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As I understand every DSLR has a separate, smaller sensor that handles phase detection focusing. Where is it located?

If this location is different for every DSLR, and therefore this broad question can not be be answered, where is the AF sensor on a Canon 550D located?

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It's generally at the bottom of the camera body. The reflex mirror permits some light to pass through it, which bounces off a secondary mirror which hangs underneath the main mirror, through a lens, off another mirror though another lens and into the AF sensor:

The reason for all of this bouncing around off mirrors is that the AF sensor has to be exactly the same distance from the lens as the main imaging sensor in order for the phase detect AF to give accurate results.

Here is a cutaway drawing from Canon of the 50D, showing the different mirrors and the AF sensor itself. It appears all Canon DSLRs use the same arrangement for the AF lightpath:

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  • \$\begingroup\$ If the reflex mirror is passing the light on anyway, why not use the main sensor? Or should this be another question? \$\endgroup\$
    – Saaru Lindestøkke
    Commented Mar 6, 2013 at 10:59
  • \$\begingroup\$ @BartArondson that would indeed be a good follow up question! The short answer is that the AF sensor sits behind a pair of special AF lenses (it's actually one piece of glass but with a moustache type profile, you can see it in the picture) to focus beams from either side of the lens onto corresponding parts of the AF sensor. To use the main imaging sensor you'd still need this lens, and you'd need a way of flipping it out of the way when taking a photo! \$\endgroup\$
    – Matt Grum
    Commented Mar 6, 2013 at 11:21
  • \$\begingroup\$ Where did you read about this @Matt Grum, and where did find this photo? \$\endgroup\$
    – D4Am
    Commented Mar 6, 2013 at 15:50
  • \$\begingroup\$ @Mr.M The image is a CAD rendering which I found using a google image search. I'm guessing Canon reps send these out from time to time as they pop up in various different resolutions on review sites, but there doesn't seem to be anywhere to download them from Canon. Can't tell you a particular source for any of the information here, but this is a good primer on how PDAF works: dougkerr.net/Pumpkin/articles/Split_Prism.pdf \$\endgroup\$
    – Matt Grum
    Commented Mar 6, 2013 at 16:07
  • \$\begingroup\$ @Matt, I quite didn't understand this part of your answer:''The reason for all of this bouncing around off mirrors is that the AF sensor has to be exactly the same distance from the lens as the main imaging sensor in order for the phase detect AF to give accurate results.'' Can you please make it clearer for me? Thank you! \$\endgroup\$
    – Morpho
    Commented Apr 1, 2014 at 12:19

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