The Autofocus

Could we photograph without autofocus? Yes, of course, but we would lose a lot of opportunities. The AF technology has been introduced more than two decades ago, and since then it has made enormous progresses. The first AF lenses had a very slow autofocus, that was useable just with static or very slow moving subjects: as you can imagine, it had a limited usefulness, but the manufacturers believed in this technology and continued to improve it. Today, the autofocus is fast, precise and very reliable, and it is almost essential for wildlife and action photography. But it is not enough to have a state-of-art technology to take great photos: you have to learn how to use AF to get the best of it.

Choosing the Right Lenses

The are mainly two types of AF motor: the "traditional" motor and the "ultrasonic (USM)" motor. The traditional motor is used on cheap lenses; it is slow and noisy. The USM motor is much better, but not every "USM" lens has actually an ultrasonic motor. Good lenses, as the Canon 300 f/4 L IS USM, have the "Ring-type USM" (the real USM), while many cheap and mid-range lenses, as the Canon 70-300 IS USM, have the "micro-USM", that has none of the advantages of the real USM: it is very similar to the traditional AF motor. If you don't have a good knowledge of lenses, it is difficult to tell if an USM lens has ring-type USM or micro-USM; the manufacturers are not very clear in this respect. Don't worry: in the Canon Lenses page I listed the main features of every lens, and I clearly specified if a lens has real USM or a traditional lens motor. 

Nowadays, I'd never buy a lens without ring-type USM motor, unless I plan to use it mainly in MF, for static subjects: for wildlife and action, the fast AF and FTM are almost essential!

Choosing the AF Area

All DSLRs have various AF areas; the number varies from 3-5 areas in the basic models to the 45 areas of the top-of-the-line Canon 1 series. You may think that all the areas are identical, but actually there are two types of AF areas: those with linear-type AF sensor and those with cross-type AF sensor. The linear AF sensor is able to detect only the horizontal detail, while the cross sensor can detect both horizontal and vertical detail. The areas with a cross sensor gives faster AF, and they are able to detect subject that would be problematic from linear sensors. In the consumer DSLRs only the central AF area is a cross-type sensor; all the other areas are linear sensor. With slow lenses (i.e. the lenses that have a small aperture), all sensors operates as linear type. The professional dSLRs have more sophisticated AF modules; the Nikon D2Xs/D2Hs have an 11-areas AF module with nine cross-type AF sensor, and the

The 45-points AF system of the Canon 1Ds Mark II and Canon 1D Mark IIn

Canon 1 series DSLRs have a 45 areas AF, with seven cross sensors at f/2.8. At f/4 all sensors except the central one convert to linear type, and at f/8 only the central sensor is active, and it works as linear sensor.

f/2.8 or faster	f/4	f/5.6	f/8
The number and the type of the AF sensors depends by the brightness of the lens. These illustrations show the how AF module of the Canon 1 series works with lenses of different brightness.

How to choose the right AF area? The are mainly three "strategies":

1 - Central area and AF lock: on every DSLR, the central area is the fastest and most reliable area. It is a good choice if you use an amateur camera or a lens with average AF, and it is the only choice if you use a lens with an effective maximum aperture of f/8, as the Canon 600 f/4 + 2.0x TC (note that only the 1 series camera can focus at f/8).
If you photograph a static subject, using the central AF area is a breeze. You just have to focus on the eye, lock the focus (if you are using AI Servo continuous AF), recompose and take the shot. For moving subject, focusing with the central area is more difficult: you don't have the time to lock the focus and recompose, so if you focus on the eye the subject will be centered in the frame, creating a static composition. You have two alternatives: you can frame the subject with some room all around, and crop the image to an off-center composition in post-processing, otherwise you can focus on the body of the subject instead of the eye. If you choose this option, pay attention to have enough depth of field to get the eye in focus; with some small subjects might be necessary to stop down to f/11 or smaller apertures.

2 - Off center area: if it is possible to get good AF even with an off-center area, you can choose the area according to your composition: for wildlife, choose the area where you want to place the eye of the subject. If you manage to place the area that you selected on the eye, there is no need to use the AF lock.

2 - Automatic Focus Point Selection (AFPS): the camera choose automatically one of the focus areas; it tries to follow the subject by changing the focus area every time that it is necessary. The exact behavior of AFPS varies depending by the camera; on Canon 1 series, the camera select the closest subject with readable contrast (in One Shot mode) or the subject identified by the central AF area (in AI Servo mode). AFPS is not as reliable as manual selection, but it is the ideal choice when you are trying to photograph a very active subject and you can't manage to center it with a single area.

Single VS Continuous: One Shot AF and AI Servo AF

Other than choosing a focus point, you have to choose between One Shot AF (i.e. single AF) and AI Servo AF (i.e. continuous AF). There are some major differences between the two AF modes. One Shot is a good choice for stationary subjects. It focuses on the subject, then it looks the AF until you keep half-pressed the shutter release. It is possible to take the photo only when the subject is in sharp focus. One Shoot is slightly more precise than AI Servo, and it is a bit more sensitive in low light, but it is not recommended for moving subjects: if the subject has moved after the focusing, you have to release completely the shutter release, and then half-press it again, to repeat the AF operation. I use AI Servo for the majority of my images. AI Servo continues to focus as long as you keep half-pressed the shutter release. It is possible to take the photo even when the camera consider the subject "out of focus" - this is a very useful option, since sometimes the camera is wrong. AI Servo works well for moving subjects, but if you photograph a static subject with the technique of "focus on the eye and re-compose", you have to lock the focus with the AF lock button, otherwise the camera re-focuses as soon as the subject exits from the selected AF area. Some cameras offers also a third AF mode, called "AI Focus" (where AI means "Artificial Intelligence" system), that chooses automatically between One Shot and AI Servo. I'd suggest to avoid it, since it is better to not delegate to the camera such important choice.

What Influences AF Speed?

Other than the AF motor of the lens, there are many variables that determine the AF speed. Knowing them allows to improve your AF technique and to avoid common misconceptions.

While the lens motor could be compared to the muscles, the camera is the "brain". As you can expect, professional cameras have more advanced AF systems than consumer cameras; the same lens gives faster AF with a 1D than with a 350D. Every new generation of cameras has a slightly more advanced AF; the Canon 1D/1Ds MarkII are the first cameras to have two dedicated CPUs (Central Processing Unit) for AF operations, one for calculation and the other to control the AF motor.

The teleconverters reduce the brightness of the lens by 1 or 2 stops, and they reduce the AF speed, too. The Canon 1.4x TC II reduces the AF speed by nearly 50%, while the Canon 2.0x TC II reduces the AF speed by 75%. The brightness of the lens (max. f/stop) influences considerably the AF speed in low light, and it determines the configuration of the AF module in every light condition, as explained in the previous paragraphs. Faster (brighter) lenses usually gives faster AF and less bright lenses: for example, the 300 f/2.8 gives faster AF than the 300 f/4. The focal length and the minimum focusing distance are other important variables. With a long lens, when something is out of focus it is *really* blurred, and often it is completely unrecognizable, while with short lenses the subject is rarely so out of focus to be completely blurred. Of course, for the camera/lens it takes more time to find and focus a completely out-of-focus subject than a slightly OOF one. Moreover, if the lens has a short minimum focusing distance, the AF is slower because the AF motor has to search the focus between a wide range of distances. This explains why a macro lens as the 180mm Macro (that has a focusing range of 0.45m-infinity) has slower AF than a 70-200 lens (that has a focusing range of 1.5m-infinity).

Great White Egret - Canon EOS 350D, Canon EF 600mm f/4 L IS USM, Canon 1.4x TC, 1/4000 f/5.6, ISO 400, tripod. This is a good example of a low contrast photo: a white subject against a light grey background. To use AF, I placed the AF area on the bill, the only area of the image that has a clear contrast of color and shapes.

The light level strongly influences AF. In low light, the AF gets slower, because it is more difficult to "analyze" the subject's detail. In very low light or in complete darkness, the AF can not find the subject, unless you use a flash with AF-assist beam (as the Canon 580EX). This beam consist in a striped pattern that allows to create some contrast on the subject, even in the darkness, and it allows to focus inside the beam range. Of course, when you take the shot the beam turns off and you won't see any patter in the photo.

The AF needs some subject contrast to achieve the focus. Low contrast subjects, as a wall, skin or a single-color dress can foul the autofocus because it doesn't see enough detail the determine if the subject is in focus. When you are trying to photograph these subject, you can either focus on the edge (if it is in the same plane of focus) or focus manually.

The AF Lock Buttons

Focus Limiter and Focus Preset

Tips and Techniques

Where to focus: when you photograph an animal, you should focus on the eyes! If the eye is out of focus, the photo loses the 90% of its value.  If the subject has very little contrast, you can try to focus on the edges.

To take this photo, I pre-focused on the bird when he was perched, then I locked the autofocus. The subject moved into the plane of focus, so it is perfectly sharp.

Shutter release: don't "mash" the shutter release. To achieve the focus in the shortest time, half-press the shutter release, and continue to focus during the action. In this way, when you press down the shutter release the subject is already in focus. If, instead, you mash the shutter release, the camera will take the photo without having the time to focus properly (in AI Servo), or it will lock up the shutter release until the autofocus has found the subject (in One Shot). Remember that keeping the shutter release half-pressed for long times reduces considerably the battery life, so take with you one or more spare batteries. Focus issues: even if your AF techniques are perfect, you might experience focus issues. Remember that the size of the AF sensors are slightly larger than the AF areas; if there is an zone with strong contrast very close to the selected focus area, the AF might focus on this zone instead on into the focus area. This is not a defect of the camera. Other than that, it is possible that the camera or the lens actually have backfocus or frontfocus issues: in this case, you have to determine if the problem is the camera or the lens, then you can send it to the manufacturer for re-calibration (that is covered by warranty).

To understand where is the problem, take some photos of a test target parallel to the camera (or a specific focus target) with various lenses. If all the images are out of focus, it is likely that the camera has focus issues. If only some images are OOF, it may be a problem of only one lens.

Focus lock for action: if you know that the subject is going to move into a certain focus plane, you can pre-focus on the estimated focus plane, lock the focus and keep it locked during the action. I have used this technique with very good results to take photos of birds that takes flight from a perch: I focus on the bird when it is perched, I lock the focus and when the bird takes flight I press the shutter release, keeping locked the AF. If the bird moves parallel to the camera, it will be in sharp focus; if I didn't lock the AF, instead, it is likely that the AF would have focused on background, since it is very difficult to keep the subject into the focus area. This technique is particularly useful with consumer cameras that have average AF.

Do you have comments or questions?

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