Autofocus Performance: Technique and Technology

I received an email recently that thanked me for the AF Micro-adjustment info, but also asked about how to test the AF function in general on the 50D. They already select the AF point manually (a key first step), but some images are in focus while others are not; also – if they shoot a sequence, there are also issues with some images being in focus and others not.  I responded via email, but figured this would make a great post.

A few things to consider:

What AF mode are you using?
You are shooting w/ a set focus point – that is key, but also ensure you are in AI Servo mode. One Shot will almost never work w/ any sort of movement. (Canon terminology, but Nikon has similar modes)

Is it out of focus? Or is it motion blur from either the camera motion or the movement of the subject?
Are you running a fast enough shutter speed? The rule of thumb for handheld for *stationary* objects is is 1/focal length… so if you are in w/ a 400 mm lens, you should be shooting handheld at 1/400th or better if the thing you are shooting is standing still… then if you add motion into the mix – you need to have great panning and a steady hand, and IS, and a faster shutter speed.

What is the color/pattern of where the focus point is?
The af systems are all based on contrast and the ability to find enough contrast to check for focus (the main reason that low light AF sucks). If your subject is consistent in color, or the light isn't great, that may give the AF system fits…

Which AF point are you using?
Keep in mind that (with the 50D – but this applies to other cameras as well) the center point has the best sensitivity – All work at f5.6 but only the center point has additional sensitivity for lenses of f 2.8 or better…  Consider using the center point, keep zoomed out just a bit from how you would crop in camera, shoot the pic, and then crop during post processing to get the final picture you want.

How fast is the thing you are trying to shoot moving, and how far away are you?
You can be shooting something fast that is far away but coming at you, or you can be shooting something slow but is close, and you can have the same problems – relative speed.  Consider this… Someone moves 10 feet closer to you in 1 second as you try to keep them in focus…  If they are 100 yards away, not a problem, you're looking at only 3.33% of the distance between you two lost… If they start 40 feet away – they've moved 25% of the way towards you… but are still moving the same speed.

Is it moving towards you or side to side?
Towards you is the AF system, side to side is your panning technique.

If towards you, is it moving towards you linearly? 
I'm not positive, but I think most of the AF routines are geared towards linear movement… I track a lot of cyclists at a velodrome.  Depending on where I am and they are on the cycling track, they may be speeding up or slowing down relative to my position at different rates, even though they are moving at a constant speed.  Then couple that with the varying distance to the subject and relative amount of distance closed per second… and the brain can lock up thinking about it all…

Which lens is key too!
How fast (bright, low f-stop) is the lens, how fast the AF motor in the lens is, etc.

The difficult thing in all of this is finding ways to test that are consistent… so that you know it is your technique or the camera, or the situation that is the issue.  Finding players in the same uniforms, in the same light, at the same distance, and so on…

I found that a lot of practice was what really helped increase the number of "keepers" I had out of any photo shoot that involved sports photography.  Practice, practice, and then practice some more. The whole time trying to focus (no puns intended) on one or two of the above items during each session. If you try to include them all early, you'll just drive yourself batty.

Camera companies all (yes, both Canon and Nikon) put their better systems in their pricier bodies. Not too surprisingly, I found much better AF performance when I started using a used 1D Mark IIn (considered by some to still have the best AF performance out there).  The additional focus points, the better AF system, coupled with a great fast lens like the 70-200 f/2.8 L IS, make for even better performance – but mainly after all the other things are sound to begin with. I've loaned my 1D to some friends who still have lousy keeper rates w/ it.

At the risk of repeating myself – practice… practice… practice.

When you are shooting a sequence by holding the shutter down, keep in mind that all of the above is still very much (if not more so) in play; especially if the subject is close. If there is little contrast to the uniform or color where the AF point is targeting, the AF system may not recognize that the subject is even moving. You get the idea…

Oh – practice… practice… practice!

Thanks for reading!

One thought on “Autofocus Performance: Technique and Technology”

  1. Practice, practice, practice!
    I (newish photographer) have heard this advise a number of times… and find it right correct.
    Then again, I’d add to this: ‘read/hear what good photographers have to say… and practice, practice, practice’
    :-)
    Cheers

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