Expanding Focus Point Size with Mark III Cameras

March 27, 2011

Expanding Focus Point Size with Mark III Cameras
… the AF system will be able to pick up enough detail... to allow autofocus calculation and accurate frame-to-frame sharpness.

What’s the benefit of having 45 focusing points instead of just a few? One often-overlooked benefit is the ability to not only manually pick from numerous AF points, but to be able to change the size of a manually-selected AF point. We will explore that option here, for Canon's top-of-the-line EOS-1D and EOS-1Ds cameras. At present, the current models are the EOS-1D Mark IV, and the full-frame EOS-1Ds Mark III. To a certain degree, users also have this option with the previous-generation versions of the EOS-1D and EOS-1Ds cameras. As you’ll see, it’s another of the benefits a photographer gets when using a professional camera body.

All of this discussion centers around options available during manual AF point selection with these cameras. This means that the user is manually choosing one AF point (it can be the center AF point, or one of the peripheral AF points). By default, this is done by pressing the AF point select button on the far right-rear of the camera body, and then turning either the top Main Dial, and/or the rear Quick Control Dial, while looking through the viewfinder. Inside the ellipse in the central area of the viewfinder, you’ll see individual red AF points light up one at a time as the dial(s) are turned; when you arrive at the location you desire, stop turning the dials, and tap the shutter button half-way to lock your choice in and be ready to shoot — using only that individual AF point. (During the selection process, if you reach the end of a row of points and the outer ring of AF points lights up, you’ve entered the Automatic AF point selection mode; this article does not deal with auto AF point selection.)


A single AF point — a blessing and a curse:

Out of the box, whenever you manually choose one AF point with an EOS-1D or EOS-1Ds series camera, you’re always dealing with only one AF point being active. This means that the AF sensor is sampling a very small area of your subject. In fact, with any EOS camera having the 45-point area AF system, the actual area covered by each AF sensor is smaller than the small rectangles you see highlighted in red in your viewfinder.

This has advantages. It means it becomes very easy to precisely target one subject among numerous surrounding objects, and it’s easy to pin-point a subject when you may have to shoot through foreground objects (think about focusing upon a bird through the branches of foreground trees, for instance). In a tight portrait, it can become very easy to precisely put the sharpest plane of focus right on the eye closest to the camera. Manually selecting and using a single AF point is a good choice for any serious photographer in many situations, especially when there’s a need to focus critically and you have the time to do it.

However, even a focus system as powerful as Canon’s 45-point Area AF system requires that the active AF point actually see real detail or texture at the part of the subject it’s reading. In situations like those mentioned above, usually no problem (especially with stationary subjects, where the shooter has time to carefully pick the part of the subject he or she wants to precisely focus upon).

Action and sports shooters, on the other hand, often don’t have this same luxury. Their subjects are often not only moving, but moving erratically. This can make it difficult or nearly impossible to keep a single, manually-selected AF point over a detailed part of the subject. If the AF sensor momentarily “sees” a plain, non-detailed part of an athlete’s uniform, without numbers, team name or logo, or even texture, it’s the same as trying to manually focus a telephoto lens on the clear, blue north sky. You can turn the focus ring back and forth, and see no difference in the viewfinder to discern when you’re sharply focused. The single AF point in these type of situations is subject to the same limitations, and it’s a frequent cause of occasional soft frames in an action sequence.

Technology to the rescue:

EOS-1D and EOS-1Ds series cameras have an answer: make the active AF point larger in size. By doing so, it’ll cover a larger area of the subject, and increase the likelihood that the AF system will be able to pick up enough detail, texture, or even folds in a uniform’s fabric to allow autofocus calculation and accurate frame-to-frame sharpness.

With the recent Mark III and Mark IV series models, this is done using Custom Function III-8.


C.Fn III-8-2 — add a ring of six points to your manually chosen AF point:

This option gives you a large active cluster of AF points. Regardless of which AF point you manually select, the system adds an invisible ring of six immediately surrounding AF points to your chosen point. You are still selecting ONE principal AF point, but the immediately surrounding AF points will be on stand-by, ready to instantly assist if your principal point ever loses detail at your subject.

With Custom Function III-8-2 active, you’ve now got a much larger potential area of a subject or scene that can be sampled for autofocus. This can also be useful in situations like run-and-gun candid shooting at events like wedding receptions or similar scenes, especially if they’re in relatively dim lighting, or if the subjects are wearing traditional, plain dark suit-jackets. Again, the point is that the AF system sees a lot more of the subject, and has a lot more information to work with in calculating AF. Understand that this isn’t just for the sports photographer; we’ve just mentioned how it could potentially be equally useful to an event or wedding shooter. The same applies to news photojournalism, wildlife shooters, and so on. From time to time, nearly any serious shooter can benefit from this technology.

C.Fn III-8-1 — smaller cluster of AF points = Even higher potential performance

Another option with the Mark III and Mark IV series cameras is to add a single AF point to the left and right of the active AF point you’ve chosen. Custom Function III-8-1 does exactly this. You’ve now got a smaller area, but still 3x as large as when using a single AF point alone.

Since this targets a smaller area of the subject, you may wonder what advantage it offers over C.Fn III-8-2 (adding a ring of six points around the selected point). Two things are important to consider:

  • With a smaller potential AF area, it can be a little easier to be precise.

During the hustle and bustle of a football or basketball game, the larger area with a total of seven AF points can be a big asset. But in situations like theater photography (for instance), you may have the time to be a bit more precise on where exactly you want to focus on your subject, yet at the same time want to minimize problems if the AF system momentarily falls upon a plain area of a subject. C.Fn III-8-1 gives you the added coverage to minimize problems with lack of subject detail, but still lets you focus with precision and confidence upon a specific part of the subject, whether it’s a large bird in flight or an actor on stage.

  • Fewer points = less processing required = potentially faster AI Servo AF

For photographers who are shooting truly challenging moving subjects, which change speed and direction rapidly, or which are moving continuously but at exceptional speed (race cars, Olympic sprinters, etc.), there’s a significant amount of AF data processing that has to occur in-camera to shoot a high-speed sequence. When you add AF points to a manually-chosen single point, you increase the amount of calculations required by the camera’s AF processor. Usually, the difference in AF performance is negligible. But with the most challenging of subjects, including tight shots of a moving subject as it nears a long telephoto lens, you can sometimes boost your performance by reducing the number of active AF points. Obviously, one option is to simply turn off C.Fn III-8, and use only a single AF point. But if you still want some added potential coverage because the subject doesn’t have a lot of detail, Custom Function III-8-1 is an extremely powerful option that Mark III shooters should consider.

New with the EOS-1D Mark IV:  Expand "All 45-Points Area":

A new option is added to C.Fn III-8 with the current EOS-1D Mark IV model.  Option III-8-3 is labeled, "All 45 points area", but it deserves some clarification.

With this Custom Function active, you still manually choose any one AF point. The camera silently and invisibly adds a broad cluster of surrounding AF points to assist your principal point -- up to 18 points, depending on whether your selected point is near the center or near the outer edges (fewer surrounding points join-in if you're out near the edges). Within this cluster of points, if your subject begins to move away from the initally-chosen point, the camera will attempt to "pass-off" the information to these surrounding points. And unlike the other AF point expansion options, YOU WILL SEE THESE POINTS ILLUMINATE IN RED IN THE FINDER, even in AI Servo AF.

So why the name "All 45 points area", if the cluster of active points is limited to 18? If your subject moves outside of this initial wide area, but is still somewhere in the AF ellipse, the entire area will begin to shift its location to attempt to keep the moving subject within its sight, and in focus

Predictably, this takes a massive amount of processing power for the AF system to achieve, and it works best in situations where there's ONE clearly-identifiable moving subject, and not a lot of sharp background detail. For some scenes where you have very erratic subject movement, C.Fn III-8-3 may be worth getting to know... there are limits to what it can do, but within those limits, it can be impressive.

AF system behavior when the AF point is expanded in size:

A vital point to remember is that no matter which option you choose to expand the size of a single focusing point, the EOS-1D and EOS-1Ds cameras always give priority to the central point you’ve manually chosen. The additional circle of six AF points, or the added left and right points, are active but on stand-by. If your primary single point doesn’t see sufficient detail at the subject, it will immediately call upon the outer points to see if any of them do, and use their results if detail is sufficient to allow accurate focus.

The outer AF points do not light up during AF operation once you’ve activated one of these Custom Functions. If one of the outer AF points is called upon in One-Shot AF to assist your primary AF point, you will see both points light up red in the viewfinder once focus is achieved. However, aside from that instance you will not see the circle of six surrounding points light up when C.Fn III-8-2 is active, for example.

Keep in mind also that in AI Servo AF, default operation is for your manually-selected AF point to light up red in the finder. But, any surrounding assist points made active as we’ve discussed here will not light up at all. This is absolutely normal.

One last thing: if you manually select a point on the outer edge of the AF ellipse you see in the finder, the camera can only add AF assist points to the inside of your selected point. In other words, if you're using C.Fn 8-2 (ring of six surrounding poins), and manually select an AF point in the central area of the AF ellipse, you get a true surrounding ring of assist points active.  But if you've chosen an outer AF point, only 3 or 4 surrounding AF points will be active, since there are no AF sensors outside of the ellipse area. This still can be very useful in the situations we’ve described above, so don’t be put off by this.

Further fine control with “AF tracking method”

With a larger focusing cluster, there’s always a chance that something may be picked up by the focusing system other than your primary, intended subject. There’s another Custom Function that can impact how the AF system deals with another subject that suddenly enters your active AF area, if you’re already tracking something in AI Servo AF mode. It’s especially useful if you’ve expanded your AF point to include six surrounding points, since this now sees a fairly broad area of the scene.

  • C.Fn III-4-1 — Continuous AF track priority

Generally, the recommended setting if you’ve expanded the size of an AF point, and are shooting in AI Servo AF mode with moving subjects (sports, etc). If another subject should enter your active cluster of AF points, while you’re already tracking another subject, the new subject is ignored — even if it’s closer to the camera than your original moving subject

  • C.Fn III-4-0 — Main focus point priority

If you’re tracking a subject in AI Servo AF, and a new subject enters the active AF area, the camera will try to re-focus on the new subject if it’s closer to the camera. This is default operation out of the box with Mark III and Mark IV models of the EOS-1D and EOS-1Ds. Again, for many action and sports shooters, we suggest at least experimenting with C.Fn III-4-1 if you expand the size of an AF point. (Note: this custom function is totally ignored if you manually select a single AF point and do not choose to expand its size.)

Previous versions of the EOS-1D and EOS-1Ds:

If you don’t own a Mark IV or Mark III camera, you still have some degree of control in terms of using the 45-point Area AF system to enlarge a manually-selected AF point. Please note that earlier versions of the EOS-1D and EOS-1Ds call this Custom Function "AF Point Activation Area" on the C.Fn menu.:

  1. Expand the size by adding a ring of six surrounding AF points Identical in concept to the process described above for Mark III cameras. In earlier models (original 1D/1Ds, or Mark II versions), this is done via Custom Function 17-1.
  2. Allow the system to automatically expand the AF point size, up to a maximum of 12 added points This is not carried over into the Mark III series. It allows the system to determine on its own, based on the lens in use, One-Shot AF or AI Servo AF settings, and (depending on camera) degree of detected movement, to expand the size of the active point by adding six or 12 surrounding AF points.
  3. For best AF performance: manually select two AF points You may have noticed that when all 45 AF points are available and you use the rear Quick Control Dial to manually move up or down, the first click of dial rotation causes two AF points to simultaneously light up red. If you stop right there, and don’t use any Custom Functions to further expand point size, you’ve got a powerful set-up for capturing fast action but you’ve also doubled the size of the focus area. The camera will automatically choose one of the points to track your subject, using the one that "sees" the most detail. A solid recommendation for Mark II and original 1D/1Ds shooters to consider. (Please note – this is not possible with Mark III series cameras, but it has returned with the EOS-1D Mark IV.)


Flexibility is vital for serious photographers, especially since shooting situations often change. Professional SLRs have traditionally been characterized by the ability to allow adjustments as the photographer wants or needs. Technology has moved this to a new level with the possibilities of Canon's 45-point Area AF system. Hopefully, this answers at least part of the question, “what’s the benefit of having 45 focusing points instead of just a few?”

Bride and Groom photograph © Bob Davis, 2008

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright Bob Davis, Erika Silverstein


© 2018 Canon U.S.A., Inc. All Rights Reserved. Reproduction in whole or in part without permission is prohibited.