One of the most difficult things you can ask a photographer (or a camera) to do, is get a pinsharp picture of a moving subject. If the subject’s moving, then so is your camera, and if your camera’s moving, then so are you, writes Andrew James and Jon Adams. With all the permutations and combinations involved in framing up, staying on target, and pressing the shutter at the right time, we have enough to do just keeping the thing in the viewfinder, so getting it sharp as well seems to require superhuman abilities!
But successful, pinsharp action shots have been captured for many decades, and a lot of these were bagged before the joys of autofocus were introduced on film SLRs back in the mid-eighties. With manual focus cameras, it was just about impossible to track something and keep it sharp by twiddling the focusing ring at the same time, so the method of pre-focusing was often used – in conjunction with a motor-drive – to squirt off a bunch of frames at a set focusing distance.
Providing your subject is situated at that pre-set focusing distance when the camera fires, you should get a sharp one within the bunch of frames you’ve rattled off. With AF cameras, pre-focusing is a technique that today’s photographers seldom think of using, but it still works, and is still used, especially with remote-fired cameras (such as those lined up behind the goal at a footie match), or those placed in positions that would be too dangerous to be operated directly (like right next to a jump in a horse race).
How does AF work?
AF systems work primarily by detecting contrast in the scene, and assume that where the contrast is maximised, the thing showing the contrast will be sharp. If you think of an angle on a sunlit building, you’ll have the lit part and the shaded part, and the point at which the line between the two is crispest gives the distance from that point to the camera, and is where the maximum sharpness will occur.
AF sensors are either the more accurate cross-type (where they assess the contrast vertically and horizontally) or the less accurate vertical-line type (where they only measure in one dimension), so with lots of cross-type sensors spread over the field of view, you should be able to ‘get a lock’ on a contrasty edge in the scene and achieve sharpness at that point. A lens can only focus on ONE point, though, so you need to decide on where you want the sharpness to be by making the appropriate AF sensor active. But before we look at using AF points, let’s consider pre-focusing as an option.
Pre-focusing
Pre-focusing can work when it comes to action, but it very much depends on how predictable the movement of your subject is. For example, trying to pre-focus for wildlife is difficult as you rarely have a situation where you can be certain where an erratically moving animal or bird is going to be. It’s the same with sports such as football and rugby. You know the players are on the pitch, but you can’t be certain which way they are going to run. However, if your subject’s line of movement is more regular, such as cars or athletes on a track, then pre-focusing is a valid technique to try.
You can autofocus on the track at a point you know the subject will pass, as we did in the image above, then lock that focus off by switching to manual focus. It’s useful to have a marker on the track – a white line or chevron – that you can use as a guide to when you need to press the shutter button and it is important you don’t move position because your focusing distance will change.
It’s also a good idea to give yourself some room for error by choosing an f/stop such as f/8 for greater depth of field, in case the subject doesn’t pass exactly where you expect it to, or you move slightly. You also need to make sure your camera’s drive mode is on its highest continuous shooting speed, so you can take a burst of shots as the subject passes through in the hope that at least one will be perfectly sharp. Lastly, remember to set a very fast shutter speed otherwise it won’t make a difference if you’ve pre-focused in the right spot, because your subject will still be blurred because of its movement.
The main issue with pre-focusing is that you are hedging your bets rather than tracking the subject itself, so you may find your sharp shot doesn’t capture the best moment of the sequence. The other problem is that as you’re likely to need a smaller aperture for the focusing leeway we mentioned, you may well need to boost ISO to get the shutter speed required to avoid subject blur. This can increase image noise and therefore reduce quality.
Even with a depth of field leeway of f/8 or f/11, pre-focusing won’t be as accurate as getting an active focus point on the precise area of the subject you want to be sharp. For example, with wildlife this would be the head or nearest eye, if possible. Finally, the pre-focus method relies somewhat on the number of frames per second you can fire off, because the precise moment the subject is at the focusing distance you’ve set may occur between the frames in the burst sequence. Imagine all that effort and the camera misses the perfect moment!
Using Autofocus
We’ve covered aspects of autofocusing a few times in Foto-Buzz, and most recently in the Birds in Flight article last year. But it’s always worth revisiting this subject as it is so critical to what we do, and there are some absolute basics to set up when shooting action using autofocus.
For starters you must choose the right AF Mode and for action, this is always Continuous Focus (Ai Servo of AF-C). But don’t just use it when a puffin is flying across you with its little wings beating faster than a bat out of hell – use it with anything that moves. A fidgety child whose portrait you are taking could well benefit from using continuous focus because it re-adjusts to the subject’s position, as long as you keep the shutter button half-depressed.
It’s so important to consider what an unpredictable subject might do. For example, a sleeping lion is as static as a teapot so you could use One-shot/AF-S to take a shot. But what happens if it suddenly decides to move, and you have to track it in One-shot? Most of your moving images will be out of focus because the AF system can’t track it, or you’ll miss the shot because you are busy trying to switch from One-shot to AI-Servo. Think ahead at all times and if there’s a chance of movement, set Continuous AF and stick with it.
Also, remember that you must have your drive mode set to its fastest continuous shooting mode. You want to be able to rattle off three or five quick shots when the moment is right and use your camera’s maximum frames-per-second rate to do that.
We’re also often asked when we are running a birds of prey workshop “how many AF points are you using?” It’s a valid question since for many of us we can choose from one to 153 Active AF points, depending on the system and model you have. The answer is never simple because it’s quite possible you need to switch the number of active AF points around on a reasonably regular basis.
Use the fewest Active AF points you think you can get away with and move that one or small group around as necessary. However, don’t be daft and stick with just one central active AF point if you are struggling. If you need a larger group to give yourself that extra leeway to get a lock on the subject, then do that. Obviously the more you have active, the greater chance there is of the AF picking up on the background. This is just a risk you have to take. It’s quite possible, although rarer, to have all AF points active to get a lock onto the subject, but here you have to be careful that you don’t lock onto your background instead. As a rule, a group of four or nine central active AF points is adequate for most moving targets. The shot above isn’t a good one but it demonstrates how using a small central group of active AF point allows you to lock onto the subject and ignore the background and foreground.
Don’t panic either! We see this often on workshops where a moving subject is coming towards the group, and everyone starts waving their camera around and rattling off shots! You need to hold your ground and your nerve, track the subject, and try to keep it in focus by half-depressing the shutter button. Then, when it’s about half-size in the frame you can start shooting. If you lose focus, try to get the target AF-points back on the subject as smoothly as possible. But you have to accept that sometimes, that just won’t happen. As with anything, practice makes perfect here, plus it is a fact that some camera and lens combinations are better than others at locking focus quickly. But whatever you have, it still need to be driven correctly by the person holding it.
Getting your AF point in the right place doesn’t always guarantee you a sharp shot. We’ve lost count of the times we’ve been handed a camera and asked to look at an image that should be sharp because the AF alert is showing that it locked focus where it was supposed to, yet it isn’t. The culprit? Shutter speed – because it was too slow for the subject to be rendered pinsharp. You might get away with it for an image online, but if you want to blow it up for a big print – you’re in trouble. The shot above was taken for the cover of a running magazine so it needed to be pinsharp. Taken on a 300mm lens, the shutter speed used was 1/1000sec even though technically 1/500sec would be fast enough to get a man running sharp.
Remember the reciprocal rule – your shutter speed should be at least the same as one over your focal length. Therefore, with a 400mm lens you want at least 1/400sec. Well, double it, or even treble it for action and then add some more! Ultimately, no shutter speed is too fast if you want pinsharp, frozen-in-time action shots.
Joanne
Really helpful article. Especially your comments around the number of active AF points. Thank you!
Mary Gilbert
Thank you – a timely reminder 🙂