6.5 Autofocus Modes

There are two main autofocus (AF) modes offered by virtually all DSLRs: one-shot, and servo (or continuous AF).  In servo mode, depressing the shutter-release button halfway engages the camera’s autofocus system, which actively continues to adjust the focus for as long as you keep the shutter-release half pressed.  For birds in motion, this is precisely what you want: as the bird changes its distance to the camera, the camera tracks the bird by continuously adjusting the focusing element in the lens.  Ideally, the camera will keep the bird perfectly focused for as long as you continue to hold the shutter-release half-pressed; when the bird assumes the pose or position that you want, you can simply press the button the rest of the way to take the photo.  Of course, in practice the AF function of the camera may not always be able to perfectly track the bird, depending on the speed and regularity of the bird’s motion, the capabilities of the camera model you’re using, and your own ability to keep the AF sensor(s) positioned on the bird in the viewfinder (see section 2.6 for a thorough discussion of how autofocus works in DSLRs).
    For birds that are not in motion, you may instead prefer to use one-shot AF.  In one-shot mode, as you depress the shutter-release button halfway, the camera’s AF engages just as in servo mode, but once the camera achieves a “perfect” focus it stops tracking the bird and leaves the lens’ focusing element stationary.  If the bird is truly stationary, then after the camera acquires its initial lock on the focus, no further re-adjustment should be necessary, and one-shot mode should be sufficient.  In some situations, however, servo can be useful for stationary subjects.  For a bird that is currently stationary but that might take to flight at any moment, it’s best to keep the camera in servo mode if you want to get photos of the bird in flight.



    While the bird remains stationary, servo will continue to re-assess the focus of the bird, and may make small adjustments to the lens’ focusing element, especially if the bird is moving its head or other extremities while remaining perched in one location.  While the bird remains perched you can take shots as desired, while still being ready to track the bird in flight when it eventually leaves its perch.  Unfortunately, some cameras’ AF modules get “jittery” when using servo mode to continuously focus on a stationary subject, so that the bird may go in and out of focus slightly even though it’s stationary.  If your camera is one of these, then you may want to avoid using servo for stationary birds.  You might also consider contacting your camera’s manufacturer to find out if an adjustment to the AF system can be carried out to make it less “jittery”; some users have reported less jitteriness after having the camera serviced by the manufacturer.
    There are a few things to consider when developing your focus technique in the field.  First, if you’re having trouble acquiring initial focus for birds that are moving quite a bit (such as songbirds foraging in a tree), try turning off image stabilization (IS—section 3.5), since IS can slow down the initial focus acquisition.  When shooting foraging warblers, I’ve found that turning off IS can give me a half-second advantage in terms of getting the bird in focus and rapidly snapping a few shots before losing the bird—for some birds, that half second can make the difference between getting the shot and not getting the shot.  Also, when shooting foraging birds, I like to re-aquire focus (in one-shot AF mode) for each shot, just in case the camera gets the focus wrong for one or more of the shots.  This also helps when you’re shooting with a very shallow depth-of-field (DOF) and can’t precisly position the AF sensor on the bird’s eye; by taking lots of photos with each one having been re-focused, you increase the chance of getting a shot with the bird’s eye in perfect focus.  Re-aquiring focus for each shot in one-shot AF mode is easy: simply lift your finger off of the shutter-release button after each shot and press it all the way down for each shot, to both engage the AF and release the shutter in a single stroke.  If your camera’s servo is reliable, you can instead use servo mode to achieve the same effect, though you run the risk of losing focus due to branches and leaves that get in the way.  One-shot AF gives you more control in these situations.
    It’s also a good idea to check your camera’s back LCD to ensure that you’re getting sharp images.  During any pause in shooting, it’s a good idea to review a few recent shots by zooming all the way in on the images and checking for critical sharpness.  If shots that you thought were well focused when you were snapping them look fuzzy on the LCD, this could indicate some hardware malfunction, such as a front- or back-focusing issue in the camera.  If your camera has an AF microadjustment feature (section 2.7.3), you might try tweaking that in the field to correct such problems.  (I sometimes tweak my AF microadjustment for different shooting situations and for birds of different sizes—e.g., if I’m focusing on the bird’s shoulder but want the eye to be in focus, the AF microadjustment can sometimes help by pushing the true focus point back a few inches from the shoulder to the eye).  Note that dirt particles can potentially cause AF problems, so if your camera’s AF starts to behave strangely, you might want to check for excessive dirt on your mirror or on the slide-in clear filter in your telephoto lens (if it has one).