How accurate should one expect autofocus to be?

Started Mar 30, 2013 | Questions thread
Just a Photographer Senior Member • Posts: 1,368
Re: Do you know what contrast AF really means?

SixtenB wrote:

Now to my question: I'm having problems getting sharp photos. One typical example would be taking photos of my 2 year old. I set the camera at 70mm, 1/250 (she's never still), ƒ/2.8, AF-C, d9 area mode and keep the AF-point centered on her eye while keeping AF-ON depressed (I have disabled shutter half-press AF). I then wait a few seconds for the right moment and release the shutter. The result is a perfectly sharp eye (when viewed at 100 %) only in about 10 % of the cases. The rest are either significantly back- or front focused (seemingly at random).

Probably there is nothing wrong with the camera, but user error or better said 'error of user expectation' due to lack of knowledge on how contrast detection AF works.

Your camera works with contrast detection, which means that the AF (selected AF point) of your DSLR looks for the biggest contrasts on the subject you are pointing at. Eg. if you point at the eye, it will set sharpness on the eye as it will see the iris (dark) and the eyeball (white).

Now you also say that you have set your camera to AF-C and 9-point mode.
This means that you have enabled your camera to be allowed to focus on any of those 9-points around the center point you've choosen. So you allow your camera to automatically let it focus on any of those 9 points that are active.

If your 2 year old is physically moving (eg. moving his/hear head) all the time, your camera will continuously detect different contrasts. So at times your camera might see the biggest contrast in e.g.. hair/ear or head/background instead of the eye you thought you were focusing upon. This does not mean your camera is faulty, but it just selects the AF points that has the biggest difference in contrast at the moment you press the shutter.

This can be any of those 9 AF points that are active as you have set your camera to allow it to do this. I would therefore advise you to set AF-S or AF-C 1-point as default and first try if this solves your problem.

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