7D erratic focus

Started May 16, 2011 | Discussions thread
Dewight McMullen
New MemberPosts: 17
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Re: 7D erratic focus
In reply to kyjy, Jul 5, 2011

You're right about focus points locking onto the nearest object and that often contributes to "focus problems" that folks who use auto point selection often encounter. However, the focus lock, regardless of the focusing mode, is determined by both distance and level of contrast in the area covered by the sensor of the active focus point. Now add this to the fact that the actuarial focusing sensors in the cross-type sensors used on all the 7D's focus points are significantly larger than the area suggested by the focus squares seen in the viewfinder and you have a situation where the 7D can appear to be back or front focusing because the focus square (the thing in the viewfinder falls over a low contrast area but the part of the actuarial sensor that lies outside of the focus square locks on to something that is behind the intended subject but has much higher contrast. (BTW, I know that front and back focusing can be caused by mechanical and/or software problems so please do not read this as me claiming that all focusing problems of 7Ds are due to user error as such a claim could not be further from the truth.)

Your first example (sleeping woman )suggests to me that the focus failure may be due to both low light levels and low contrast. In your second example (group standing outside a store) looks like the focusing point you used may have locked on high contrast area of the dark object that behind the right shoulder of the man the point is over. Note how much more contrast there is in that area than on the man's chin. Also note how much sharper objects well behind the group (man facing the store on the left) appear to be.

I have learned to use manual select: Spot AF (which ignores areas of the focusing sensor that lies outside of the focus point squares seen in the viewfinder, when trying to lock focus on low contrast objects that are surrounded by high contrast objects.

My general point to all this is that you can get more predictable focusing out of the 7D by understanding that the focusing systems locks on to the nearest object that has sufficient contrast in the area falling under the selected focusing sensor and the sensor is significantly large than suggest by the viewfinder's focusing squares used to represent the focus points in the viewfinder. When trying to lock on low contrast objects surrounded by higher contrast objects, consider using the spot focus mode to get better agreement between what you think you are focusing on and what the focusing system "sees" as something that it can lock on to and establish focus.

Finally, I've not yet discovered a relatively fool-proof means of determining if a focusing problem was due to user error or a mechanical/software problem. Therefore, anyone who has spent time (a week or two of shooting should be enough, particularly if you're upgrading from an earlier Canon camera) mastering the 7D's focusing system should consider having their camera and/or lens checked out for mechanical problems.

b) About a thrid are severely back-focused, often by several meters. In the back-focus issues I notice that the camera does not obey the Canon method of focusing on the nearest subject.

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