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I recently purchased a Canon 70D for its dual pixel autofocusing system and ISO performance. I am in love with this feature rich-camera; however I found the camera's autofocus fails to function correctly with a 50mm 1.8 prime on manual zone AF mode.

The camera's manual single point AF focuses tack-sharp and quick; similarly the complete auto also lives up to its expectations. However when I set the AF method to manual zone select, it fails to result sharp pictures; the focus points get highlighted and shows to have captured the desired point, but when I preview it on my laptop, I see the focus is slightly under calculated (example: subject at 5ft would be focused around 4.8ft), as I mostly shoot with a lower F number to achieve shallow depth, I often get unusable results.

I am on one-shot mode on AF method. If I switch to manual focus on lens and use the zones for reference, I am able to achieve wonderful results.

I borrowed a 50mm prime from other fellow photographers and the issue still persists. Have I got a bad piece of 70D or is there something I am doing wrong?

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Although the question concerns a different camera body and lens, the issue is similar and this answer may help you understand how the PDAF focus system in the 70D, which is very similar to the 7D, works. photo.stackexchange.com/a/41179/15871 –  Michael Clark Oct 29 '13 at 23:46
    
Another issue that may be affecting your results with the EF 50mm f/1.8 II is that the distance between 'steps' in the focus motor is wider than any other current Canon prime lens. –  Michael Clark Oct 29 '13 at 23:51
    
Keep in mind that the dual pixel AF only works in video/live view. When focusing through the viewfinder you're using the same AF sensor that existed in the 7d. –  tenmiles Oct 30 '13 at 0:46
    
I am having the same issue with 50mm f/1.4 and 50mm f/1.8. Also, I discovered that the problem changes over time. So there is more than just "being out". You might have to microadjust, but then later you check it and its like 5 points away. I am suspicious of heat causing this - I haven't investigated why it changes over time yet - but it does change. –  user23019 Nov 2 '13 at 3:23

1 Answer 1

The thing you have to remember is that the areas of sensitivity for each focus point are larger than the representation of those points in the viewfinder. This is especially true when using zone focus. The camera will focus on the area of highest contrast within the entire area of sensitivity. This will not necessarily be the area directly behind the little square you see in the viewfinder.

This is a map of the 7D focus system. The 70D is almost identical. To see a full explanation of the data on this chart, please see http://photo.stackexchange.com/a/41179/15871

enter image description here

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It should be noted that the 7D AF system is known to have precision problems. I have used the 7D for a couple of years now, and because of the actual design of it's AF sensor, small differences in the actual focal plane for a selected point or zone (i.e. 4.8ft vs. 5ft) are pretty much to be expected. You also get a slight amount of jitter when focusing with the same AF point at the same spot on the same subject over and over. AF won't remain "locked"...it shifts forwards and back by a very slight amount each frame. Using AI Servo, I tend to take bursts of 3-5 shots every time... –  jrista Nov 2 '13 at 20:18
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...to make absolutely certain I get at least one frame in sharp focus. Ironically, while the AF system is one of the 7D's (and now 70D's) biggest draws, it is also it's achilles heel...it just isn't precise or consistent. The only way to fix that problem is to move up to a better camera...the 5D III or 1D X, which have very high precision and consistent accuracy. –  jrista Nov 2 '13 at 20:20
    
I've found that if you manually run the focus past infinity before each shot you get more consistent results with shot to shot variation because the focus is always moving in the same direction. –  Michael Clark Nov 2 '13 at 23:53
    
If you have a larger phase differential, then yes, it is usually more accurate. However, when you are tracking a subject with AI Server AF and shooting at higher speed, that is simply not an option. I've tracked birds and wildlife, and had sequences that range from five frames to thirty frames long. You would expect, so long as the subject remains underneath the selected AF zone or expansion point, that it would remain in sharp focus for each frame. Sad fact is that the 7D (and apparently 70D) is simply not capable of that kind of consistency or precision...it jitters. –  jrista Nov 3 '13 at 3:17
    
Because of this issue, I have seriously considered getting a 1D X for its high frame rate and phenomenal AF system. I have held off so far, in hopes that the 7D II will have an improved and more accurate AF system, maybe something like a 41pt AF sensor akin in design to the 61pt system found in the 5D III and 1D X. For anyone who needs consistent accuracy and precision for action work, Canon's 19pt AF system really doesn't cut it in the long run. (It seems AMAZING when you move up from a 9pt system, but for critical work, it just has that annoying flaw that really ruins the whole camera.) –  jrista Nov 3 '13 at 3:19

protected by John Cavan Mar 5 at 17:43

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