How can I tell focus errors from subject motion blur or camera shake in figure skating photos?

Question

I’m shooting figure skating with a Nikon D90 and Sigma 70-200mm f/2.8, usually around f/2.8, 1/320s, and Auto ISO up to 1600. Some images look softer than expected, and I’m trying to work out whether the cause is:

- missed focus or focusing on the wrong subject
- shallow depth of field
- subject motion blur
- camera shake

I can usually spot obvious focus misses when the wrong object is sharp, but sometimes the image is just generally soft and it’s hard to tell why. At 1/320s, I wouldn’t expect huge camera shake, and I’m not sure how much motion blur a skater should show at that shutter speed.

Can you tell from the look of an image whether softness is caused by focus error, subject motion, or camera shake? Also, can a skater move enough between focus lock and shutter release to fall out of focus, and what shooting techniques help reduce that problem?

user2719 · Accepted Answer

Both [subject] motion blur and camera shake will have a directionality to them. If you can see more than one direction of motion, it's you.

Figure skaters can move pretty quickly, but 30mph (13 m/s, 48 km/h) is trucking along pretty darned quickly (even when setting up for something like a triple-triple combination), and at 1/320s, 30mph (13 m/s, 48 km/h) translates to just a little over one and a half inches (4 cm) of movement during the exposure. If the skater is moving towards you or away from you (within a few degrees), that doesn't translate to enough motion to give much of a radial motion blur (due to apparent size change) or give the subject enough time to escape your DOF unless you are framed very tightly. Overall fuzziness is more likely a focus error (or it's telling you that your camera's predictive autofocus is not up to the task).

Looked at from the side, though, that one and a half inches (4 cm) is a lot of lateral displacement, and it will cause a significant blur -- but that blur will have an obvious directionality to it. The same thing applies to spins and jumps, except that you'll see the axis of motion staying relatively in focus while the distal portions of the skater blur a bit (which is an effect you probably want to capture a lot of the time). A camel spin is just about the only spin you can hope to freeze.

If linear motion blur is more than an inch and a half (4 cm), or if it's got a "hook" to it, then it has to be camera motion. You just can't move that fast on picks, but at 200mm, a little bit of camera motion translates to a lot of linear motion at the subject plane. It could be from panning too quickly or too slowly, from the arena temperature, or from coffee used to compensate for arena temperature. (For those not familiar with the environment, community arenas tend to be cold throughout -- it's expensive to maintain a cold-on-the-ice, warm-in-the-stands environment.) And if my experience is still valid, figure skating tends to happen at ungodly times except when a major meet is scheduled -- it helps to be wide awake without being wired on caffeine. Your monopod will help a lot.

How can I tell focus errors from subject motion blur or camera shake in figure skating photos?

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