AF point not right

Started Apr 15, 2013 | Questions thread
Jerry-astro
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AI Servo and contrast
In reply to bterlaan, Apr 15, 2013

bterlaan wrote:

I do a lot of horse photography and one of th emost popular subjects is foals racing around the paddock. Why is it that on more than half of my pictures the fences in the background are razor sharp and the foal on which I had centered my focus point isn't? I do panning, of course. My tech data: 50D, high speed series, Av, but I also tried the automatic sprts mode. No difference. My lens is a Canon EF28-300 3.5-5.6 L IS.  A friend of mine who has a highe rend camera says she sees the same. I read something about the difficulty for th ecamera of combining data transport and adjusting the AF at the same time, Can that be the answer? Or can it bethat my focus point goets off the foal without me noticing it? I try to keep dthe cenral point on the foals eye or thereabouts. Does someone have an idea?  It is so disappointing to see an otherwise great picture spoilt by a not wuite sharp foal and a razor sharp fence in the background! Photoshop helps, but it is not the solution!

One obvious question: which focus mode are you using?  If it's one-shot, that almost certainly explains the issue.  That mode is designed for single composed shots, not ones that involve tracking moving subjects.  That's what AI-Servo mode was designed for.  With that mode you need to first acquire your subject (give it a little time to lock focus), then track movement as you have, trying to keep the area you want in focus on your focus point as you fire shots in HS mode.

Another issue to consider is whether the area you've chosen has good lighting and sufficient contrast to achieve proper focus.  Contrast is key, and that's how the focus system works.  If the head of the foal you're focusing on is dark (say black) and there is insufficient contrast between the coat color and the eyes, the camera may struggle to find focus.  That might explain why it selected the more contrasty fence, for example.  If that is an issue, then you might consider finding something on the foal at a similar distance that is more contrasty and easier for the camera to lock onto.

There could be other issues as well, but let's start with those two and see if either can explain the issues you're experiencing.

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