Canon T3i Autofocus during movie shooting

[JJasd]

Hello, 

I am new to this forum so apologies if I posted this in the wrong place or something like that. 

I was trying to shoot a video on my Canon T3i, but I found that the camera wouldn't autofocus while taking the video. For example, I could focus the camera before starting recording, press the button to start recording, but then the focus would be locked. If I focus onto a different subject or the initial subject moved farther/closer, there is no way to autofocus it. I would appreciate any help, thanks!

[Waddizzle]

The camera does not have Movie Servo AF, which is only found in Canon camera bodies with Dual Pixel AF image sensors.  You would need to manually refocus during movie recording.

[JJasd]

Hello, 

Thanks for your response. To me this is a little hard to understand as a beginner. If the camera can autofocus in the first place before starting the video, shouldn't it be able to do so while recording video? It doesn't sound like a hardware limitation but rather a software block to me. It's probably more complicated than I am thinking it is, but it still confuses me anyhow. 

Thanks for the response though! 

[Waddizzle · Accepted solution]

A DSLR uses different AF hardware for shooting stills vs shooting video or Live View.  The AF system for video and Live View is much slower to auto-focus.

When you shoot stills, the AF sensor is located at the bottom of the mirror box.  The mirror reflects light downward toward the AF sensor and upward toward the OVF, Optical View Finder.

When you use Live View or record video, the mirror swings up and out of the way.  The image sensor is used BOTH for image capture and auto-focusing.  

If the camera is capturing video, there is only a very brief moment between video frames for refocusing.  It is insufficient time to perform an accurate focus under all scenarios.

I have no idea how Dual Pixel AF actually works.  There are actually two light sensing elements comprising each pixel.  One pixel could be dedicated to image capture, while the other pixel could be dedicated to auto focusing during each frame, instead of between frames.

[JJasd]

That makes so much more sense, thank you so much!

I did have to re-read it a couple of times, but that helped me out a lot. Thank you very much!