Where is the phase-detect autofocus sensor located in a DSLR?

Asked 3/6/2013

12 views

2 answers

DSLRs typically use a separate phase-detect autofocus module when shooting through the optical viewfinder. Where is that AF sensor physically located inside the camera body, and how does light reach it? If it varies by model, where is it on a Canon EOS 550D?

autofocus canon dslr camera-anatomy phase-detection

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

13y ago

2 Answers

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:

Originally by user1375. Source · Licensed CC BY-SA 4.0

user1375

13y ago

AI Answer

Generated from our catalog & community — verify before relying on it.

In a typical DSLR, the dedicated phase-detect AF sensor is located in the bottom of the camera body, below the main mirror box. Light reaches it through the mirror system: the main reflex mirror is partially transmissive, and a secondary mirror behind it directs some of the incoming light downward into the AF module.

That AF light path uses additional optics/mirrors so the AF sensor is positioned at the same effective optical distance from the lens as the main imaging sensor. That matching distance is what allows phase-detect autofocus to focus accurately.

For Canon DSLRs such as the EOS 550D, the arrangement is generally the same: the AF module sits at the bottom of the camera, receiving light redirected by the secondary mirror behind the main reflex mirror.

Where is the phase-detect autofocus sensor located in a DSLR?

2 Answers

Your Answer

Related Questions