Why no video autofocus on DSLRs

[Seight_Returns]

Why (with a couple of exceptions I know) do DSLRs not have autofocus when shooting video - and likewise why is the liveview autofocus generally so pitiful ?

Yes I know the autofocus sensors on a DSLR are in the viewfinder housing and so blinded when the mirror's up - but my S100 seems to do a pretty good job of contrast detect focusing driven off the main image sensor - both for video and still images - but my 7D is manaual focus only for video and the liveview auto focus is all but useless, as I beleive is the case with most DSLRs. Why is this so ?

Edited by Seight_Returns on Friday 27th July 16:36

[LocoBlade]

It's something I wondered when buying my DSLR a few weeks back whilst looking for something that also did half decent video. The only thing I can think is the manufacturers dont want to add to the development cost by spending time and money developing a top notch AF system into the sensor for live view AF when it's only needed for a secondary function and something a lot of people probably don't use at all.

[PiB]

I'll be of no help because I was on holiday in Finland and I asked the same to a friend of mine - he gave a very descriptive reason why - something to do with the type of sensors or where the sensors are located verses a cam corder - but alas I was on holiday and didn't really follow all he was saying. There seemed to be a very logical reason why based on how the DSLRs are designed.

[sgrimshaw]

Think you've pretty much answered your own question.

In most DSLR's the mirror blocks the AF Sensors when up.

This doesn't happen with the Sony SLT range, for example, as the mirror is Translucent so it can Autofocus even when recording Video.

[RobDickinson]

650d has on sensor pdaf focus now during video.

Not sure why slr's are poorer for contrast af but remember they are dealing with a much much smaller depth of field than something like the s100 so its a trickier job.

[Ari]

Mine has. Nikon D7000

[LocoBlade]

A lot of cameras like your D7000 and my D5100 can do it, it's just not very good compared to the AF on a dedicated camcorder or even a good mirrorless stills camera

[GetCarter]

Can autofocus on D800, but it's slow.

[Simpo Two]

Well as a stills person I find this a bit like saying 'My lawnmower doesn't cut hedges very well'.

[GetCarter]

Yea but if you know what you're doing:

http://www.youtube.com/watch?v=E3doNm4bH7U

All filmed on a D800

[LocoBlade]

I read somewhere that DSLRs can offer as high video IQ as pro video cameras costing a few grand, not sure how true it is but some of the demo videos I've seen shot on DSLRs are impressive

[RobDickinson]

LocoBlade said:

I read somewhere that DSLRs can offer as high video IQ as pro video cameras costing a few grand, not sure how true it is but some of the demo videos I've seen shot on DSLRs are impressive

For fast lenses and low light SLR's have much bigger sensors than any cheapish pro video camera. This makes a huge difference.


If you've bags of money for a motion picture then its not really what your after, cameras costing 100,000's and lenses similarly priced plus filters that cost more than an SLR and lens are the goto items.

For lower end pro stuff SLR's are breaking new ground.

[Super Slo Mo]

RobDickinson said:

For fast lenses and low light SLR's have much bigger sensors than any cheapish pro video camera. This makes a huge difference.


If you've bags of money for a motion picture then its not really what your after, cameras costing 100,000's and lenses similarly priced plus filters that cost more than an SLR and lens are the goto items.

For lower end pro stuff SLR's are breaking new ground.

The snag with SLR's for video use is that they 'handle' like an SLR not a video camera.

There's a new generation of camcorders out now that offer SLR levels of image quality coupled with the handling benefits of a proper video camera. Most professionals manually focus anyway, and it's things like being able to zoom with your right hand (that's also supporting the camera), and ride the iris in shot, which is tricky on an SLR.

Panasonic AF101, Sony FS300 & 700 spring to mind. They're the step between an SLR and the RED camera system, and are becoming popular amongst professional film makers.