Focus problem vs. motion blur vs. camera shake - how to tell the difference?

Question

I'm not sure if the title of the question captures what I mean, but it's hard to be succint in the space allowed.

Suppose you have a picture that is not as sharp as you think it should/could be. (And, I'm not even sure that sharp is the right word here).

I can think of a number of reasons:

• out of focus (entire frame)
• depth of field (focus on wrong element)
• motion blur (not fast enough shutter speed)
• camera shake (hand-held, shutter too slow)

Can you tell, by looking at a photo, what the problem might be?

Specifically, I am shooting Figuring Skating with a D90, and a Sigma 70-200mm F2.8, usually at F2.8 and at 1/320s, with auto-ISO up to 1600 (based on lighting conditions in the rink).

So, at F2.8, I sometimes have DOF issues (the auto-focus grabbed the wrong element, and the parts I was interested in are not as sharp as I would like). I can usually tell this because I see the wrong elements in focus.

Sometimes, the auto-focus just wacks out (and focuses on something too close, like glass, or too far, like the stands). I can tell this because the focus is really off.

Other times, it isn't as easy to tell. When the photo is not sharp, I can't tell whether the focus is just off a bit, or if it is motion blur or camera shake.

At 1/320s, I would think that motion blur for Figure Skating wouldn't be too much of an issue, but then I wouldn't think that camera shake would be much of a factor at that speed either.

Sometimes I feel like it focused correctly, but by the time the shutter is released, the subject has moved enough that they are no longer in perfect focus. Is that possible?

What would be telltale signs of bad focus vs. camera shake vs. motion blur?

Are there ways to test/compensate for this? (I've just bought a monopod to try to reduce camera shake).

UPDATE:

Here are some photos of Synchronized Skating: Winterfest 2011

Here are some photos of Figure Skating: Celebration On Ice 2010

Here's an example of a "bad" photo:

Here's an example of a "better" photo:

Neither are great shots, but the first is softer for sure.

Answer 1

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.

Answer 2

Yes, you can tell what went wrong:

• out of focus (entire frame)

This can be a problem because of close-focus, i.e., the lens and camera never did find something to lock onto before you fired and was focusing ahead of everything in the scene. Sometimes this is because your lens focuses too slow, sometimes it's because there wasn't enough light to provide enough contrast to allow the camera to lock-on so it did it's seek-cycle, trying to find something, then gave up, AND then you fired. You'll KNOW if you did this, and will cuss yourself, but we all do it. Once.

For a close subject, it might be too close and the lens can't focus close enough. Move back, or tell the subject to step closer to the canyon behind them. It could be the result of your lens being set to not allow it's closest focusing range because it slows the seek. Canon's 70-200L f2.8 has a switch to control that, and it occasionally caug... strike that. I never had that problem.

It can even be the result of a damaged body or lens mount, making the image never focus on the sensor/film correctly - I've been there and done that and it was really annoying to diagnose, then get fixed, and, with a pro-body and lens, can be very expensive. Uh. Yeah. I did that running to get away from a bull while shooting pro-rodeo and flipped over the fence. Broke my Nikon D1x and 70-200f 2.8 at the same time but the bull didn't get me.

• depth of field (focus on wrong element)

This is easier to diagnose. Something in the image is in focus, but it's not what you wanted. Sometimes it's the result of having a different sensor chosen than you thought. My favorite stunt.

It can happen because you need continuous focus because your subject is moving toward or away from you, and the camera locked focus, instead of following the subject. This is a problem that bites all sports-action photographers periodically, usually after they have been shooting something else, or loaned their camera to their wife or girlfriend, who messed with the settings. And, no, I have NEVER experienced THAT. :-)

• motion blur (not fast enough shutter speed)

Usually this shows up as the main subject is a streak in the scene, but could be that their extremities are blurred if they were moving hands, feet or maybe their head. The torso moves less than the other parts and usually is not (as) blurry.

An alternate, but much more desirable result, is when you are panning with the subject, fire, realize the shutter is taking too long, but you keep panning. Your background will blur but the subject will still look good. I used this a lot for my sports photography - doing it deliberately, not like Peewee Herman, where "I meant to do that."

• camera shake (hand-held, shutter too slow)

This occurs most when you're using a long-lens in lower-light. Generally the motion blur is equally squiggly all over the frame. If the problem occurs with a still-life or landscape and you have IS, you might be able to fix it by turning that on, unless it was already on. A tripod could help. Set up some big strobes and light the heck out of it. Increase your ISO, open the aperture more... do something to increase the available light hitting the sensor.

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Regarding learning how to pan

Panning is really easy, it just takes some practice and knowing a trick.

Here's a good way to practice: Set your camera to its shutter-priority mode (whatever it's called in your camera) and set 1/60 as your exposure time. Using a long-ish lens, in the 200mm range, stand a little ways from a busy road, maybe 50 feet (15 meters), and as cars pass try to get a photo of a wheel or the door handle.

The trick to panning correctly is to sync to the speed of your subject quickly, so face the roadway, then pivot at your waist/hips to view the approaching vehicle. As soon as you see it, start tracking its motion, then, as it nears being right in front of you, fire and continue panning. You should see your target in the view finder before you fire, and it should still be there when the shutter opens again. If so, you were panning at the right speed for the subject and you should have little to no motion blur of the target. If the target wasn't in the same place as it was when the shutter released then you didn't pan smoothly.

It's important to stand in a balanced position, at least until you've been doing it a while and it's all second nature. Similar to shooting a rifle, you want your feet about as far apart as your shoulders. Face the road where you intend to release the shutter, not where you'll see the car first. You need to pivot smoothly from the hips as you learn, because swinging your arms or rotating your upper-body will make the camera rotate, leading to photos with verticals that are not vertical, leading to post-processing, which is no fun.

And, again, the goal is to have the target, whatever you're shooting, be in the viewfinder before and after the shutter opens and closes.

Answer 3

Out of curiosity, are you using any kind of Servo AF? When photographing moving subjects, particularly those that may move closer/farther away while you pan and frame the shot, you should be using an AF mode that continually focuses. (I think the D90 calls such a mode AF-C.) Usually when using such a mode, the camera will lock focus onto something, then try to maintain focus on that subject so long as the shutter button is at least half down. This is either done with basic contrast and shape matching, or possibly more advanced AI algorithms.

Usually, higher end cameras, such as the Canon 7D or 1D series or the Nikon D7000 or D3X, will have much more advanced AF systems that are specifically designed to track and maintain tracking on a subject. These cameras come equipped with much higher AF points (the D90 has 11 w/ 1 cross type, while the D7000 has 39 w/ 9 cross type w/ 3D tracking, the 7D has 19 cross-type or 63 adaptive zones grouped into 5 selectable areas, the Canon 1D IV has 45 w/ 39 cross type, and the D3X has 51 w/ 15 cross type). More points, particularly more AF cross-type points (these detect phase in both the horizontal and vertical planes or in both diagonal planes, rather than just the vertical plane), contribute to AF lock-on performance and AF tracking performance. The more control you have over the selected point, the more you will be able to control how and where your camera locks onto focus. With an AF system like the 7D, you have the ability to select dynamic zones that will automatically focus on subjects only in that zone, or even adapt the zone as the subject moves. The Nikon D7000 has similar technology that it calls 3D tracking (although I am not sure if it has the adaptive zones.)

The D90 has only a slightly more advanced AF system than my (now rather dated) Canon 450D, which has a 9-point AF system. While it does have "3D tracking", with so few AF points, and only cross-type point (usually, only the center point is cross type), the success rate for AF locking onto the subject you intend it to lock onto, and maintaining that lock, is considerably lower than AF systems designed to lock onto and track the subject you intend to be in focus throughout the frame.

While it is possible to determine if an image has locked incorrectly, knowing that you missed a shot because AF missed its lock doesn't help you replace the lost moment. If you do a lot of action photography, shooting things like figure skating, you might want to look into a camera with a better AF system. You probably won't find a camera with such an AF system for less than about $1200, however if it means you nail those perfect shots far more frequently than you do now, and nail them with clearer focus than you do now, that $1200 may be well spent.

Answer 4

Camera shake is the easiest to narrow down with simple tests. Take test shots of a static subject with something that makes it easy to measure sharpness (books on a bookcase). Take bursts at decreasing shutter speeds and see how far you can get while maintaining acceptable sharpness. With a long non-VR lens you might be able to get down to 1/60s before shake is an issue. With a VR lens maybe down around 1/15s. Try it handheld and with the monopod, that will give you a baseline to go off of. If you are shooting faster than your baseline then you need to worry about motion blur and your focus.

Answer 5

You might also try a smaller aperture--it could be that your particular lens is not at its sharpest when wide open, and that could be worsened and made more evident by your trying to capture a figure in motion.