Autofocus reloaded. AF-S, AF-C, AF-A, AF-F. 11, 39 and 51 point AF systems. Dynamic area AF, focus areas and 3D

It's been more than five years since I wrote this article (in Spanish) on autofocus and although in many areas the innovation of DSLR’s has slowed down since, autofocus systems are still making unimagined jumps.
I don’t know if this is because we have lost our sense of awe about innovation or because the big camera makers keep devoting considerable resources to AF, but the advances in this area are probably among the most significant in modern DSLR camera development.
If only five years ago we were talking about a second-generation system (Multi-CAM 1000), the latest Nikon models already use fourth generation systems that are considerably faster, more versatile and accurate.

In this article we review the 51-point system, which was introduced in late 2007 with the Nikon D3, D300 and the evolved version of which is used in the D7100, D750, D810 and D4S (Advanced Multi-CAM 3500).
Cameras such as the D5300 and D610, among others, come with the Multi-CAM 4800 39-point system.
Apart from having 12 fewer focus points and the fact that its implementation is slightly less sophisticated in the D5300 –where all modes must be set in the menu– , the two systems are virtually identical in technical terms.
Although the step-in models are still using Multi-CAM 1000, this system has also evolved in comparison with 5 years ago.
Today, its implementation and management is very similar to the more expensive cameras, even though without the convenience of being able to control the AF modes via the command dials.
Still, for the so-called “Baby Nikons” the jump from the 3 focus points available on the D40 to the 11 points in the latest generations can also be considered as a tremendous improvement in operability.

Note that all references to menus are from the D7100 manual. That said, the terms used are universal for all Nikons since the D3, D300, D3000, D5000 and D7000.

Selection and management of AF modes

The first three generations of Nikon digital cameras, from the D1 and D100 to the D3X and D300S, have a focus-mode selector on the front of the body to set the focus mode to either AF-S, AF-C or MF and from the D2, D200 onward a selector on the camera back to designate so-called dynamic area modes (picture below).

This changes with the D90, which employs a switch with only two positions, AF and M, but incorporates an new central button that works in combination with the command dials and has been implemented in all new models since then, including the flag-ships.
This change allowed the removal of the dedicated selector for the dynamic area modes, which are now assigned to the sub-command dial, while the AF modes to the main (D7100 defaults).
Although on the D90 this freed-up space is designated to the focus selector lock switch, on next generation cameras a new one, dedicated to live view, replaces it.

Thus, all recent Nikon DSLR’s feature operationally very similar autofocus systems, with 11, 39 and 51 points, respectively, offering even in its simplest version (AF11) four AF modes (+ M) and four area modes: Single point, Dynamic area, 3D and Auto.
More sophisticated models –with AF39 or AF51 systems– furthermore feature different dynamic area modes that use 9, 21 and 39 or 51 combined focus points, respectively.
There are another additional four focus modes available in live view, of which the most useful is so-called Face-priority AF, which can detect up to 35 faces simultaneously.

Phase Detection vs. Contrast Detection AF

All modern Nikon DSLR cameras use TTL (through the lens) for both exposure metering and autofocus, while cameras that have live view and video incorporate two different AF systems.
For viewfinder photography the camera employs so-called phase detection, while for live view contrast detection.
The reason is simple: the primary and secondary mirrors are required to redirect part of incoming light to the AF sensor, and in live view both are elevated out of the light path.

Phase detection is achieved by dividing the incoming light into image pairs and comparing them.
The system measures the distance to the subject using a beam splitter together with a small secondary mirror (2), which redirects part of the incoming light toward the AF sensor (5), located at the bottom of the camera.
Two micro-lenses capture the light rays originating from opposite sides of the lens, creating a simple rangefinder, similar to the split-screen system used in old film cameras.
These two images are then analyzed to find similar light intensity patterns (peaks and valleys) and the separation error between them is calculated to determine if focus is in front or behind the subject, as illustrated in the image below. Source: Wikipedia

This, in turn, generates the instructions for the direction of the required adjustment (forwards or backwards) and an estimate of the amount of movement required in the focusing system.
These operations are performed in micro-seconds and are repeated until correct focus is acquired.
When the camera is set to AF-C, this process is permanently repeated in a so-called closed loop, which adjusts the focus continuously to match subject movement, until the camera fires or the user stops half-pressing the shutter release button (or AE-L/AF-L).
If the system does not achieve proper focus within a determined amount of time, a problem called hunting occurs, where the system repeatedly shifts focus back and forth, while unable to successfully acquire focus-lock.

Unlike phase detection, contrast detection does not utilize distance measurement to the subject, but is achieved by calculating the measure of contrast within a determined area of the sensor.
Because the difference in intensity between adjacent pixels increases or decreases naturally with the varying sharpness of the image, the optical system repeatedly adjusts to determine maximum contrast.
Despite being generally adequate to achieve proper focus, contrast detection presents important challenges while tracking moving subjects, since the loss of contrast gives no indication about the direction of the motion, i.e., whether the subject is moving toward or away from the camera.
Even though contrast detection systems have improved substantially in recent years due to the rise of the mirror-less camera, phase detection is still considered to be superior to contrast detection, both in accuracy and speed.
For more information, I suggest you review this article on Wikipedia.

Focus Points

With focus points we refer to the squares, or rather, sensors, that are available in the viewfinder, wether they are 11, 39 or 51.
In more sophisticated cameras the user can choose to activate all focus points (AF39, AF51) or alternatively 11 (AF11) through custom menu (CSM) a:6.
AF51 provides very accurate focus point selection which, in practice, virtually eliminates the need to recompose, while AF11 allows to quickly navigate between the available focus points, without losing the dynamic area options.
This is particularly useful when the main subject dominates the frame, when it is near the thirds intersections of the frame or when assigning an exact focus point is less important than speed.

In all modes the focus point is assigned with the multi-selector, which in Single point AF mode (S) means moving the focus point to any position among 51, while in the dynamic modes the central focus point moves in conjunction with the associated 9 or 21.
Furthermore, pressing the center (OK) button on the multi-selector can return the focus point to the center position, if you choose this setting in CSM f:1.
In the custom menu you can also configure how the focus point moves past the edge of the grid, where wrap-around means jumping from the right side to the left, from top to bottom and vice versa (CSM a:5).
Finally, it is important to note that the focus points can also influence exposure metering, because in spot mode measurements are made in a circle of ± 3.5 mm around the selected focus point.

Cross-type and linear sensors, maximum aperture and crop modes

Apart from the considerable increase in the number of available focus points in the viewfinder, the other significant development in recent years has been the incorporation of an increasing number of cross-type sensors, which – unlike standard vertical linear sensors – measure contrast at the focus point in both the horizontal and vertical plane, resulting in increased accuracy.  The 39-point system counts with 9 sensors of this type in the central area of the viewfinder, while the 51-point system with 15 (blue).
Moreover, whereas until recently AF systems were limited to work effectively with lenses with a maximum aperture of at least f/5.6, the latest top cameras have at the center of their focus point formation from one (D7100) to eleven points (D810, D4S) that allow achieving accurate autofocus and electronic range-finding with lenses or sets of optical elements (lens plus tele-converter, extension ring, etc.) with a combined f/ up to 8 (red).
In practice, this means that the camera, even with, for example, the 70-200mm f/4G plus the TC-20E III tele-converter – with a combined f/8 – ought to focus smoothly when using the f/8 sensor(s).
Finally, the 1.3x crop mode available on the D7100 and the 1.5x DX-crop available on the D810 and D4S gives us viewfinder frames that are almost entirely covered with focus points, an improvement which, together with the previously mentioned, accounts for the significant advancements that AF has made over the past half decade.

Focus Modes, settings

The different focus modes can be selected by pressing the center button on the AF mode selector in combination with the main command dial, while the dynamic area modes through the sub dial (D7100 defaults).
In the D3K and D5K series cameras these modes are only available through the menus and exclusively in PSAM.
AF is activated when half pressing the shutter release and/or pressing the AE-L/AF-L button, which locks the registered auto-exposure and focusing distance, depending on the camera settings and the focus mode selected.
Independent of focus mode, the camera always shows correct focus acquisition, illuminating the >●< point in the lower left corner of the viewfinder (4).
You can configure most of the Nikon DSLR’s also to beep to confirm correct focus acquisition, even though only in AF-S mode (CSM d:1).

Release Priority vs. Focus priority 

How the camera behaves when the AF system acquires focus or not, can be set in CSM a:1 and a:2. Both in AF-S and AF-C there are two options: focus priority and release priority.
While in AF-S mode it does not make a whole lot of sense to have the camera fire when the subject is not in focus, in AF-C it depends entirely on the situation.
When shooting spray-and-pay bursts it can be frustrating that the camera refuses to fire, because under certain conditions some shots may be in-focus while others not, as happened to me on more than one occasion when photographing groups of birds in flight, for example.
Although in most cases the camera ought to be set to focus priority, in particular cases it may be more productive to choose release priority, which is, in any case, the default setting of the D7100 for AF-C mode.

Subject tracking, predictive focus and lock-on

When in AF-C mode the subject moves from its initially acquired position, the camera automatically activates predictive focus tracking, which aims to predict its future position.
When the subject follows a more or less regular trajectory, predictive tracking can be quite effective, however, if the subject – like a hummingbird, for example – is moving less predictable or erratically, it may be necessary to activate an area mode wider than single point, such as d9, d21 or even d51, 3D or Auto, discussed later on.
While we are tracking or panning along with one or more subjects against a neutral background this might be sufficient, however, if there are interruptions in the fore- or background, such as trees or buildings, it may be necessary to modify the amount of time that AF waits before reacquiring focus.
When something temporarily blocks the view of the subject, when the distance to the subject changes abruptly or when there are elements in the background with sufficient contrast, AF typically tends to either refocus on the wrong subject and/or completely lose focus, which can be avoided by modifying lock-on time: long (5), normal (3) or short (1), depending on the subject (CSM a:3).
For nature photography you should generally leave this option between normal and long, to prevent that the camera unexpectedly refocuses on something that goes through the frame, however, when it comes to sports photography it may be more appropriate to leave lock-on between normal and short, to allow immediate refocus on wherever the action is.

AF Modes

AF-S: single-servo

In AF-S, the camera maintains the focus unchanged once acquired, and while the shutter button is half-pressed (or AE-L/AF-L), regardless of whether the subject or camera change position or distance. Preferred for stationary subjects, because it consumes less battery. Dynamic area modes available: S and Auto.
Note: do not confuse AF-S autofocus with the AF-S tag used on certain Nikkor lenses. The “S” on a lens indicates that it incorporates an ultrasonic focusing motor or SWM.

AF-C: continuous-servo

In AF-C the camera permanently adjusts the focus, once acquired, according to the distance information provided by type D and G lenses and the selected focus point. Useful for moving subjects, even if they move only a little. Dynamic area modes available: S, d9, d21, d51 (d39), 3D and Auto.

AF-A: auto-servo

The camera decides what is the most appropriate focus mode for the scene. Chooses between AF-S and AF-C. AF-A offers the same dynamic area modes as AF-C.
AF-A is the default setting for the majority of the so-called scene- or auto modes, and cannot be modified in most, depending on the model of the camera and the selected auto-mode.

AF-F (Live view, video): full-time servo

Live view offers two focus modes, AF-S and AF-F. Similar to AF-C, in AF-F mode the camera adjusts focus constantly as long as the shutter button is half pressed. Dynamic area modes are available in both focus modes: face priority, wide-area, normal-area and subject tracking.
The latter is similar to AF-C 3D, though less effective, in my experience, because it uses contrast detection instead of phase detection, as explained earlier.

MF: Manual focus

For obvious reasons, this mode has no configurable options.
It is typically used in combination with manual focus (AI, AI-S) non-CPU lenses, which can be used only with A or M metering and after the user stores the focal length and maximum aperture in the camera database (Setup menu > non-CPU lens data).
MF can also be used with autofocus lenses when we need a more precise or selective focus, when AF does not give satisfactory results, for example in low light or low contrast situations, with subjects that contain complex patterns or when there are obstacles in the field of view.
It is important to note that, although with non-motorized AF-D lenses it suffices to select M mode on the camera to release its built-in focus motor, with motorized AF-S lenses (D & G type) it is necessary to set the A-M selector on the lens to M to unlock the focus ring, except if it has manual AF override, as is the case with some of the more expensive AF-S models.

Dynamic area modes

The different dynamic area modes are subject to the chosen focus mode, AF-A (auto), AF-S (single) or AF-C / AF-F (continuous).

AF-S – Single Point (S)

The camera focuses once on the subject in the selected focus point. Useful for subjects that are either stationary, move slowly and predictably, or are proportionately small enough to not present framing and/or tracking problems.
Independent of the chosen focus mode, single point is – in spite of all the sophisticated dynamic alternatives available – still the most predictable and reliable.
It ensures that the focus point is exactly where we want it, which is particularly important when we are shooting with fast lenses at or near their maximum aperture, since wide-open or even slightly stopped down the depth of field (DoF) is often so paper-thin that proper focusing is key.
The photo above is a good example: while the eye at the focus point is 100% sharp, the other is already woolly, i.e., shows traces of focus-blur.
AF-S single point is most suitable for studio photography, still life, florals, portraits, landscapes and architecture, but useful in almost any situation in which we need very precise focusing on a subject that moves little or not at all.

AF-C – Single point (S)

Identical to AF-S with the only difference that the camera continuously adjusts focus according to the distance to the subject, as long as the user keeps the shutter release or AE-L/AF-L pressed.
AF-C single point is the appropriate mode for photographing subjects with a predictable trajectory, such as individuals or objects in motion, subjects which move unpredictably but over short distances, like a flower blowing in the wind, and also to focus on a specific individual within a group on the move, like a platoon of cyclists, a group of runners or a herd of horses, for example.

AF-S – Auto

The cameras focuses only once and on the closest subject, while the focus point or points are determined by the camera.
Several focus points may light up initially, but then turn off, leaving only the main focus point lit.
Although not documented by Nikon, the Auto mode apparently uses – both in AF-S and AF-C mode – an implementation of what used to be called closest subject priority.
Additionally, the increasingly sophisticated Scene Recognition System (SRS) is, together with type D and G lenses, capable of distinguishing humans in a scene.
Although not as effective as the face priority mode available in live view, auto usually successfully focuses on people's faces.
AF-S Auto can be used for photographing consistently and predictably moving subjects in parallel to the camera position as long as they are well isolated from their environment.
Nonetheless, because of the blur risk involved in focusing and/or tracking moving subjects, I consider that in most cases AF-C Auto is preferable, which is why I have rarely used AF-S Auto so far.

AF-C – Auto

Again, identical to AF-S, with the difference that the camera adjusts focus continuously.
AF-C Auto is like matrix metering applied to AF and indicated for photographing moving subjects, whether predictable or more erratic.
These should be well separated from their environment, there ought not to be other subjects closeby to throw off AF and/or affect composition and the background must be free of significant distractions.
Allows the efficient focus acquisition on the subject closest to camera, regardless of where it is in the frame.
AF-C auto can be used, for example, to capture birds in flight against a neutral background (sky or other low contrast background) and also to isolate the main subject within a group, such a runner who is well separated from the others or any other subject that approaches the camera with a reasonable separation from possible others.
If a group is moving parallel to the camera position, keep in mind that the camera may lock focus on any subject that happens to be closest to camera.
If focus on a particular subject within a compact group is critical, it is recommended to use alternatively the single point mode or a dynamic area mode like d9, depending on how large the subject is in the viewfinder, the compactness of the group, the aperture used, and how important it is to visually isolate a main subject.

AF-C – Dynamic area, d9, d21 and d51

These dynamic modes group multiple sensors into what are called focus areas, activating a determined number of focus points surrounding a central point.
When the subject temporarily leaves this point, the camera uses the surrounding points to maintain focus, while indicating the activation of the new focus point in the viewfinder (red).
These modes are used to keep the focus on erratically moving subjects that have sufficient distance and/or contrast relative to the background.
They are also convenient when we use long and/or heavy lenses with which it is sometimes hard to keep frame when tracking fast moving subjects.
If the back- or foreground contain defined and/or changing contrasts, it is likely that autofocus will hunt, trying to focus on any contrasting element from the foreground to the background.
In that case it may be recommendable to modify lock-on time (CSM a:3) and/or to select AF-C single point, even when in that mode it may be (a lot) more difficult to keep the subject both framed and in focus.

The number of neighboring points to be used is determined by how fast and unpredictable the movements are, although it must be kept in mind that the more focus points are activated, the greater the risk is that the camera changes focus away from the main subject, especially when there are other possible subjects in the scene.
In most cases the effectiveness of these modes is highly dependent on a good separation between the subject and the background.
As an example may serve that, in my experience, for subjects such as hummingbirds or insects, which typically fly very unpredictably and near flowers or shrubs, none of these modes gives satisfactory results.
By using all focus points available, it appears that the d51 mode is identical to Auto.
However, unlike the previous, which does not allow any intervention, in d51 the user can choose a particular focus point, even though the camera might end up using any of the other, depending on the number of elements with sufficient contrast present in the scene.

AF-C – 3D

The 3D mode is very useful for AF Tracking a subject throughout the frame.
The camera combines the distance information from the focusing system with the information obtained from the RGB sensor to maintain focus on a subject in rapidly changing motion, independent of its position in the frame.
To operate successfully, the scene should be well lit, while the main subject should be well separated from the background and have a color that clearly distinguishes it from the environment, like, for example, a red car coming down the road or a white-clad tennis player against gravel or grass.
3D is activated when we place one of the focus points on the subject and press the shutter button halfway.

AF-S / AF-C – Group area AF

Available only with the Advanced Multi-CAM 3500FX system of the D750, D810 and D4S.
Group area is operationally identical and has the same advantages as other dynamic area modes (d9, d21, etc.), but with the difference that it is user configurable.
The advantage is that the user may place only five focus points on the main subject (four of which are visible), which decreases the risk that the camera accidentally focuses on something adjacent. In AF-S mode, the camera will detect and focus on faces if there are people in the scene.

Conclusions

Modern AF systems are becoming ever more effective and efficient, but also increasingly require more expertise from their users.
For those who feel overwhelmed by the increasing sophistication of these systems and the difficulty associated with their handling, I can recommend the scene modes, which do not only handle AF properly, but also correctly configure metering, aperture and – if necessary – flash.
With modern cameras one does not have to be an expert, nor be worried about not being up-to-date with the latest advances in technology.
Technology is there to take advantage of and, in the case of digital photography, may result in amazing photos even in amateur mode – if the brain behind the viewfinder has the necessary vision and creativity –, whatever it is that people whom use the “A” word may mean by it.

Happy shooting!