in focus and takes a bit longer to do so. 

The next time you switch from manual focus to autofocus on your lens, just remember what's actually happening inside your camera for it to do what you're asking from it. 

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6 Comments

Rod Kestel's picture

OMG, 133 fanfold computer paper, an Uber Nerd. I totally do not have a pile of this too. He'd get higher marks if he'd written it in COBOL instead of Python.

Anyway, interesting to learn about autofocus. Didn't know it was different when using live view. Is it my imagination, focussing using live view is slower than through the viewfinder?

Robert Nurse's picture

Nope, not your imagination. It does take slightly longer, at least with my camera. But, for me, Live View is more suitable in certain situations.

Depends on the camera. Sony's SLT cameras (all SLR style cameras they released in the past 10 years), always redirect part of the light to a secondary sensor with PD. But with many Canon and Nikon cameras it will be noticeably slower.

Timothy Gasper's picture

Thank you sir.

Ziggy Stardust's picture

And there's mirrorless with both including on-sensor PD. And Panasonic with DfD to address the shortcoming of CD.

WOW,very interesting and good to know