Thursday, April 19, 2012

Canon 5D Mark II - AF Point Area Expansion Issue


I went last week to the Egyptian countryside to spend some relaxed time with my family, away from the city and the fast-paced lifestyle. I decided to get to know my newly acquired equipment as well.

I was shooting some daylight portraits with the 5D Mark II and my currently most favorite lens, the Canon 200mm f/2.8 L II USM (expect a review soon), and as I reviewed the pictures on the camera's back LCD, I found that all of my shots were out-of-focus!!! From experience I knew that my 200mm f/2.8 focuses very quickly and accurately, and I took some wickedly sharp portraits with it, so what was the problem? More after the jump.

The out-of-focus shots destroyed my concentration, and I started fiddling with the Micro Focus Adjust to try and rectify the consistent front-focusing. As you can imagine, MFA is not that easy to calibrate in a few seconds in bright daylight on the back of a small LCD screen, and as a result, I lost lots of good moments that I went out to shoot in the first place, it was very frustrating.

C.FN III-7 "AF point area expansion"

I was reading the day before about this custom function over the forums, and users were reporting that it helped them track moving subjects better, so I decided to give it a try and turned it on, here is what the Canon user guide say about this custom function:

AF Expansion Points (in red)
C.Fn III-7 "AF point area expansion"
0: Disable (default)
1: Enable

 When you select AI Servo AF and the center AF point, the six Assist AF points will also function. Seven AF points will track the subject. This is effective for subjects that move erratically, making it difficult for only the center AF point to track it.

I found a shaded place and sat down to try and figure out what went wrong with the lens, then I remembered that the only change I made was switch the above mentioned custom function on, so I turned it off, and suddenly everything went back to normal, the lens focused correctly as it ever did.

Once I returned back home, I searched online for other people having a similar experience, and to my surprise, I found two or three reports mentioning the exact same issue, but no one tried to do a more thorough analysis. I decided to do it, and here is what I found.


First of all, the custom function in question should only be effective in AI Servo AF mode, and should not make any difference in One Shot AF mode. And since the AI Focus AF mode switches automatically from One Shot to AI Servo once the subject starts moving, then it might use the expansion points in certain situations.

For the record I was using AI Focus AF (I find it to work the best with the 5D2, but never with the 60D) when I encountered the front-focusing.

I setup the camera on a tripod with a 2-second timer, my subject was a book put to an angle to be able to demonstrate the exact focus. I tried two lenses, the Canon 50mm f/1.4 USM and the Canon 200mm f/2.8 L II USM, both lenses were shot wide open.

I only used the center focus point. I tried all three AF modes on each lens, and I repeated the process three times for each AF mode, between each trial I moved the focus ring manually to infinity so that the subject is out-of-focus, and the lens has to re-focus each time.


I tried the Canon 50mm f/1.4 first, and the lens focused at the same spot each time no matter what the focus mode was, but with the Canon 200mm f/2.8, I got the results I expected.

AF Point Expansion OFF - AI Servo or AI Focus

AF Point Expansion OFF - AI Servo or AI Focus

With the custom function turned on, the camera consistently front-focused with AI Servo and AI Focus AF modes, as you can see in the second image. What astonishes me, is that when I used AI Focus mode, although both the camera and the subject were stationary, the camera decided to use AI Servo and used the expansion points. Also as expected, turning this function on or off, makes no difference at all with One Shot AF mode.


What to conclude out of this?

Since I find the AI Focus mode to work the best with the 5D Mark II, and I hate switching between One Shot and AI Servo modes, I decided to leave this function turned off. Maybe some day in the future I will give it a try with moving subjects, but I guess that would be hard to judge accurately.

I would really like to know why did the front-focusing occur with this custom function turned on, even in AI Servo mode? And why did it appear with the 200mm lens and not with the 50mm?

I would appreciate it if any 5D Mark II users shared there experience to get to the bottom of this.