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I believed that T4i's phase detection is really a winner for taking videos with auto-focus, until I read this post.

Has anyone else experienced any practical difference in the quality of videos, (while auto focusing moving objects) between phase-detection empowered AF cameras like the T4i and contrast-detection AF cameras like the D5200?

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Note on the down vote. I vote down questions about shooting video on this site. – dpollitt Feb 27 at 1:50
@dpollitt, well, before posting my question here I read this post from FAQ [link] (meta.photo.stackexchange.com/questions/28/…). I believe that this question goes well with the majority of opinions reflected in that post. It would probably be better if you express your points of view in that post. – cybergeek654 Feb 27 at 2:10
Yes I'm aware of it. I still downvote questions about videography. I enjoy shooting video on my DSLR and also enjoy learning about it. I just like this website to be on topic and about taking photos ie photography. This is specifically about video AF - something that really doesn't matter when you take a photo. – dpollitt Feb 27 at 2:12

closed as off topic by dpollitt, mattdm, MikeW, Itai, John Cavan♦ Feb 27 at 2:57

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1 Answer

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My understanding is that the phase detection hardware on the T4i lets auto focus work at all during video. Contrast based detection is typically unsuitable for video since it requires guessing and checking until the ideal contrast is found. Phase detection works based on phases of light entering the sensor and doesn't require as much guess and check process in theory. I have not, however, seen the T4i in practice to know how the performance compares to a dedicated video camera.

In general principal however, contrast based AF is not suitable for video where as Phase Detect can be used for to make the results more passable by narrowing the range that contrast detection has to be used over.

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Not entire sure what you mean in your first sentence but the T4i only used phase-detection to approximately set focus and then hunts with contrast-detect to finish focusing. The phase-detection areas on the Canon sensor are apparently not sufficiently precise. – Itai Feb 27 at 1:41
Itai is correct, Canon currently uses a hybrid AF method in video. It only uses the phase-detect pixels to generally determine the phase shift (PD pixels are not as accurate as dedicated AF units in DSLRs), so once the phase-detect part is done, it still has to use contrast detection...the only difference is that the range of contrast detection (where it hunts) is much smaller than it might otherwise potentially be. – jrista♦ Feb 27 at 2:09
@Itai - thanks for the info, good to know that it isn't entirely done using the phase detect. My only info about the T4I had been second hand from a co-worker. I have updated my answer to include the fact that the PD only narrows the range searched. – AJ Henderson Feb 27 at 13:51

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