Thursday, October 29, 2009

Nikon D300 Auto-focus for Sports

I shoot a lot of soccer games with a Nikon D300 and a 200-400 f/4.  I am regularly asked how I set up the camera for best auto-focus.  Here's what I do:

1) Set the camera to AF-C (continuous focus).  This is done via a small lever on the front of the camera near the lens mount.  You want the setting marked "C" (continuous) as opposed to "M" (manual) or "S" (single).  This will allow you to track the action and have the camera continually focusing whenever you trigger focus.

2) Create a shooting bank for Sports.  Since you will want to return to these sports settings regularly, but won't want to use them all the time, it's a great time to make a custom setting bank.  First, create a normal shooting bank.  At the top of the "Custom Setting Menu", pick one of the four banks and name it "Normal".  Then pick one of the other ones and name it "Sports".  Make it the current custom setting bank.  Now all settings we create from now on will be part of the Sports bank.

3) Set custom setting a1 "AF-C priority selection" to "Release".   Since I assume you're going to want to shoot in continuous mode with multiple frames per second, this setting allows the camera to deliver the desired frame rate.  If you set it to "Focus", the camera will wait until it is sure it has acquired focus before firing every frame.  Except in very bright and very contrasty conditions, the D300 will sputter and not shoot as many frames as you want.  You WILL miss the point of action you wanted.  The D3 has fewer issues with this setting, but the D300 needs it set to Release.  In reality, if you have good focus tracking technique, the camera will have focus most of the time and you will get more keepers this way without missing the key point of action you wanted.

4) Set custom setting a3 "Dynamic AF-area" to 9 points or 21 points.  There is no hard science here on which is better.  I think that the more you can constrain the AF sensors while still managing to keep them on the desired focus target, the less likely the camera is going to pick a focus target you don't want and the less computation you are asking it to do so the more likely it can keep up with the action.  So, if you have good focus tracking technique and can keep at least some of the center 9 points on your target as it moves, then I recommend 9 points.  If you have trouble doing that, then switch to 21 points.  I do not recommend 51 points.  There are just too many things for the camera to think about switching the focus point to.

5) Set custom setting a4 "Focus tracking with lock-on" to "Short".  There are many long discussion threads about the effect of this setting on the responsiveness of focus tracking.  Without some real technical documentation from Nikon it's hard to know how it really works under the hood.  Here's what we know.  If you set it to "Off", the camera will immediately respond to a new focus target with no delay.  The moment the 9 or 21 focus points fall off your subject, the camera will snap to a new subject.  If you set it to "Long", the camera will wait a significant amount of time (waiting to see if it sees your subject back on the target focus points).  If it does, it will continue tracking it without having switched to another subject.  If it doesn't see the subject come back after some wait time, then it will pick a new subject and jump to that.  Short and Normal (the other two settings) are obviously in between.  When shooting soccer, I've found that I really don't want a long delay before it snaps to my current subject.  I may want to pick a new subject without releasing the focus trigger so I want it to be responsive.  But, at the same time, if you shoot with the most responsiveness (this setting "off"), the camera is very demanding for accurate focus tracking.  You HAVE to keep the 9 or 21 points on your subject.  If you slip off, the camera will acquire a new focus target and that will probably not be what you want.  I have shot games with it on "off" before.  With intense concentration on the focus tracking, I can get very good results.  But, I'd rather have it on "Short" because I don't feel like I'm sacrificing any responsiveness to a purposeful target switch, but I still get a little forgiveness if I vibrate off the subject for a brief moment.  Is shoot with on "Short".  If you are having trouble tracking your subject, you probably first want to go to 21 points.  If you still need more help with tracking, then change this setting to "Normal".  Nikon says that this setting is designed for situation when an object might momentarily go between you and your desired subject (temporarily obscuring it).  With a long tracking delay, the camera will not jump to the new subject, but will wait a period of time for the temporary object to leave.  There appears to be more to this setting that just that type of case because it also affects the responsiveness when you just drift off your intended target (even if it's still in the frame, but not on the 9 or 21 points you have configured).

The controversy in this setting is that some people believe that the longer you set this setting to, the worse focus tracking is on objects that are changing distance rapidly (come towards or away from you).  The people who believe that is a strong influence, recommend setting it to "Off" to get the best focus tracking performance.  In practice, it takes a lot of skill to successfully track an unpredictable subject like a soccer player or a darting bird in flight on 9 points so it's nice to have just a little forgiveness.   I'm getting very good results on "Short".

6) Select the "Dynamic Area AF" Area AF mode for focusing.  On the back of the D300, just to the right of the viewfinder is a level that has three options for focus area mode.  These three options are Single Area AF mode (only use one of the 51 sensors), Dynamic Area AF mode (use a group of sensors to make intelligent decisions as the subject moves) and Auto Area AF mode (the camera decides what to focus on).  I use Single Area AF mode for things like portraits or landscapes.  You know exactly what you want to focus on and it's not moving so Single Area AF mode allows you to unambiguously do that.  Dynamic Area AF mode tells the camera that you're going to acquire the subject you want on the selected focus sensor, but if the subject moves to one of the other focus sensors within your range (9 points or 21 points from the previous settings we discussed), then the camera should track it to that other focus sensor and keep focusing on it.  This is what works best for sports.  Auto Area AF mode is what point and shoot cameras do.  The camera is going to look at the scene and try to guess what you want to focus on.  I can't imagine how this could ever work consistently for sports.  When two players are in the scene, there's simply no way for the camera to know which one you want to focus on.  I use Dynamic Area AF (the middle setting) for sports.  It works well.

7) Select a "cross hatch" focus sensor as the center of your focus targeting.  The 51 focus points are not all the same.  If the camera is in the horizontal direction, then the three center columns of 5 sensors each are all "cross hatch" sensors.  These cross hatch sensors can work in two directions which makes them work better on a some subjects.   The other focus sensors are unidirectional and don't work as well on some subjects (it depends upon the geometry of the target and whether it has vertical or horizontal contrast lines in it).  Without really understanding all of this, I've found drastically different results in focus performance when using the center 15 sensors vs. the outer ones.  I shot 4 hours of hurdles in a track meet one day with the camera in the vertical orientation and the focus point purposely up two rows (which puts it on the first non-cross-hatch sensor).  My focus tracking results were horrible.  I had nearly 60% of my shots with unusable back-focus.  Switched back to the center sensor and the problem disappeared.  The outer sensors seem to work fine for less demanding tasks (such as a static subject like a portrait), but I never use them for action.  Further, I've decided that I want all the sensors around my chosen sensor (which is what I'm using in 9 point mode) to also be cross hatch.  That pretty much means I'm always using the center sensor for action.  Because I don't always want the focus target in the center of the shot, I may just shoot a little looser (zoomed out a bit more than normally required) and then fix the framing with a small crop in post processing.  An action shot that doesn't nail focus is going to trash bin so that I feel like I have to do what I have to do to nail the focus.  For action shooting, there are some benefits to including a little more of the scene outside the intended subject.  If an extremity (arm or leg) suddenly gets throw up or out from the body for balance, your shot may still have room for that without cutting it off.  Likewise, you sometimes capture other good things with the ball or another player in that extra room.

8) Set custom setting a8 "AF point selection" to "51".  The D300 has 51 focus points.  If you want to be able to select any of the 51 focus points as your central focus point, you must set this to 51.  If you set it to 11, then the 4-way button that lets you move the desired focus point skips every other sensor and moves in bigger jumps.  Because of the previous point I made about getting on a cross-hatch sensor, I want to be able to move in fine granularity, not big jumps so I always set this on "51" to let me pick any sensor.  I should add that for sports shooting, I very rarely ever change the active focus sensor.  With all the concentration required for focus activation, good tracking, anticipation of where the action is going, awareness of what is happening outside the frame and trying to trigger the shutter at just the right time, I don't find it a wise use of my brain cycles to be thinking about moving the focus sensor around.  Occasionally I will move it when there's a pause in the action and I know what's coming up next (corner kick, penalty kick, etc...) and I want to for the shot.  But, usually I just shoot a little looser, put the center sensor on the desired focus point and crop a little in post.  For good sports shots, you're going to be cropping a little in post anyway because you can't possibly frame everything perfectly.

9)  Turn VR off.  If your lens has VR, turn it off.  VR helps you stabilize an image when you are using slow shutter speeds and don't have rigid camera support.  VR does not help you keep a moving subject from being blurry.  The only way you can do that is with a faster shutter speed.  Once you have a faster shutter speed, you no longer need VR.  Further, VR has a few potential downsides:
  • It can add shutter delay.  Because the camera has to stabilize the VR system before it takes an image, it can sometimes add some shutter delay that is not helpful for capturing the exact moment you want in action photography.
  • It uses more battery.  This might or might not matter to your situation, but the VR action uses more battery power.  In a sports setting where you are frequently tracking for long periods of time, the VR will be engaged a lot more than other kinds of shooting.
  • It can degrade the bokeh in your image.  Bokeh is the quality of the out-of-focus areas in your image (typically the background behind the subject in a sports setting).  Because VR works be decentering a lens element that is especially designed such that the decentering action can be used to exactly counterbalance lens shake.  When this lens is decentered, the quality of the out-of-focus areas in a shot can take on some weird artifacts.  Exactly what happens and how noticeable it is depends upon the particular lens design, what aperture you're shooting at and the relative distances of subject and background. On my 200-400 VR lens, the difference is pretty noticable.  I get weird doubling-type artifacts in the out-of-focus background areas if VR is on.  I don't get them if it's off.
10)  Shoot as tight as you can.   In point 7 above, I said to use the center sensor and don't worry if you have to crop a little to make the framing of the shot exactly what you want.  This is the corollary to that.  A focus sensor will only be able to give you the best results if the focus target is larger than the focus sensor.  If you're trying to take a picture of a player all the way across the field and the focus sensor is twice as big as the target you're trying to focus on, you can imagine how hard a job it has.  The same focus sensor is getting some reflected light from your intended target and some reflected light from your background.  It's getting mixed signals for what to focus on.  Usually, it will favor the thing you're actually aiming at, but I've see regular back-focus errors where it ends up with some sort of compromise that isn't exactly on your subject and isn't exactly on your background, but is somewhere in between.  It is common for newbies at action sports to not zoom in enough.  They take a wide shot of the field and the desired subjects are only in the very middle of the frame.  If you're taking an image of a single athlete, try to get their height to fill at least 2/3 the height of the frame.  That will get them large enough to get good data on the intended focus sensor.  Don't be afraid to take some shots with just their upper bodies.  Shots in this tight are harder to track, but can also come out really nicely.

11)  Practice and learn how to do focus tracking.  One significant skill in action shooting is learning how to acquire focus on your target and then to track focus on that target while you follow the subject waiting for the exact moment that you want to fire.  Besides developing the skill to do this, you also need to be able to do it subconsciously such that the main part of your thinking brain is thinking about framing the shot with the right zoom and aim, thinking about which moment you actually want to fire the shutter and thinking about what's going to happen next in the action.  It is initially hard to think about all these things at once.  You need to practice the focus tracking enough that it just becomes second nature.  In portrait shooting people will talk about getting the focus "on the eyes".  In action sports shooting, you aren't aiming for anything that precise.  Usually, I'm aiming for anything on the body of the person.

12)  Pick the best types of focus targets.  The focus sensors work off reflected light.  The more reflected light there is from your desired focus target that gets to the focus sensor, the better chance they have at giving accurate results.  The two main things that affect how much reflected light comes off your subject are the overall amount of light in the scene (bright day vs. late evening) and the reflectivity of the target (black soccer jersey vs. white soccer jersey).  If it's really bright out, you don't have to worry so much about this.  But, if I'm shooting in marginal lighting conditions, I may adjust what I'm aiming the focus sensor at to get it the best light.  If I'm shooting a team with black jerseys and white shorts and the overall light is low, I'm going to aim more at the shorts than the jersey even though the jersey is a bigger target.  This really makes a difference.  I regularly shoot a team that has black jerseys and my keep rate goes way down if the overall light is low and I'm only targeting the black jersey.  If the shots are any brighter go for them when the light is low.

13) Get yourself some stability.  It is way, way easier to track a moving target when you have some support for your camera/lens that you can adjust from.  Shoot for a couple hours and that support becomes even more important (your muscles will get tired).  I predominately shoot sports from a monopod.  It gives me a base of support, but still has plenty of versatility to aim anywhere I want.   As we said before, focus tracking skill and accuracy is very important to getting good results and some form of stability really helps.

14) Think carefully about your depth of field and background.  This article is about focus technique and focus settings, but one characteristic of an image that can have as much to do with the perception of sharp focus as anything is how your image is positioned in the background.  In a sports setting, it is rare to have a nice clean background.  Usually there are fans, stadium seats, fences, benches, coaches, other players, even sometimes houses and cars and powerlines in the background.  None of these complications help your image.  The cleaner a background you can get, the more likely your subject will stand out and appear sharp.  So, the first thing to consider in this regard when setting up for shooting is to try to position yourself for as clean a background as possible.  Sometimes I get lucky and one direction has a nice empty grassy hill that forms a non-complicated, non-competing background.  Sometimes, due to the logistics of lighting and where you are allowed to be, there are no good choices, but you should at least think about it and see if you can improve your situation.  A clean background will make your subject look better.

Then, think about your depth of field.  The best thing you can do with a less-than-ideal background is to make it go blurry.  A relatively sharp subject on a blurred background really, really looks nice.  In fact, you don't even have to have killer sharpness in your subject for it to look pretty sharp because the contrast in sharpness is so great.  There are two ways you can get the background to go blurry.  The most effective (it works at any aperture) is by using relative distance.  If you are close to your subject, but the background is a long ways behind your subject and you focus on your subject, the background will go blurry, even at f/8.  And, the opposite is true too.  If your background is right behind your subject (very close to it), even if you're at f/2.8, you may have trouble getting the background to blur.  I generally shoot sports around f/4 because I find that that is usually enough depth of field for my shot, but gives me some good background blur as long as I have some decent relative separation in distance.  Shooting at a larger aperture (f/2.8 - f/4) requires more focus accuracy because there's less depth of field to absorb focus errors.  But, once you've developed the technique for it, you can blur away some of the complication in the background and your subject will look sharper.

I find that shooting for a lower height (often kneeling on the ground) gives me several advantages.  To this point, because I'm shooting at a little bit of an up angle, it raises up the point in the background that's in the shot and skips over near parts of the field that would be in the shot if I was shooting down.  This is even more important with little kids where a standing position means you're angled down to shoot them, but works for all ages.  The slight up angle also sometimes lets you skip over things on the other sideline so that you get more relative distance for your background.  And, lastly when you're shooting from a lower angle, you are more likely to be able to catch their eyes and face.   Particularly in soccer, players head and eyes are often pointed downward.   A lower angle lets you see more of the face.

15) Think about how you want to trigger focus.  This is a more advanced topic and is not required for getting good results.  I include it here for completeness and because it's an additional focusing option that I like to use.   On the D300, you have three main options for trigger the start of continuous focus:
  • A half press of the shutter.
  • The AF-ON button
  • The AE-L/AF-L button.
By default, the camera comes set up for a half press of the shutter trigger continuous focus to start so that's what most people do.  If that feels comfortable to you and you like it, there is nothing wrong with doing it that way.  You will not get better focus accuracy by doing it any other way.  There are, however, some other options.  I won't talk about athe AE-L/AF-L option just because I haven't studied it, but I do really like the AF-ON option.

To use the AF-ON option, you tell the camera that focus is triggered ONLY by the AF-ON button.  A half press of the shutter does not trigger focus.  While this creates a little bit more to think about (you have to press a different button to start focusing) and it takes several days of shooting for it to become completely natural and automatic, it has some significant advantages.  As you will recall, item 1) above is to set the camera to continuous focus (AF-C).  That means that once you activate auto-focus, the camera continues to focus until you stop activating auto-focus.   That means that you can't really acquire focus, reframe, then shoot.  When you reframe, the continuous focus will keep changing focus and you won't be able to keep the focus on the original target.  But, there are times in sports shooting when you'd like to pre-focus on a particular spot, frame your shot and then just fire the shutter at the right moment.  In soccer, I want this behavior at a couple different situations in the game.  For example, if I'm trying to get a shot of a player making a corner kick, I'd like to pre-focus on the ball placed on the ground and then be able to just fire three frames as they come through on the kick.  If I'm trying to shoot the goal area during a corner kick (expecting the ball to come through the air near the goal with lots of players up in the air), I like to prefocus on the goalie and then just fire off shots as I see the ball come into the frame.  So, if you're using continuous focus and have a half-press of the shutter activating focus, you just can't do this without switching your lens to manual focus or switching to the AF-S focus mode and then immediately switching back for the next part of the action.

The answer is to stay with AF-C (continuous focus) but move the focus activation off the shutter button and onto the AF-ON button.  For regular focus tracking of a moving player, you simply press and hold the AF-ON button.  Point the center sensor at your target, press and hold the AF-ON button to activate focus tracking, track your subject, the press the shutter when you want to take the shot.  So far, it's not a lot different except there's another button involved.  Now, imagine the corner kick scenario from above.  You point the center sensor at the static soccer ball on the ground where the corner kick is going to be.  You press the AF-ON button until it focuses on the ball, then you release it.  You are now pre-focused on the soccer ball.  Because you released the AF-ON button, the camera is no longer continuously focusing.  You can now follow the player with whatever framing you want for the shot and just fire the shutter at will.  The lens will stay focused at the distance of the ball.  Anytime you want to start focusing again, you just press and hold the AF-ON button again.

In a nutshell, you get to switch between AF-C continuous focusing and AF-S static focusing without changing any settings on the camera, but merely by how you press and hold the AF-ON button (continuous focus) or press and release it (static focusing).  It gives you the best of both worlds.  In fact, I now use AF-ON focusing for all types of shooting.  It works great for macro shooting to.  I can press the AF-ON button to do initial focus, then release it, then decide if I want to do any fine tune on the focus by turning the lens barrel.  In that case, I'm getting a combination of static focusing and manual focusing again without changing any camera settings.  For spontaneous portrait work, I aim at the eyes, press and release the AF-ON button to acquire focus on the eyes, then reframe for the portrait and shoot.

To set your camera up for AF-ON focusing, you set a5 "AF activation" to AF-ON only.  As we already suggested above, a1 "AF-C priority selection" must be set to "Release" because otherwise, it won't fire if your focus sensor isn't currently focused on the target because you pre-focused and reframed.