Probably the most important technological advance of all time in the field of nature photography will turn out to have been the development and continual refinement of the autofocus (AF) mechanism for SLR cameras.  Indeed, it could be fairly strongly argued that today’s standards for fine-art bird photography could not be met by even the most adept pro photographer without autofocus, especially for birds in flight (BIF) and other intense wildlife action scenarios.  Action obviously requires quick focusing, since subjects in motion will tend to rapidly move in and out of focus.  The bigger problem, though, is that manually focusing by looking through the viewfinder and turning the lens’ focusing ring by hand is severely limited by the fact that the eye can’t resolve (through a standard camera viewfinder) nearly as much detail as today’s high-resolution imaging sensors.  Human focusers (e.g., you and I) are simply incapable of being as exacting as today’s high-resolution electro-optical instrumentation can be, in real time.  Even if your interest is in capturing portrait shots of static (non-moving) subjects, a high-precision AF system will be able to produce sharper images, on average, than you’ll be able to do by manually focusing the lens by hand (and by eye). 



    Because different camera models can differ quite substantially in their AF capabilities, understanding the basics of the underlying technology can help you to better choose a camera that will deliver more in-focus (and therefore “sharper”) photos of birds.  Our goal in this section is to develop a basic understanding of the different types of autofocus systems in use today by the major players in the DSLR arena, so that you can intelligently choose between them so as to suit the type of bird photography you want to do.  Understanding this material will also be very useful later (in Part II of this book) when we discuss techniques for obtaining the sharpest possible images in the field.

2.6.1 How Autofocus Works

First, we need to understand a few things about how lenses work—in particular, about how they focus an image onto a film plane or a digital sensor.  Since lenses are discussed in much greater detail in the next chapter (Chapter 3), the description that follows will be appropriately brief.


How A Lens Focuses an Image

You probably remember from high-school physics that when a ray of light (i.e., a stream of photons all traveling in a single line) strikes a glass surface, it is refracted, meaning that the ray of light changes its angle slightly.  For the simplest optical lenses, which are shaped in somewhat of an oval (or biconvex, parabolic) shape, this means that light rays originating from the same point but following different paths to the lens will be bent so as to reunite at some particular point on the other side of the lens. 
    In Figure 2.6.2, below, you can see this illustrated by the red lines, which represent light rays originating at some point on the bird’s head, passing through the lens at different locations, and then finally intersecting at some common point in the focal plane on the far side of the lens.  The blue lines in the figure also originate at a common point on the bird’s leg, and also rejoin at a common point in the focal plane, despite having taken different routes through the lens (the “focusing element” shown in the figure).  The property that light rays originating at a single point, but following different paths through an idealized, biconvex lens, will under ideal conditions rejoin at a single point on a common plane some distance beyond the lens is a basic mathematical result deriving from the geometry of lenses.
    If you were to place your head in such a position that the retina of your eyeball was perfectly superposed over the focal plane as defined by the geometry of a given lens (and the position of a given subject imaged by that lens), you would perceive a perfect view of the subject, though the image would be upside-down (this is a minor detail that we’ll ignore from here on, since the electronics inside a camera can easily deal with this issue by swapping all the pixels in the image from top to bottom; we’re also ignoring the natural lensing properties of the human eye for this discussion).  If, however, you were to stand slightly forward or backward from this ideal position, so that your retina did not perfectly coincide with the lens’ focal plane, the image you perceived would be blurry.
    To see why this is the case, note first that the rays of light which converge at the focal plane in Figure 2.6.2 continue on past that plane, diverging from each other and producing a scattered image on an imaging sensor placed some distance behind the focal plane.  This scattering of the rays of light which originated at the same point on the bird distribute the color information from that part of the bird over a wide region of the imaging sensor, rather than focusing it at a single point.  That’s why the resulting image is a blur.  The problem is that the imaging sensor is not positioned at the focal plane.  If it was, the rays of light would properly converge and the image received would be a sharp representation of the subject in focus.





In order to rectify this problem, we’d need to move either the imaging sensor, the focal plane, or the bird.  Given that most birds don’t follow commands very well, the latter option can generally be ruled out.  Likewise, since the imaging sensor in a DSLR is generally mounted in a fixed position within the camera housing, moving the sensor is out of the question too.  The remaining option is to move the focal plane, and this we can easily achieve by moving the focusing element.
    In practice, camera lenses typically contain many optical elements (each of which we would typically call a “lens”, but which we’ll call an “optical element”, to avoid ambiguity).  Some of those optical elements can move, and this is how the lens is able to focus on objects at different distances from the lens.  In the figure above we’ve simplified the scenario by assuming there’s just one focusing element within the lens assembly.  As that focusing element is shifted toward the imaging sensor or away from it, the focal plane also shifts.  It’s important to understand that the focal plane is defined by the position of the subject, relative to the position of the lens.  By appropriately adjusting the focusing element inside the lens, we can shift the focal plane of a particular subject so that it coincides with the imaging sensor, thereby producing a perfectly in-focus image of the bird.  This is, at the grossest level, how autofocusing works: it shifts the focusing element until the image focused on the sensor appears as sharp as possible.


AF Sensor Points

The autofocus modules used in DSLRs don’t assess the sharpness of the entire image when they’re adjusting the focus.  Since the image is made up of literally millions of pixels, accurately assessing image sharpness (i.e., “in-focusness”) for the entire image would require too much work for the tiny computer chips embedded in the camera.  More importantly, since a tiny bird like a warbler typically doesn’t fill the entire viewing frame, we’d like to be able to tell the camera to just make sure that the bird is in focus, even if the surrounding scenery isn’t.  The way that we do this is via autofocus sensor points.
    If you look into the viewfinder of a DSLR camera you should see a set of black squares spanning the viewing field, as illustrated in part A of Figure 2.6.3, below.  These indicate (roughly) the positions of the autofocus sensors.  For any particular camera model, the exact number and arrangement (and sizes) of these AF sensor points can differ to a greater or lesser extent from the one shown in the figure, which is based on the Canon pro-sumer line of DSLRs.  The pro models of both Canon and Nikon have many more AF points, and those points are typically much more accurate at assessing focus than those on the consumer and pro-sumer bodies, as we’ll discuss later.




In most camera bodies, it’s possible to select an individual AF point to use for autofocus, or to utilize all of the points simultaneously.  For bird photography, the former (single AF points) is typically best for stationary birds, while the latter (all AF points in use simultaneously) is typically most useful for birds in flight, though there are exceptions to both of these generalizations.  We’ll discuss strategies for choosing AF points in much greater detail in section 6.6.  For now we’ll limit our discussion to the case in which a single AF point has been chosen to perform the autofocus function.  In that case, the other AF points are disabled, and all focusing information comes from the single AF point that we’ve selected.
    In part B of Figure 2.6.3 (above) we show the overall structure of the camera’s light path.  Recall that during focusing, a portion of the incoming light passes through the semi-transparent main mirror, and is diverted downward to the autofocus circuitry located (typically) at the bottom of the camera.  For whichever square box you’ve selected in your viewfinder, there is a corresponding AF sensor in the autofocus module that will be active during focusing.  Our goal in the sections that follow will be to explain how that AF sensor is able to assess the sharpness (or “in-focusness”) of that part of the scene (hopefully the bird) that the AF square is positioned over.  Once that focus information has been collected by the AF sensor, the rest of the autofocus circuitry then decides what needs to be done in order to bring the subject into better focus, and we’ll discuss below how this happens too. 
    The actual process of adjusting the lens’ focus is a mechanical issue, which we’ll largely ignore for the present.  As we’ll discuss later, there are some important differences in the methods used by various lenses for moving their focusing elements, including methods that rely on in-camera versus in-lens motors, and also methods that rely on gears versus ultrasonic vibrations to move the focuser.  First, we’ll focus (no pun intended) on the AF sensors, and how they differ between DSLRs and point-and-shoot cameras.


Contrast-based Autofocus

In compact, point-and-shoot cameras (including a few DSLR-like models), the method used by the AF circuitry for selecting the best positioning for the focusing element is to search for the most contrasty image.  That is, the camera shifts the focusing element back and forth, observing how the movements of the focusing element affect the image’s contrast, and then (eventually) settles on the focus setting that produces the image with the highest contrast.  This is largely a process of trial and error, and can be quite slow—typically too slow for serious action scenarios.  For this reason, DSLRs use a faster method called phase detection or phase comparison, which we’ll describe shortly.
    To see how contrast can guide the camera in focusing an image, consider the series of images shown in Figure 2.6.4, below.  These images show a portion of a Prairie Warbler’s face, including the bird’s eye and two black stripes below the eye.  The images are blown up so that the individual pixels are apparent.  The leftmost image is in focus, while the images to the right have been progressively blurred in software, to simulate the effect of taking the lens increasingly out of focus. 




Below the images are contrast measurements, which are simply the average intensity differences between all neighboring pixels, expressed as a percentage of the maximum intensity (and then re-scaled by a factor of 10).  The important point is that the contrast measure used here is based on local differences in pixel intensities, where the intensity values reflect only the brightness of a pixel, not the hue (i.e., color).  You can see that, based on brightness alone, the contrast decreases very rapidly as the bird is taken slowly out of focus, so that an in-focus maximum contrast of 81% decreases to only 6% when the bird is taken completely out of focus.  The exposure histograms on the right side of Figure 2.6.4 show that the luminosity (brightness) distribution in the image becomes more “peaked”, or concentrated (i.e., less uniform) as the image is taken out of focus, indicating a loss of “information” (or what computer scientists call entropy).  The horizontal axis of these exposure histograms corresponds to different levels of brightness—from very dark at the left end to very bright at the right end.  The vertical axis represents frequency, so that higher peaks in the graph mean that that particular brightness value occurred more often in the image than other brightness values.
    Note that although the second image is only slightly out of focus, the measured contrast is over 30% less than for the in-focus image.  Local contrast (i.e., differences in intensity between neighboring pixels) can thus be a very sensitive indicator of “in-focusness”, and explains why contrast-based focusing systems, despite being slow, can be very accurate.  The problem with contrast-based systems is that the contrast measurement gives no indication of the direction in which the lens element needs to be moved, nor the amount by which it would need to be moved in that direction in order to achieve perfect focus, so that the focusing circuitry has to adopt a trial-and-error approach to finding the correct focus.  Most DSLRs therefore use a more sophisticated approach, called phase comparison, which we describe next. 


Phase-based Autofocus

The phase-based approach utilizes a neat “trick” arising from the geometry of light paths through a simple lens.  Consider a bird’s eye—typically the most important feature of a bird when adjusting focus.  Ambient light rays can strike the bird’s eye from any direction, and they will reflect off of the bird’s eye in just as many directions.  As shown in Figure 2.6.5, below, those rays coming from the bird’s eye that happen to strike the lens (focusing element) are refracted upon entering and leaving the glass element so as to be focused on a single point inside the camera.  Assuming that we’ve selected a particular AF sensor point and positioned that point over the bird’s eye in the viewfinder, the rays of light coming from the bird’s eye should pass through the opening in the AF point screen (at left in Fig. 2.6.5) corresponding to that selected AF sensor point.




If the focusing element is correctly positioned, the “eye light” (light rays from the bird’s eye) will be focused precisely at that opening in the AF screen, as suggested by the figure.  After passing through that focal point, they will again spread out in a cone.  That cone of “eye light” passes into the AF module shown at left in Figure 2.6.5.
    In Figure 2.6.6, below, we show an expanded view of the AF module.  In part A, in which the bird’s eye is perfectly in focus, the light rays pass through the AF point screen, and are either blocked by a “blocker” element or are captured by one of two “mini-lens” elements (which may be implemented as prisms in some designs).  The blocker element ensures that the only rays of light reaching the minilenses are those which passed through either the uppermost portion of the main lens or the bottommost portion.  Those from the bottommost portion end up reaching the upper minilens and are then focused on the upper AF sensor positioned behind that minilens, and conversely for the rays coming from the uppermost portion of the main lens.




In part A of Figure 2.6.6, we show the case for a subject that is perfectly in-focus.  The split light path forms two identical images, one on the upper AF sensor, and one on the lower AF sensor (both of these “AF sensors” correspond to a single “AF sensor point” as viewed in the viewfinder, not to two different AF sensor points).  But notice how the light rays striking the upper AF sensor are angled upward, while those striking the lower AF sensor are angled downward.  That’s important.
    In part B of Figure 2.6.6, we show the case where the focusing element is slightly mis-positioned, so that the bird is slightly front-focused (i.e., the empty space in front of the bird is in focus, but the bird itself is more-or-less out of focus).  In this case, the light rays striking the upper AF sensor will be shifted upward somewhat from their normal position, while those striking the lower AF sensor will be shifted slightly downward (the blue lines in part B of the figure show where the light rays would be if the focus had been set correctly).  The images formed on the two AF sensors will still be roughly identical, but the distance between the two images, as measured along the AF sensor plane, will be larger than expected for an in-focus image.  This condition of larger-than-expected distance between the two images indicates a front-focusing condition.  Had the bird instead been back-focused, the two images would have been somewhat closer to each other than expected.  Thus, by measuring how much the images are shifted on the paired AF sensors, and in which direction (i.e., closer or further away from each other), the AF circuitry can determine both the direction and the amount by which the focusing element needs to be moved in order to achieve “perfect” focus.
    Now we need to explain in greater detail some things that we glossed over in the preceding discussion.  If you’re satisfied with the explanation given above, you can skip to the next section, titled “Practical Consequences”.  Otherwise, just keep reading.
    The first “fine point” we’ll consider is the issue of how the AF sensor is able to measure the distance between the two images formed on the twin AF sensors for a given AF sensor point.  Obviously, the computer chips inside the camera don’t know that you’re photographing a bird, so they can’t find the bird in the two images and measure the distance between them.  What they can do, fairly rapidly in fact, is to compare the intensity profile along a single row or column of pixels with the corresponding profile from the other AF sensor.  In most DSLRs today, the individual AF sensors are exactly that: a single row or column of pixels oriented horizontally or vertically.  If you imagine the thin image strips we showed earlier in Figure 2.6.4 (above) being thinned down to a single column of pixels, then you’ll have some idea of what the AF sensors actually see, except that that the pixels would be seen in shades of gray since color information is typically not used by AF sensors.
    In Figure 2.6.7 (below) we consider the task of figuring out how much the twin images on the paired AF sensors are shifted toward or away from each other, for the case where the sensors are horizontal rows of pixels.  In part C of the figure you can see that the image is indeed slightly out of focus, because the two images are shifted by about one pixel from each other (a half pixel in each direction), so that properly aligning the two images requires an overhang of one pixel on each end.  Obviously, this comparison is done electronically in the camera; we’ve arranged the pixel arrays side-by-side in the figure just to make it easier for you to compare them visually.




In part A of the figure we show the intensity profiles of the twin AF sensors (marked “upper” and “lower” in the figure) as graphs, where the height of the curve at any point shows the intensity (brightness) registered by that pixel (don’t confuse these intensity profiles with the exposure histograms shown in Figure 2.6.4; in these profiles, the horizontal axis corresponds to pixels in the AF sensor, whereas in the exposure histograms the horizontal axis corresponds to different brightness levels).  The green curve in the middle shows what the profile would look like if the image had been in perfect focus.  As you can see, because the image is out of focus, the upper profile is shifted one way and the lower profile is shifted the other way.  Of course, the green curve isn’t available to the camera, so the camera has to compare the upper and lower profiles directly.  In part B we show the two profiles aligned so that their peaks and valleys line up.  In order to make the peaks and valleys line up, we had to shift the upper profile to the right by some distance.  Once the camera figures out this distance, it can compute the exact amount (and direction) that the focusing element in the lens needs to be shifted in order to bring the image into perfect focus (or near-perfect focus—see below).  Thus, the two curves are considered to be out of phase by some amount, and this amount of phase difference is a measure of how out-of-focus the image is, and in which direction.  Hence the term phase-based autofocus.
    So, all the camera has to do is to find the best alignment of the two profiles in order to deduce how much the images are shifted relative to each other (and in which direction—i.e., whether they’re shifted away from each other or toward each other, corresponding to front-focusing or back-focusing).  This is sometimes referred to as the problem of image registration.  How can the camera find the best alignment between the two profiles?  And how can it do so very, very rapidly?  Precisely how this is implemented in a given camera model may differ, but a straightforward technique is to have an array of parallel circuits each of which computes a correlation score for a particular alignment of the two profiles.  For example, one subcircuit would compute the correlation for the two profiles aligned with a shift of one pixel to the right, another would compute the correlation for a shift of two pixels, another for a shift of three pixels, etc.  Circuits would also be included to handle shifts in the other direction.  These circuits all compute their correlation scores simultaneously, and the circuit producing the highest correlation score dictates the optimal offset for the alignment, and therefore the desired adjustment for the focusing element.  These correlation scores (or a similar score called the coefficient of determination) can be computed almost instantaneously using analog circuitry that combines solid-state elements for multiplication and addition.  In this way, phase-based AF methods are generally unrivaled in their capacity for raw speed.
    Another fine point requiring a brief explanation is the slight difference in the shapes of the profiles shown in part B of Figure 2.6.7.  Not only are the two curves shifted relative to each other, but their peaks and valleys are somewhat deformed relative to each other.  In fact, if you compare these two profiles to the “correct” profile for an in-focus image (the green curve in part A of the figure) you’ll see that the peaks and valleys have been somewhat eroded and skewed.  This is because the twin images for these profiles have each become somewhat out-of-focus (remember, the main image itself is front-focused in this example), but since the paired AF sensors receive light rays from different parts of the main lens (as we showed previously, in Figure 2.6.6), the image data will be “smeared” in opposite directions in the two images.  For this reason, the twin images won’t look exactly the same, and neither will their intensity profiles, which is why the correlation score is needed to find the best alignment between the profiles, since there generally won’t be a “perfect” alignment between them.  Note, however, that the degree of distortion in the twin images due to being out-of-focus can be mitigated by using a relatively small aperture in the AF minilenses.  As we’ll eventually discuss in section 6.1, using a small aperture results in greater depth-of-field (DOF) which in turn allows slightly out-of-focus images to appear more in-focus.
    Other assumptions of the phase-based system that we’ve largely glossed over up till this point are that the subject is parallel to the sensor plane (so that the portion visible through the AF sensor point is either completely in focus or completely out of focus) and that there are no intruding elements (such as twigs or branches) passing in front of the subject.  In practice, these assumptions can often be violated to varying degrees without catastrophic results, but in order to obtain the most accurate performance, especially in poorly-lit scenes, it’s good to try to keep these considerations in mind.  Whenever possible, it’s best to avoid focusing on a part of the bird that isn’t oriented as a flat surface parallel to the sensor plane, or to focus through intervening foliage.  Even if you think there are enough “windows” through the foliage to get a good exposure of the subject (i.e., with intervening foliage being rendered out-of-focus and nearly invisible in the final image), the greater depth-of-field of the AF sensors’ mini-lenses can result in the autofocus module being more confused by the intruding elements than you might otherwise expect, based on your view through the viewfinder.
    Note that while the phase-based AF technique is in theory capable of moving the focusing element directly to the precise location needed to achieve perfect focus, without the need for any “searching” as done in the contrast-based method, in practice some fine-tuning of the focus can be necessary even with phase-based AF, due to slight measurement errors in the distance calculation.  Another source of error in the phase-based system is imperfect factory calibration.  Because phase-based systems are much more complicated than contrast-based systems, they require precise calibration in order to work correctly.  Both Canon and Nikon are known for selling cameras that are not always perfectly calibrated at the factory and need to be sent in to the manufacturer’s service center for re-calibration.  Even the pro models from these companies are not immune to this: a recent “voluntary recall” by Canon of its $4000 pro body (the 1D Mark III) was carried out due to faulty calibration of some of the outer focus points in select units of that model.  This is one of the hidden costs of the increased complexity of these systems.
    It should be noted that while DSLRs generally use phase-based autofocus instead of the simpler contrast-based method, there are a few exceptions.  The first exception occurs in the context of Live View.  Recall from section 2.1.2 that Live View is a mode of operation which is increasingly becoming available on DSLRs, in which the shutter is left open and the image formed on the main image sensor is shown in near-real-time on the camera’s back LCD screen.  The first generation of live view capable DSLRs required manual focusing when using live view, but newer models now allow the user to invoke a contrast-based autofocus function when in live view mode.  The reason phase-based autofocus can’t be used during live view is that the light path can’t be split during exposure so as to direct some of the light to a dedicated AF module with paired AF sensors as required for the phase-comparison method. The other notable exception is the new crop of “compact” DSLRs and DSLR-like cameras being offered by various second-tier and third-tier companies which utilize contrast-based autofocus.  These cameras, unfortunately, mostly use the so-called four-thirds imaging form factor, which involves a 2x crop factor and therefore limits photosite sizes and places limits on the noise characteristics of the sensor.



Practical Consequences

    Now let’s consider how knowing any of this can help you to either choose a better camera (if you’re in the market for a new one) or to operate your existing camera better.  First, regardless of whether a camera uses contrast-based or phase-based AF technology, the subject you’re trying to focus on needs to have some contrast in order for the camera to properly focus on it.  This is obviously true in the case of contrast-based AF methods.  In the case of phase-based methods, the intensity profile as seen by the twin AF sensors (i.e., part B in Figure 2.6.7) has to have some peaks and valleys in it, or the correlation score will be useless for aligning the two profiles.
    Keep in mind that the autofocus procedure is applied only to the portion of the image covered by the selected AF sensor point(s).  If the bird is either large or very close and the AF sensor point is positioned over a part of the bird that is of a uniform color and has no visible contrast, then both methods described above will usually fail.  Remember also that the actual AF sensors are, in most DSLRs, just a single strip of pixels oriented either horizontally or vertically.  If your subject (or the part of your subject you’re trying to autofocus on) has visible features that are parallel to the AF sensor’s pixel array, rather than perpendicular to it, then autofocus will again tend to fail.  For example, if the AF sensor point you’ve selected is a horizontal-type sensor and you’re trying to focus on a part of the bird that has horizontal bars and nothing else, then the intensity profile seen by that AF sensor point won’t have any strong peaks or valleys (no contrast), and the phase-comparison method will fail because the twin profiles have no strong features that can be aligned.
    Many pro and pro-sumer cameras, and now even some consumer-grade cameras, feature one or more cross-type AF sensors, in which the sensor has both a vertical array of pixels and a horizontal one, so that the problem just described shouldn’t happen as often.  Unfortunately, these cross-type sensors are typically limited (except in pro bodies) to just the central AF point, with all other AF points being strictly horizontal, or strictly vertical.  Furthermore, in many cameras having one or more cross-type sensors, these sensors typically revert to working as strictly horizontal or strictly vertical whenever the main lens has a maximum aperture smaller than f/2.8 or so, meaning that most consumer-grade lenses force the cross-type AF points to lose their cross-type functionality.  One way around the horizontal/vertical AF sensor problem is to rotate your camera 90 degrees whenever focusing a subject with strong horizontal or vertical bars/stripes.  It’s therefore good to know which types of AF sensors (horizontal or vertical) are in your camera, and/or to know which sensors are of which kind (if your camera has both), in case you find a strongly striped or barred bird and have difficulty getting the camera’s autofocus to lock on to the bird in either portrait or landscape orientation. 
    Finally, the above discussion highlights two reasons why you should always try to choose lenses with the largest aperture (smallest f-number).  First, a larger aperture will obviously let in more light, and any contrast that is present in the subject will be better captured and more effectively utilized by the AF module when more light is collected.  Note that what we’re talking about here is the maximum aperture of the lens, not the aperture used for actually taking the photograph.  An f/2.8 lens can of course be “stopped down” to f/11 in order to produce an image with greater depth-of-field, but the actual stopping-down to f/11 doesn’t happen until the mirror flips up and the shutter opens (stopping-down is discussed in detail in Chapter 6).  During autofocus, the aperture is always kept wide open, so what matters is the maximum aperture that the lens is capable of opening up to, not the aperture setting that you dial in on your camera for a particular exposure.
    Second, for the phase-based AF method, recall that the twin AF sensors in each AF sensor point collect light from opposite sides (top and bottom, or left and right) of the main lens, and they utilize the difference in angles between these two sets of light rays to deduce the correct AF adjustment.  For lenses with a maximum aperture of f/5.6 or f/8 or f/11, the rays of light coming from opposite sites of the lens aren’t forming such wide angles as they would if you were using an f/2.8 or f/4 lens.  This means that the shifting of the intensity profiles from the twin AF sensors as described above and illustrated in Figure 2.6.7 will be reduced to smaller distances that will be harder for the camera to detect and precisely measure.  For very small (maximum) apertures like f/11 or f/16, the shifting of the intensity profiles can be so small that the camera may not detect it at all.  In fact, most consumer-grade and pro-sumer cameras won’t perform autofocus at all for lenses having a maximum aperture of less than f/5.6 (which is one reason why the use of teleconverters on many lenses causes autofocus to be disabled).  Pro cameras can typically perform autofocus with lenses having a maximum aperture of f/8, but not with lenses having smaller maximum apertures like f/11 or f/16.  Even if your camera does support autofocus at f/5.6 or f/8, in poorly lit conditions the combination of low light levels and reduced profile shifting distances due to shallow light angles can severely affect the accuracy of the autofocus.
    Remember that in phase-based systems, the total amount of light passing through the main lens is severely reduced before it reaches the AF sensors: it’s first reduced by the tiny “window” in the AF screen corresponding to the selected AF point, the portions coming from the middle part of the main lens are blocked out, and then the remaining light is split (i.e., halved) between the upper and lower (or left and right) AF sensors.  This is why autofocus in low-light conditions is very difficult for DSLRs, and why using a lens with a large maximum aperture can very profoundly affect the ability of the camera to accurately perform autofocus.


Mechanical Considerations

As we mentioned earlier, there are a few mechanical issues related to the implementation of autofocus in different cameras and lenses that may be of interest to the aspiring bird photographer.  The first is whether the motor that moves the focusing element is located within the camera or in the lens.  Most lenses today have the motor located inside the lens assembly, but some older lenses (such as Nikon’s 80-400mm VR zoom lens) utilize a motor located inside the camera.  In the latter case, the camera’s AF motor articulates (connects) to the focusing element in the lens via a long rod that spins like a motorized screwdriver, clockwise to focus out and counterclockwise to focus in.  The motor-in-the-body solution has been largely abandoned by the leading manufacturers because having a dedicated motor embedded within each lens, while seemingly redundant, does allow for much faster and perhaps more accurate control of the focusing element. 
    Another technological innovation is the use of ultrasonic motors (USMs) for focusing, which utilize vibrations to turn the motor drive.  Though Canon pioneered this approach and uses it in all of its large telephoto lenses, other manufacturers have followed suit with their own versions of the technology, under a variety of different names—i.e., hypersonic motor (HSM) by Sigma, silent wave motor (SWM) by Nikon, etc.  The advantages of USM-type motors are the near silent operation of the motor (which can be important when working in a bird-blind or similar close-quarters situation, where birds may be scared away by mechanical noises), and sometimes a faster and possibly more accurate control of the focusing element, which can obviously be important as well.

2.6.2 One-shot Versus Servo

Most DSLRs offer two main AF modes: one-shot, and servo (or continuous AF).  The former, one-shot, is for static subjects, while continuous AF (servo) is for shots of subjects that are in motion.  With one-shot, the camera attempts to achieve focus of the subject and then beeps after focus has been achieved.  At that point, the AF module basically goes to sleep; if the subject suddenly moves, the camera will not re-focus the lens so as to re-establish focus.  If the subject moves out of the focus plane after one-shot has finished focusing, you need to re-initiate focus by lifting your finger from the shutter-release button and pushing it back down (halfway) so that one-shot AF starts up again and then beeps to confirm that it has regained focus.  In servo mode, the AF module never goes to sleep, as long as you have your finger on the shutter-release button (i.e., holding it down halfway).  As soon as the bird moves, the AF module will detect that the image is no longer in focus and will take action to restore focus.  For birds in flight, servo is essential for keeping the bird continuously in focus throughout its trajectory. 



    Note that servo can also be used for stationary subjects, and one-shot can sometimes be used for subjects that are not completely still. When photographing warblers I often use one-shot instead of servo, even though these tiny birds are almost always moving.  As long as the bird stays perched in one location, one-shot can be used to focus on the bird because its movements (e.g., eating, stretching the wings, etc.) are not taking the whole bird in and out of the focus plane.  Conversely, when photographing a bird that is currently stationary but that might suddenly take flight, I’ll often use servo, since servo can (usually) work just fine on stationary subjects, and will obviate the need for me to quickly switch focus modes when the bird suddenly leaps into the air.  Just note that on some bodies, the servo mode might be “jumpier” than on others, so that for a stationary subject the AF module might constantly invoke tiny changes in focus as it searches for a better focus of the stationary bird.  Some units of Canon’s current pro body, the 1D Mark III, exhibited this type of “jumpy” behavior when using servo mode for stationary subjects, until Canon corrected the behavior via a firmware update (i.e., an update to the software running on the tiny computer inside the camera that controls various camera functions such as autofocus).
    The accuracy of servo mode can differ substantially between camera models.  Whereas some models feature a “predictive” servo, which actively tries to predict the trajectory of the subject in order to more closely track it during focusing, others are more “reactive” in that they wait to re-adjust the focus until they detect that the subject has already moved.  Reactive servo can suffer from poor tracking, because the AF module is always “playing catch-up”.  Predictive servo tries to act more intelligently by incorporating an additional correction factor into its focus changes, so as to account for the subject’s velocity relative to the camera.  For birds flying erratically, however, this predictive approach can fail because the camera overcompensates and behaves too confidently in its prediction of the bird’s next location.  The best way to assess the effectiveness of these various technologies is to test them out in the field yourself, or to wait until a camera is professionally reviewed by an online magazine or product review site that will test the camera on moving objects.  Unfortunately, many of these review sites test their cameras on either athletes or moving vehicles, rather than birds.

2.6.3 Customization and Special Settings

One important difference between various camera bodies is the degree to which the AF system’s behavior can be customized to suit your shooting style.  For most consumer-grade and pro-sumer bodies, little or no customization is possible.  In contrast, pro bodies typically offer many options that can be set via the camera’s “custom functions” menu.  So many options are available on some pro bodies, that many users—even pro users—don’t know what all of the options do.  We’ll briefly review some of the options that are typically offered on current pro bodies, so that you’ll know what to look for when reading up on a prospective model’s features.
    An important option for photographing birds in flight is the ability to adjust the camera’s sensitivity to large changes in focus during servo tracking.  This comes into play when you’re tracking a bird in flight and you momentarily allow the AF sensor point to slip off of the subject in the viewfinder.  This happens a lot when tracking erratically flying birds, especially when you’re hand-holding a lens (i.e., not using a tripod), since smoothly tracking a bird in your viewfinder can be quite difficult for fast-moving subjects, or when the subject quickly changes direction.  Turning down the tracking sensitivity gives you more time to re-acquire the bird in your viewfinder, because the camera pauses for a longer interval when it detects a large change in focus.  This can also be useful when tracking a bird that may fly behind a tree branch or telephone pole; with a low enough sensitivity setting, the camera won’t try to focus on the branch or pole, but will instead wait for the bird to come back into view.


    Another important option for birds in flight (BIF) is to allow the camera to switch to nearby AF points when the subject has drifted off of the selected AF point.  This is sometimes referred to as AF point expansion.  In the case of simple expansion, the camera may allow you to restrict expansion to any immediately neighboring AF point, or to just those AF points above or below the currently selected one, or perhaps to the ones left or right of the current one.  If instead of having a single AF point selected you enable all AF points, the camera may (depending on model) allow the subject to drift to any AF point and keep tracking the subject there.  In the pro bodies you may also be able to fine-tune this behavior by specifying, for example, what to do if two AF points register subjects (or parts of the same subject) at different distances from the camera (some cameras can indeed compute the approximate distance to a subject, based on the setting of the focusing element). 





    Yet another option on some bodies is whether the AF system should keep searching for a subject by racking the focusing element in and out, when a subject hasn’t yet been successfully focused, or whether the AF system should give up after one search over the full range of the focusing element.  Though phase-based AF systems are described as not requiring a search-based approach to achieving focus (unlike contrast-based systems), that’s true only if all of the assumptions are satisfied—i.e., the subject is parallel to the focus plane and fills the whole AF sensor point, there are no extraneous intruding elements in front of the subject, etc.  Also, if the subject is extremely out of focus when you first engage the autofocus, a search is often necessary to find the “neighborhood” of the correct focus, after which a direct focus adjustment based on the phase-comparison method described above may be sufficient to jump directly to the correct focus point.
    Perhaps the most useful feature to recently emerge in AF systems is the ability of users to calibrate the focus system via the so-called AF microadjust setting.  As we remarked earlier, phase-based AF systems are complex devices that require precise calibration at the factory.  Many cameras, especially consumer-grade models, are only roughly calibrated at the factory, with fairly liberal “tolerances” in component settings.  For example, I’ve read that consumer-grade models are typically calibrated with a tolerance of +/- one DOF (depth-of-field), whereas pro bodies are generally calibrated with a much more exacting tolerance of +/- one-third DOF or less.  The AF “microadjust” feature (for those cameras that offer it) allows you to calibrate the focusing system yourself, by dialing in a number typically ranging from -20 to +20.  This number is added in to the camera’s focus computations so as to bias the setting of the focusing element away from where the camera would normally set it.  This is useful if you find that your camera is consistently front-focusing or back-focusing, because you can compensate by consistently adjusting the focus one way or the other (backward or frontward), automatically.  On some camera models you can set a different microadjust value for different lenses, to account for focusing biases that are due to defects in the lens’ calibration rather than the camera’s.  Finding the optimal microadjust setting for your camera is discussed in section 3.11.

References

Kerr, DA (2005) Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras.  URL: http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf