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I was testing the video mode with my 600D, and I noticed autofocus was not working. I then enabled it via the menu and now you can focus when pressing the main button. I was wondering whether this is the only possibility. Is it not possible to have a fully automatic autofocus which automatically focus when zooming in without manually pressing the button. Can't seem to find this anywhere.

I am sorry if this is a every basic question, but I'm only a beginner.

Thanks!

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closed as off topic by dpollitt, mattdm, Imre, Nick Miners, Itai Jan 13 '13 at 22:57

Questions on Photography Stack Exchange are expected to relate to photography within the scope defined by the community. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about reopening questions here.If this question can be reworded to fit the rules in the help center, please edit the question.

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I voted this down. I don't like questions about video recording on DSLR's on a photography website. So I vote them down. I really don't think the intricacies of autofocus on a DSLR while in video modes needs to be covered here. avp.stackexchange.com should cover that. –  dpollitt Jan 10 '13 at 23:39

2 Answers 2

up vote 3 down vote accepted

This isn't possible on the 600D; you have to half-press the shutter to get autofocus. As you'll discover, this still isn't an ideal solution as you'll tend to get a lot of "focus hunting" as the contrast-detect autofocus looks for the best lock, and (if you're using the kit lenses) a lot of noise as the autofocus motor drives.

I'd recommend trying to learn how to use manual focus in video if you're interested in video with an SLR, or perhaps just buying a camcorder :-)

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Thanks for your answer! I understand how to use manual focus, but I am never sure that the object I want is in complete focus (the screen is way too small to be completely sure about that). –  Bram Vanroy Jan 10 '13 at 20:52
    
With video, getting really sharp focus is generally less important than it is for stills for a couple of reasons as people won't (generally) be looking at individual frames. –  Philip Kendall Jan 10 '13 at 20:58
    
I understand that, but because I will be capturing video with a tripod (no stabilisation needed) and the focus will always be on one person, it would be nice if that person's always sharp. –  Bram Vanroy Jan 10 '13 at 21:06

As @PhilipKendall said full-time autofocus isn't possible with DSLRs. This is due to the fact that when shooting video the mirror is raised to allow the sensor to record footage. The autofocus sensor relies on the mirror to reflect the image from the lens on to it. Basically if the mirror is up the autofocus sensor isn't getting information.

There are however cameras from Sony labelled DSLTs. The difference between the R and T is that the R stands for reflex whereas the T stands for translucent. The mirror in an SLT is in a fix position all the time. This is possible because the mirror is translucent. In Sony cameras I believe that ~33% of the light coming is reflected on to the autofocus sensor while ~66% is always hitting the sensor. This means that not as much light hits the sensor but you get full time autofocus which allows you to record video while leaving the autofocus on or shooting up to 12fps(on certain models) with each frame being separately auto-focused.

If you really want full-time autofocus you would need to switch systems which could be expensive. I am a Sony shooter(A77) and can honestly say that the autofocus is fast and pretty accurate.

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I suppose I would be better off with a camcorder. It is a shame though, because I liked the full HD video of the 600D. Thank you for your explanation! –  Bram Vanroy Jan 10 '13 at 21:25
    
The sony's do shoot full HD video, but if you find yourself shooting video more than stills you should probably invest in a camera designed to shoot video. –  CyberKnoy08 Jan 10 '13 at 21:30
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More accurately, full-time phase detection autofocus isn't possible with a traditional DSLRs: some Nikon DSLRs (the D3x00 series at least) have full-time contrast detection autofocus, but it suffers from all the usual focus hunting and noise problems. The new generation of DSLRs with PDAF elements on the sensor itself may help the focus hunting issue, and lenses like Canon's new STM lenses will help the noise issue. –  Philip Kendall Jan 10 '13 at 21:33

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