Understanding Video Autofocus

[3kramd5]

I rarely shoot video, but was messing around with it today (5D3). I noticed that, if I try to autofocus in video (and probably live view stills too, but I haven't verified), it gets significantly brighter while achieving focus, even if I'm already shooting wide open. What causes that to happen? Does it momentarily drop the frame rate? Can it be avoided? It looks extremely jarring.

Thanks

 

[fatmanmedi]

What i'm about to say some people will disagree others will agree.

Do not use autofocus when shooting video.

this link will give you the best instructions on how to shoot great video

http://lifehacker.com/5575142/how-to-record-great-video-with-your-hd-dslr-camera

fats

 

[Badger]

http://lifehacker.com/5575142/how-to-record-great-video-with-your-hd-dslr-camera

Thanks!

 

[3kramd5]

What i'm about to say some people will disagree others will agree.

Do not use autofocus when shooting video.

this link will give you the best instructions on how to shoot great video

http://lifehacker.com/5575142/how-to-record-great-video-with-your-hd-dslr-camera

fats

Sure, the few times I've shot video, I focused manually. I understand video AF still lags far behind still AF, but really I am asking why it happens. It seems weird. If would be one thing were I stopped down, but I was shooting wide open. Pressing the AF ON button, the scene brightens dramatically, like more than a stop or two. What causes that to happen?

 

[Sanaraken]

I never tried video on my 5DIII, but on my 5DII before I always use manual focus. I never like using the auto focus.

 

[rossgumbrell]

I thought it was to do with the aperture opening up to achieve accurate autofocusing with more available light.

 

[eyeland]

OP is not asking IF he should be using AF in Video, but rather about a specific behavior of the camera.
I use manual focus for video, but I have been wondering about the brightening phenomena as well.

 

[fatmanmedi]

when you use AF the camera uses contrast detection as it's method to bring the image in to focus.

Now when taking still photos the camera will open the Apature to the widest to get as much light in to the sensor as possible to enable it to find and achieve the focus possible. Now that is great if the image is a still one but when the camera is shooting video, it can cause problems when filming, the effect for the user is the image goes brighter. Another item the camera will use to enable it to find focus is the AF assist light that can come on as well to help the camera to find focus.

Both items make the image momentary brighter and can ruin a shot.

So with the link i posted earlier it's important to use manual focus and learn how to do it properly.

I hope that this helps.

Fats

 

[DanThePhotoMan]

Some good replies so far and I think the above people nailed it. I think it also comes down to the fact that the 5Dmk3 was not designed to autofocus while you are shooting, in the sense of continually tracking a shot. IMO, it's much easier to just magnify in and manually focus, just so there's no discrepancy incase you happen to focus on a background object by accident. Nailing focus on a DSLR screen can be terribly hard if you're just judging by the screen alone, especially since you don't have any built in focus assist. If you already know what you want to shoot and know the subject will move, just have them move from point A and B and mark your focus so you know how far to pull. If not, you're probably not working on a major production so just play around!

 

[3kramd5]

when you use AF the camera uses contrast detection as it's method to bring the image in to focus.

Now when taking still photos the camera will open the Apature to the widest to get as much light in to the sensor as possible to enable it to find and achieve the focus possible. Now that is great if the image is a still one but when the camera is shooting video, it can cause problems when filming, the effect for the user is the image goes brighter. Another item the camera will use to enable it to find focus is the AF assist light that can come on as well to help the camera to find focus.

Except in this case I was already wide open (f/2.8 on an f/2.8 lens), and without a flash installed (5D has no popup). Something else is happening.

 

[Tonyphotos]

Is your ISO set to AUTO? That could be it...

 

[ahab1372]

Maybe the camera changes shutter speed and/or ISO during the autofocus.
To test for the former, you could film something moving at constant speed, like a slowly rotating fan, and see if the amount of motion blur changes in the frames that were taken during the autofocus. It would probably take some trial and error to get the rotational speed right.

 

[fatmanmedi]

i don't own a 5d i own a 1dx so i downloaded the 5d Mk3 manual and on page 228 it explains the issue, i quote below.

• The camera cannot autofocus continuously like a camcorder.
• Autofocusing during movie shooting may momentarily throw the focus far
  off or change the exposure.

I hope that this helps.

Fats

 

[3kramd5]

i don't own a 5d i own a 1dx so i downloaded the 5d Mk3 manual and on page 228 it explains the issue, i quote below.

• The camera cannot autofocus continuously like a camcorder.
• Autofocusing during movie shooting may momentarily throw the focus far
  off or change the exposure.

I hope that this helps.

Fats

Thanks. My best guess is that it lowers the framerate to allow more time to grab focus. Guess it can't be avoided (well, other than by keeping your thumb off the AF ON button )

 

[joema]

...
Except in this case I was already wide open (f/2.8 on an f/2.8 lens), and without a flash installed (5D has no popup). Something else is happening.

In Live View or video shooting, there is only contrast detect AF. That becomes even more difficult in low light, or a low exposure. Probably to help the one-shot video AF obtain a focus lock, the algorithm is "if current exposure is below a certain level, momentarily increase ISO". It may also momentarily decrease shutter speed -- I don't know. It does cause a momentary brightening on the LCD, but not always.

Since the 5D3 AF system is designed primarily for stills, this fluctuation has no effect in that shooting mode. In video it does cause a slight disruption, but that was likely not a design priority several years ago when the design was frozen.

The 5D3 AF behavior is obviously not necessary, as other contemporaneous designs like the D800 don't do that. However they don't have perfect video AF either, and for serious DSLR video you typically use manual AF.

Back when the 5D3 was designed (possibly around 2010), the designer's viewpoint may have been a high-end HDSLR is normally not used for casual video, most serious videographers manually focus, and the then-available technology did not permit fast reliable video AF in a shallow DOF situation at an achievable price point. Thus they may have used a somewhat rudimentary single-shot video AF system, and the brightness variation during video AF was a stopgap to improve AF lock performance. I'm just guessing.

Time and technology march on, and it's likely the 5D Mark IV will have a greatly improved video AF system. Newer cameras like the Lumix GH4 and Canon 70D show improved video AF is possible in DSLR or similar cameras. However those cameras (even though newer) don't have the low-light performance of the 5D3.

The problem is at that market segment you have to deliver superb low light performance, and anything you put on the sensor like phase detect pixels could theoretically degrade that. There are also issues with legacy lenses and how well they can be made to work for video AF. E.g, the 70D or Lumix GH4 only produce best video AF performance with certain lenses.

 

[pablo]

It boost ISO massively to acheive a focus-able image.