Best AF practice for video

[HowardLane]

I'm using a EOS 7D for shooting 1080p HD video. New to the camera and DSLR for video. I've looked at several tutorials but still don't have a good understanding of what the best practices are for focusing.

If I put the camera in AI Servo will it auto focus if the subject moves away or toward me with out me depressing the back AF-ON or shutter button? Or must I press one of those buttons to make the camera continue focus?

Do you know of any tutorials or video teachings that go through using focus with video?

Thanks in advance!

[MikeSowsun · Accepted solution]

I don't know of any tutorials but most serious videographers say it is best to use only manual focus.

Trial and Error is a good teacher when it comes to DSLR Video. You will quickly learn that Video Auto Focus is really bad with the 7D and most people don't use it all all.

The method that works for me is to use AF before you start shooting the video, and then manually adjust if you need to as you shoot the video. Using f/8 or f/11 helps keep everything in focus without having to keep adjusting. 

Newer Canon DSLR's like the T4i, T5i, 70D, and SL1 have a "Hybrid AF" that does a much better job of AF during video, but it still isn't as good as a Video camcorder

[MikeSowsun · Accepted solution]

I don't know of any tutorials but most serious videographers say it is best to use only manual focus.

Trial and Error is a good teacher when it comes to DSLR Video. You will quickly learn that Video Auto Focus is really bad with the 7D and most people don't use it all all.

The method that works for me is to use AF before you start shooting the video, and then manually adjust if you need to as you shoot the video. Using f/8 or f/11 helps keep everything in focus without having to keep adjusting. 

Newer Canon DSLR's like the T4i, T5i, 70D, and SL1 have a "Hybrid AF" that does a much better job of AF during video, but it still isn't as good as a Video camcorder

[TCampbell · Accepted solution]

The 7D is not going to perform continuous auto-focus in video.  AI Servo mode is for still photography.  The 70D _will_ perform continuous auto-focus during video and does an outstanding job of it (due to it's ability to do phase-detect AF directly on the camera sensor.)

Every camera *other* than the 70D (including all other brands -- the 70D is unique right now) have to do "contrast detect" auto-focus.  Contrast detect has to analyze contrast differences between adjacent pixels, make an adjustment and then evaluate if the contrast increased or decreated.  That tells the camera if it's even focusing in the right direction... then it has to re-focus, re-evaluate, re-focus, re-evaluate... until it finally optimizes contast and that means the image is finally focused.  This results in a game of "focus hunt".  It's slower and anyone watcing the video will see the camera hunt for focus.

The better way is to manually focus the camera.  If you were making a more serious video you'd even pre-plan the focus changes -- and wrap a piece of tape around the focus ring and use a marker to place the focus points on the ring (you pre-focus the camera to each position you plan to use during that clip).  Having done this you can smoothly move focus from subject to subject just by looking at the marks you made on your focus ring.  This isn't going to help you in ad-hoc video clips -- it's what you'd do if you were thinking about film-making.

The 7D will re-focus if you press the AF-ON button... but it will focus hunt when it does this.

[HowardLane]

Thanks to both of you. I figured I was probably going to get "Manual" as an answer but had to ask. I typically use manual with a camcorder too, but have used auto in some situations where I had the subject moving at me quickly or I was doing a run & gun. I'm getting the feeling that DSLR may not be the way to go in those types of situations, especially since focus is hard to see without several accessories (bigger monitor indoors, or LCD viewfinder outdoors). Still learning...

[TCampbell]

If you want camcorder-like auto-focus capability, the only DSLR that can do that is the 70D.  

The camera has a unique on-sensor focus system that gives it "phase detect" auto-focus on the sensor (previously, on-sensor focusing could only use the "contrast detect" system).  

Phase detect focus is much faster, but more important... when something is out of phase, the camera can tell which direction it needs to go to fix focus and it can even tell how far it needs to go to fix focus.  This results in a system which is both faster and does not need to "hunt" for focus -- and yet you get the benefit of the DSLR lenses for a higher film-like quality and less of a camcroder-like quality result.

The 70D has a touch-screen -- and you tap the screen to switch focus points (e,g. changing from near to far subjects in the same scene).  For video... it's the bees knees right now.

[HowardLane]

thanks again. I'm leaning more towards one of the canon HD camcorders that can shoot 1080 p at 24 FPS for those types of shots that I described. Usually they don't require a short depth of field because of the movement. The camcorder will have other uses too such as event video. The DSLR will be great for the somewhat static shots and close ups. Thanks again for your help. Do you know of any good tutorials for DSLR video?

[MikeSowsun]

The 70D is not the only Canon DSLR to have Continuous AF in Video mode.

The T4i was the first with the "Hybrid CMOS AF"  followed by the T5i. Then came the SL1 with improved ""Hybrid CMOS AF II". And now the the 70D has "Dual Pixel CMOS AF". 

Each version is slightly better than the previous version, but they all will do a respectable job at continuous Video AF.

[TCampbell]

I forgot about that -- there's even a Powershot with that feature.  It's not nearly as good as the 70D implementation, but it does offer "continuous" focus.