6.7 Selecting AF Points

As we described in section 2.6, the standard autofocus mechanism used by DSLRs (except when in live view mode) is based on one or more tiny strips of pixels called autofocus sensors, or AF points.  In many cameras, you can see a representation of the AF points by looking into your viewfinder: they’ll appear as tiny squares or diamonds, usually arranged into a regular pattern that spans much of the visual field.  In some cameras they’re only visible when the autofocus is actually engaged, and may change from black to red when the camera believes it has achieved perfect focus.  Note that these squares or diamonds that you see in the viewfinder only represent the general locations of the individual AF sensors; in general, the actual AF sensor may be larger or smaller than the glyph seen through the viewfinder.
    There are a number of factors that need to be considered when deciding which AF point(s) to use in a given situation.  Obviously, you’ll always want to make sure that the selected AF point(s) are positioned on the bird in the viewfinder, rather than on other features of the scene such as branches or leaves, or even other birds besides the one you’re concentrating on capturing most prominently in the image.
    Perhaps the simplest strategy to begin with is to use only the center AF point, and to always keep the bird positioned in the center of the frame.  When autofocus is engaged (by pressing the shutter-release button halfway), the camera will focus on the bird, since the center AF point should be positioned on it.  If you’re close enough (or using a lens with enough magnification), the bird may be large enough in frame that you can position the center AF point on just the bird’s face, or just on its eye.  This is typically what you want, since bird photos almost always look best when the bird’s eye is perfectly in focus, even if some other parts of the bird are out of focus.



    If the bird is very large in the frame, then you may find that positioning the center AF point on the bird’s eye results in some important part of the bird’s body being clipped (i.e., not visible in the viewfinder—keeping in mind that your viewfinder might only show you 95% or so of the actual frame seen by the camera’s imaging sensor).  There are two solutions to this problem (other than using less magnification).  The first is to simply use another AF point.  If your camera has lots of AF points, you can simply tell the camera to use the one closest to the bird’s eye (precisely how to select AF points differs from camera to camera, but usually involves pressing a button and then turning a dial until the AF point that you want is highlighted in red).  If the bird is stationary and you’re not worried about getting rapid follow-up shots as the bird takes to flight, then selecting a non-center AF point closest to the bird’s eye is generally ideal.  If you think the bird may soon take to flight, and you're hoping to track it during its flight (using servo AF mode), then there may be some risk involved in using a single, non-central AF point, since that point might not be ideally positioned for AF tracking of the bird in flight.



    The other option for focusing on a part of the bird that is not positioned in the center of the viewfinder is to use the focus-and-recompose method.  In this method, you first position the central AF point on the bird’s eye, press the shutter release halfway until the camera achieves focus (usually indicating the fact via an audible beep, or by flashing the AF point in the viewfinder), and then recompose—that is, adjust the camera’s angle so the bird is now positioned however you want it within the frame.  As long as you keep the shutter release pressed halfway (and assuming you’re in one-shot AF mode), the bird’s eye should stay more-or-less in focus even though the enabled AF point is no longer positioned on the eye.  When you're ready you can press the shutter-release the rest of the way and take the shot. 
    The focus-and-recompose method is very popular, and can be extremely effective.  It’s important to be aware of a potential problem with this method, however.  The problem is best understood by first viewing the situation geometrically, as in Figure 6.7.3, below:



In the spirit of theoretical physics and its much-celebrated “perfectly spherical cow”, let us assume a perfectly flat bird, occupying a single plane, as depicted in part A of the figure above.  As shown in part B, after recomposing, the plane occupied by the bird no longer coincides with (though it does intersect) the plane the camera is focused on.  As a result, the bird may appear slightly out of focus in the resulting image.  Exactly how much out of focus depends on the depth-of-field (DOF).  Recall from section 3.13 that DOF can vary, based on aperture and distance (as well as pixel size—but we’ll ignore this fact, since for most DLSRs the pixel size is effectively fixed).  Thus, in the figure above we should in practice replace the thin lines (representing planes) with thick, fuzzy lines (representing plane-like volumes).  If the thickness of the resulting plane-like volumes is great enough, and the angle involved is small enough, the act of recomposing may not result in any discernible loss of sharpness in the final image.  And this is indeed what is often seen in many cases, which is why focus-and-recompose continues to be a popular method.  Just be aware that focus-and-recompose has at least the potential to result in out-of-focus images, especially at wide apertures and short distances, and check for this regularly when shooting in the field.  Whenever I use this method, I always zoom in on the bird’s eye on my camera’s LCD screen (after taking the shot), to make sure the eye looks sharp.
    One other issue to be aware of when using focus-and-recompose, if you’re not using manual exposure, is that the method can sometimes result in exposure problems.  On some cameras (and for some metering modes), the camera’s metering function is linked to the selected AF point.  If after acquiring focus you then recompose, with the AF point now positioned elsewhere in the scene (i.e., no longer on the bird), the measured light level read by the camera’s meter may represent the luminance of the background more than that of the bird.  As a result, the bird itself may end up being over- or under-exposed.  There are a number of solutions to this problem.  One is to simply use manual exposure and find the proper exposure for the bird; then you can recompose all you want and shouldn’t have to worry about confusing the camera’s meter, since you’re not using it.  If you prefer to use spot metering, then the best solution would be to use your camera’s auto-exposure lock, or AEL, feature.  When metering is linked to the selected AF point, you can use AEL to force the camera to take its meter reading at the same time that it performs autofocus (or when you press the AEL button)—i.e., when the AF point is positioned on the bird, before recomposing.  On some cameras this can be a bit tricky, because you may have to press and hold an additional button (AEL) while still holding down the shutter release halfway, all the while concentrating on the most aesthetically pleasing angle for recomposition.  Another option is to use a different metering mode, such as evaluative metering, which should be less affected by recomposition (on average).
    Instead of using just one AF point, on most DSLRs it’s possible to enable all points, or just a subset of all available points.  The subsets that can be selected are often arranged in ring patterns, so that you can, for example, select just the centermost cluster of points, or just a ring of points around this central cluster, or perhaps a ring of points more toward the periphery of the frame.  When multiple AF points are selected, the camera assesses the initial focus of the scene through all the selected AF sensors, and then makes focusing decisions based on the information from those selected sensors.  Precisely what strategy is used for achieving focus based on the set of active AF sensors differs between cameras.  Depending on the camera model, the AF system may try to achieve perfect or near-perfect focus for the largest possible subset of selected AF sensors, or it may try to ascertain which AF point is positioned over the closest part of the scene and then perform focusing based on just that AF sensor.  Regardless which strategy your particular camera uses, it’s important to note which AF sensors (among those that you’ve enabled) the camera has chosen to actually use—these will typically be indicated via red flashing inside the viewfinder.  If, for example, you enable all AF points, you may find that the camera chooses a subset of points corresponding to the branch that the bird is perched on, rather than the bird itself.  In that case you’d obviously want to try enabling a different subset of AF points, if possible, or to switch to single-point focusing.  I typically don’t use more than one AF point unless I’m trying to track birds in flight, and even then I often use just one AF point.



    Note that on many high-end cameras there are “invisible” or “helper” AF sensors that you may not see in the viewfinder, or may not be able to explicitly select or enable/disable, but that can influence the focusing strategy of the camera.  These can typically be globally enabled or disabled via a “custom fuction” setting deep within the camera’s menu system.  As noted in section 2.6, features such as “AF point expansion” that intelligently modify your selected set of AF points can sometimes help to keep a flying bird in focus, especially if you’re having trouble keeping the AF sensor(s) positioned on the bird in the viewfinder during its flight.  However, these features can sometimes be more of a hindrance than a help, since they may, for example, result in the camera moving the selected AF point from the bird you’re following to another bird in the frame.  These issues are very camera-specific, and will generally require some research and experimentation in order to become adequately familiar with how your camera behaves under different custom function (CFn) settings.  Some camera companies publish “white papers” giving advice for how to optimally set the various CFn’s on a given model, based on the shooting situation.  
    On most cameras there are situations in which you’ll be limited to using only the center AF point, such as when using teleconverters or a lens with a maximum aperture of f/5.6 or f/8.  I don’t find this to be a severe limitation, since I typically use only the center AF point, except when tracking erratically-flying birds, in which case I may switch to enabling an appropriate group of points (such as a ring-shaped array), with or without automatic point expansion. 



    Note that for birds that don’t fill the entire frame, an alternative to focus-and-recompose is to simply keep the bird positioned in the center of the frame during shooting, and then to later crop the image on the computer.  While cropping you can choose a crop window that places the bird in the most pleasing position in the resulting frame.  Even though I only have a 10 megapixel camera, I crop most of my photos to 50% or less of the original size, and typically do my recomposing (i.e., positioning the bird outside the center of the resulting frame) then rather than while actually shooting.