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How DSLR autofocus works and why cross type points are important

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December 15, 2017 by John Aldred 2 Comments

For most people, exactly how phase detection autofocus in a DSLR works is a mystery. All we know is that the AF in the camera we’re using either sucks or it’s awesome. While autofocus can indeed suck, it often fails us because we don’t understand it enough to work around its potential pitfalls. This video from ZY Productions demystifies DSLR autofocus and explains why cross type AF points are a big deal.

Just to be clear, he’s talking about phase detection autofocus in DSLRs. Not the autofocus systems built into mirrorless cameras, which work very differently. It’s a great introduction to autofocus systems, which explains why the type of AF points is often far more important than the number of AF points.

So, in a nutshell, this is the overview of how DSLR autofocus works. As we know, light enters the lens, bounces off the mirror, up into the pentaprism (or pentamirror), and out through the viewfinder eyepiece so you can see what your camera sees. But none of this has anything to do with the AF. A translucent section of the mirror controls that, which reflects light down toward the AF sensor below.

Before the image hits the AF sensor, optical elements split the image into two. These two images are projected onto the AF sensor, and the further apart they are, the more out of focus various parts of the scene are. As the subject you wish to focus on becomes more in focus, those two images merge into one.

It doesn’t look exactly like this, because your lens is looking at a scene in three dimensions. So, whereas above, everything is doubled, in the real world, even though it might be seeing two of the subject you want to focus on, there might only be one set of monitors showing in the background. Then as the focus comes toward your subject, the subject becomes one, and the monitors would split.

This is why DSLRs struggle to focus on things which are a solid shade of colour (or black/grey/white). Because when you blur, for example, a solid grey backdrop, it still looks just plain solid grey. It doesn’t know whether or not it’s in focus because it all looks the same. This is why they need to see detail.

The difference between regular AF points and cross type (and multicross type) AF points, well, you’ll have to watch the video for that. It’s much easier to understand in a video demonstration than it is to try to explain it in text.

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Filed Under: Gear Tagged With: auto focus, autofocus, dslr, ZY Productions

About John Aldred

John Aldred is based in Wishaw, Scotland and photographs animals in the studio and people in the wild.

You can find out more about John on his website or follow him on Facebook and YouTube.

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  • Mike Aubrey

    Twenty years ago, this was simply to explain by showing someone the different types of focusing screens.

  • Jia Chen Lu

    Makes Harry Potter reference and can’t stop laughing. See him trying to hide his grin. I think he had to cut the video cos he burst out laughing. I would laugh too lol. Great video

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Dunja Djudjic is a writer and photographer from Novi Sad, Serbia. You can see her work on Flickr, Behance and her Facebook page.

John Aldred is based in Wishaw, Scotland and photographs animals in the studio and people in the wild.

You can find out more about John on his website or follow him on Facebook and YouTube.

JP Danko is a commercial photographer based in Toronto, Canada. JP
can change a lens mid-rappel, swap a memory card while treading water, or use a camel as a light stand.

To see more of his work please visit his studio website blurMEDIAphotography, or follow him on Twitter, 500px, Google Plus or YouTube.

JP’s photography is available for licensing at Stocksy United.

Clinton Lofthouse is a Photographer, Retoucher and Digital Artist based in the United Kingdom, who specialises in creative retouching and composites. Proud 80's baby, reader of graphic novels and movie geek!
Find my work on My website or follow me on Facebook or My page

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