Canon’s 45-point Autofocus (AF) System
Text and images copyright Arthur Morris, all rights reserved

Canon’s professional camera bodies (the EOS 3, 1V, 1D, 1Ds, 1D Mark II and 1Ds Mark II) feature a 45-point Autofocus System. The 45 sensors are arrayed in a roughly elliptical pattern that is, of course, centered in the viewfinder. You can activate all 45 points by first pressing the focus-grid button on the upper back-right of the camera with your right thumb and then turning either the index-finger dial or the thumb-dial until all the sensors on the outer edge of the ellipse become illuminated in red. Even though only the outer sensors are lit up in red (producing the “ring of fire”), it is important to understand that all 45 sensors are activated. When all 45 points are activated you are in Automatic Focusing Point Selection (AFPS) mode; the camera automatically chooses the best sensor or sensors in a given situation. When using AFPS in AI Servo mode (for moving subjects), the active sensors will not be illuminated while focusing as they are when using AFPS in One-Shot mode (for static subjects).

Marbled Godwit in flight, San Diego, CA
Image copyright 2005 Arthur Morris/BIRDS AS ART

Canon 600mm f/4 L IS lens with 1.4X TC and EOS 1D Mark II on Gitzo CF 1325 tripod. 45-Point AF (AFPS) ISO 250. Evaluative metering at zero: 1/800 sec. at f/8.

When walking down the beach, I teach folks to mount the 1.4X (not the 2X) and choose AFPS so that they are ready for unexpected action. Doing so allowed me to capture this image when the subject unexpectedly attempted to fly the coop.

I almost always set AFPS in conjunction with AI Servo AF when walking down a beach as this combination usually does quite well with action and fairly well with flight. I also use this same AF set-up when making tight horizontal head-shots of herons, egrets, or cranes as it enables me to place the bird’s eye above the horizontal centerline. AFPS/AI Servo AF also performs well when creating vertical images of these same birds (regardless of how tightly they are framed). On the other hand, using AFPS on a swimming duck is a total waste of time because the system will usually attempt to focus on the water in front of the duck. You simply have to know your equipment. Once AF is acquired, you can easily move the subject around in the frame to create a pleasing composition without losing focus. If you do lose AF in this, or any other situation, it is best to release the shutter-button and then depress it again to re-acquire focus.

When using AFPS it is relatively easy to attain focus on flying birds and child’s play to maintain focus (except when birds at close range are flying directly at the camera). With the EOS 1D Mark II, AFPS does pretty well with subjects flying against backgrounds other than sky, and the results are often exceedingly sharp. Autofocus tracking accuracy, however, is not as consistently precise as it is when using the central sensor only (CSO), and there-in lies the rub: which do you want, the more accurate focusing offered by CSO or the ability of AFPS to acquire and maintain focus? I often find myself switching back and forth between the two when faced with consistently good flight photography opportunities. I must confess, though, as I get older, each day I rely more and more on AFPS.

My relationship with AFPS has long been of the love/hate type, and continues to be so even with the Mark II. With this amazing new camera, however, the love predominates by far. In the short time that I have been using AFPS with the Mark II, it has helped me capture some truly amazing images that I could simply not have created with other cameras. Its ability to grab a moving subject and focus-track accurately (even when working against backgrounds other than sky) often borders on the astounding. On my last trip to Nome, Alaska, AFPS helped me make some wonderful images of a Black-bellied Plover flying about on the tundra and another of a hen Northern Shoveler blasting off (thanks to Matt Hagadorn!) from a roadside pond. When AFPS is good it can be really, really good. Thus the love.…

On the same trip, we had a moose standing on the tundra a good distance from the car. It was dingy, foggy day. I reached for the 400mm f/5.6 which had a Mark II on it set up with AFPS. I focused easily on the moose but just as I depressed the shutter button the system lost focus. The moose (if a moose can scurry) scurried off and I bemoaned my fate. CSO would have held focus easily, as would any sensor with One-Shot. AFPS had a problem with the low contrast. Thus the hate.…

In short, AFPS may have difficulty acquiring and maintaining focus in low light/low contrast situations and this is true even with static subjects (like the moose). It is strongly recommended that users switch to One-Shot AF mode and use the CSO only when photographing static subjects in these conditions.

The EOS 3 film camera was the first to feature the 45-point AF pattern and from the moment that I first held one I felt that the 45-point AFPS system was not—to coin a phrase—as central sensor-dominant as it should have been. Even with the Mark II (amazing as is it) I believe this to be true. At times, even when working in bright light with high contrast situations, AFPS fails to “see” a large-in-the-frame subject and fails to acquire focus even when the bird is in the center of the frame. (This pretty much never happens when using the central sensor only. Do note that the central sensor is more effective than any of the other 44 focusing sensors.) If you have a problem focusing on a fairly large subject in the center of the frame, try reframing so as to move the subject away from the center of the frame. This often saves the day. When using the EOS 1n with all five of its focusing sensors active, users were advised to begin AF with the central sensor. That worked well because the 1n’s AF system was central-sensor dominant. I do hope that it is possible to tweak the 45-point AF system in the next generation of Canon cameras so that it works in a similar fashion (and have repeatedly urged the folks at Canon to do so).

When you are using autofocus but not using AFPS, you will need to manually select one (or two) of the 45 points. To do this, press the focus selection grid button. You can move the active sensor side-to-side by rotating the index-finger dial, or up and down with the thumb-dial. To select two side-by-side sensors, first select the single sensor immediately above or below the two that you want to activate and then rotate the thumb-dial. If you rotate it clockwise, you will light up the two sensors below the one that was previously selected. If you rotate it counter-clockwise, you will light up the two sensors above the one that was previously selected. I often use the two sensors immediately below the central sensor to photograph swimming loons and ducks so that I can place them a bit down in the frame. Do realize that when I activate a sensor other than the central sensor, I am virtually always working with AI Servo AF. When I am working in One-Shot I work with the central sensor more than 95% of the time.

Activating and using only the central sensor (CS) when photographing birds in flight will almost always yield the sharpest images, and AF tracking accuracy with subjects flying towards the camera will be maximized. The downside of using the central sensor only (CSO) is that it is often difficult for some folks (including me) to keep the sensor on the subject at all times. This is especially true when the bird is flying quickly or erratically. When doing flight photography, it is ideal to place the CS on the bird's head, face, neck, or eye(s). If the sensor falls off of the subject and detects distant trees or fields or mountains as background, it will—in most cases—lose focus on the subject; it will do exactly what it is supposed to do--focus on whatever is detected by the active AF Sensor. I once mentioned during a full-day seminar that lots of practice would improve one's flight photography and that fine-motor control was a key factor in keeping the CS on the subject. Someone asked, "Does Canon sell Fine Motor Control?”

Bald Eagle in flight Image copyright 2005 Arthur Morris/BIRDS AS ART Canon 300mm f/2.8L IS lens with EOS 1D Mark II. ISO 400. Evaluative metering off the sky + 2 2/3 stops set manually: 1/1000 sec. at f/4.5. Using the central sensor only and placing it on the bird’s eye, face or neck is the way to go when photographing birds in flight, especially those flying directly at you. (Do note that this is a pano-crop from the original, left.)

On my first trip to Homer, Alaska, in February 2005, I alternated using AFPS and Central Sensor while photographing hundreds of Bald Eagles in flight and realized once and for all that the percentage of images critically sharp on the eye while using CSO far exceeds the percentage of sharp images attained when using AFPS. While I did get some exceptionally sharp images with AFPS, the performance of CSO was clearly better overall.

With sky-only backgrounds, the AF system will almost always stay locked on the subject even if the active sensor falls off the subject. Many of my sharpest flight images were made with CSO in cases where the CS was not on the subject. (Breezebrowser's "Show Focus Points" feature often confirms that the CS was nowhere near the bird at the instant of exposure! On a related note, however, it is strongly recommended that folks using any one of Canon’s pro bodies set Custom Function 17-1 which activates the sensors immediately surrounding the active sensor (or sensors). If you have done so, remember that Breezebrowser will show only the selected sensor, not the surrounding sensors (that were activated by CF 17-1). Setting and using CF-17-1 will improve AF performance.

Brown Pelican, LaJolla, CA Image copyright 2005 Arthur Morris/BIRDS AS ART Canon 100-400mm L IS zoom lens (at 350mm) handheld with the EOS 1D Mark II. ISO 250. Evaluative metering at zero: 1/2000 sec. at f/5.6. Whenever the subject presents itself relatively parallel to the back of the camera, AFPS performs superbly.

If the background is uniform and of low contrast, it is sometimes possible to hold focus on the subject when using AI Servo AF even when the sensor does not fall on the subject. This can be a huge advantage when you are using a 2X TC with an f/4 lens. With smaller-in-the-frame subjects, it is sometimes possible to focus on the subject and then recompose so that the subject is well off-center. If the background is low contrast, AF will often continue to hold even though the sensor is off the subject and you are in AI Servo AF mode! I discovered and used this technique recently while lying in the sand on my belly on Sanibel photographing shorebirds. I was trying to come up with a way to make a pleasing composition without having to reach up and toggle from AI Servo to One-Shot AF. And I did!

When using a Canon pro body, it is—in virtually all cases—best to use AI Servo AF for moving subjects because (when it works as it is supposed to) it not only tracks the subject but predicts where it will be at the exact instant that the shutter opens. The decision to use One-Shot AF or AI Servo AF when photographing perched or otherwise static birds can be a difficult one. (By the way, AI stands for “Artificial Intelligence.”) When using One-Shot AF, focus on the bird’s eye and then recompose. If the bird moves its head at all, there is a danger that the eye may not be in sharp focus. If you choose AI Servo AF, it is best to manually select the AF point (or points) that will yield a pleasing composition when the sensor is placed on the bird’s head, face, neck, or eye. The danger in this approach is that if the bird changes its posture or position you may be left out in the cold. Another option that works well when photographing small or medium-sized birds at close range (especially against clean backgrounds) is to use the CS, choose a small aperture in the f/11 to f/16 range, focus on the near-wing, and then rely on having enough depth-of-field to sharply render the eye. (I do not like using AFPS for small birds at close range.)

Note: The Élan 7 series bodies and the EOS 10D (digital) camera body feature a seven-point AF array, and the EOS 20 D features a nine-point AF pattern. With all of these bodies the speed of initial focus acquisition is adequate or better (best with the 20D) but not as fast as with the pro bodies. With each of these camera bodies, however, AF tracking accuracy is excellent, especially with birds flying directly at the camera. AF accuracy in these situations surpasses AF accuracy in all of the above-noted bodies except possibly for the Mark II, and this was true whether using the central sensor only or activating the entire array. Initial focus acquisition, however, is much faster with these cameras when using the central sensor alone than when activating the entire array. On clear days, when the wind and light are from approximately the same direction, I like to carry an intermediate telephoto lens with me into the field so that I can be ready for handheld flight and action opportunities. I mount one of three lenses (the 400mm f/5.6, the 100-400mm L IS zoom lens, or the 70-200 L IS zoom lens—the latter if the birds are fairly close) onto a 20D and sling the rig over my shoulder via the camera body strap.

Arthur Morris, a NatureScapes.Net columnist, is a highly accomplished and inspirational photographer specializing in birds. His writing and his photographs have been published in hundreds of books, magazines, and calendars. Art now photographs, travels, speaks, and teaches extensively in North America. For more information please visit his website at www.birdsasart.com.

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