March 26th 2013 by Andrew S Gibson
This is the third in a series of articles about the Canon EOS autofocus system.
The EOS 7D is a revolutionary camera. Before its release the only way you could purchase an EOS camera containing Canon’s most advanced autofocus system was to buy a One Series camera. These are expensive cameras built for professional photographers. Naturally, not many enthusiasts own one, and had to ‘make do’ with the autofocus arrays found on all other models.
That changed with the release of the EOS 7D in October 2009*. It has a 19 point autofocus (AF) system that is much more versatile than the nine point AF system found on most other non-professional cameras. It is designed for photographers who photograph moving objects and need an autofocus system capable of accurate tracking. However, photographers that only ever photograph non-moving subjects will still benefit.
*At the moment, the EOS 5D Mark III is the only other non-professional EOS camera body to have an advanced autofocus system.
Let’s take a look at some of the features that set the EOS 7D’s autofocus apart from other APS-C cameras:
Cross-type AF points
If you own an EOS 40D, 50D, 60D or 650D (or the new 700D when it becomes available) then your camera has nine cross-type AF points. The centre AF point becomes a high-precision AF point when the camera is used with a lens that has a maximum aperture of f2.8 or larger (you can read about this in more detail in my previous autofocus article).
If you own one of these cameras and you mainly take photos of stationary subjects, then your autofocus is probably good enough to cover just about any situation. But the EOS 7D gives you more flexibility, as we will see shortly.
The other APS-C EOS cameras (plus the EOS 5D and 5D Mark II) only have one cross-type AF sensor, located in the middle of the autofocus point array.
19 Cross-Type AF Points
The EOS 7D has a 19 point AF array, and is still the only APS-C camera to do so. Each of these AF points is a cross-type AF sensor as long as the lens mounted on the camera has an maximum aperture larger than f5.6.
Here’s a diagram showing the AF point array found in most EOS APS-C cameras:
Here’s a diagram showing the AF point array in the EOS 7D:
One benefit of having more autofocus points is that it gives you more freedom when placing the part of your subject that you want to focus on. For example, when I take a portrait with my EOS 5D Mark II I frame the image in such a way that the eye I want the camera to focus on is covered by an AF point. If I had the EOS 7D, I would have ten more AF points to chose from (see below). This would give me more freedom with the composition.
Furthermore, as AF point is a cross-type they should behave equally in terms of autofocus accuracy. The only exception is the centre AF point, which becomes a high precision AF point when the camera is used with a lens that has a maximum aperture of f2.8 or wider. High precision AF points have twice the sensitivity of other AF points. This helps the camera focus prime lenses used at wide apertures (where the depth-of-field is very narrow) accurately.
If you use a lens that has a maximum aperture that is smaller than f5.6 with the EOS 7D some of the outer AF points become single line sensitive only. Canon doesn’t sell any lenses with a maximum aperture of f8, but you might get there if you use Canon’s 1.4x or 2x EF Extenders with an existing lens. These magnify the image, but at the cost of a light loss of one and two stops respectively. Using the EF 300mm f4L lens with the 2x EF Extender, for example, effectively gives you a 600mm f8 lens.
The same also happens if you use a Canon 35-80mm, 35-105mm or 80-200mm zoom lens with a maximum aperture of f5.6 at the telephoto end.
This diagram shows how the number of cross-type AF points is reduced to seven with one of the affected lenses or lens and extender combination:
AF Point Size
The squares that you see in the viewfinder of the EOS 7D* to indicate the AF points don’t show the true size of the AF point. In reality, the area covered by the AF sensor is larger than the square.
* I suspect this applies to all EOS cameras, not just the EOS 7D. But I haven’t found any information to confirm this.
You can’t do anything about this on other APS-C cameras. But the EOS 7D lets you both expand or decrease the area covered by the AF point.
In Spot Autofocus, you select an AF point and the camera tries to focus on a smaller area than usual. A small square is displayed within the AF point square to indicate that you are using Spot Autofocus (the actual area covered by the AF sensor is still a little larger than the AF square, even with spot autofocus activated).
This is a feature that may come in useful when taking portraits using a prime lens. Unless you are close to the person you are photographing, the active AF point covers both their eye and the area around it. You never quite know where the camera will focus. It may focus on an area near the eye instead of on it. Switching to Spot Autofocus helps prevent that (see photo below).
Another situation where spot autofocus may help is if you are photographing an animal or bird that is partially obscured by grass or tree branches. Switching to spot autofocus reduces the chance that the camera’s autofocus will try and focus on the grass or branches rather than the animal or bird.
Spot autofocus doesn’t work well in low light or with moving subjects. Therefore it’s default position is off, and you have to enable it if you want to use it.
Expanding AF points
On the other hand, you might find it useful to expand the size of the area covered by the active AF point. Canon have come up with a couple of clever ways to help you do this:
1. Zone autofocus. The 19 AF points are split into five zones. You select one of these zones, and the camera uses all of the AF points within that zone to achieve focus. The five zones are shown below:
2. AF point expansion. You select an AF point. If the camera can’t focus on the subject using the selected AF point it activates the surrounding AF points and uses those instead until it finds focus.
Note: This feature is also available in AI Servo mode via the Custom Functions menu in the EOS 5D Mark II, but not in any other APS-C camera.
Both of these are designed to help the camera track moving objects. The benefit is that if you use your camera to photograph any type of moving subject (such as sports or wildlife) then the EOS 7D’s autofocus is faster and more accurate than any other APS-C camera.
There are a couple of other interesting features you should know about:
- The EOS 7D is the only EOS APS-C camera that has a dedicated microprocessor to handle autofocus calculations. This helps the camera track moving subjects accurately at shooting speeds of up to eight frames per second.
- The EOS 7D uses a transmissive LCD screen to display the AF points (at the moment the only other EOS camera to do so is the 5D Mark III). On other EOS cameras the AF points and spot metering circle are etched into the viewfinder, and the display doesn’t change (except when the active AF point turns red to indicate focus). The transmissive LCD screen of the EOS 7D can display whatever the camera tells it to. The display changes according to the autofocus settings selected. This keeps the viewfinder display uncluttered. The only disadvantage is that you can’t remove the focusing screen and replace it with another one.
EOS 7D Mark II?
It’s been more than three years since the release of the EOS 7D and that suggests that Canon may release an EOS 7D Mark II later this year. It is possible that it will include some of the advanced AF features found in the EOS 5D Mark III. But we will only find out when Canon release it.
This article only touches on the EOS 7D’s autofocus system, but I’m sure you can see that is more sophisticated than the autofocus on most other EOS cameras. It is the most cost effective way to get your hands on some of Canon’s more advanced autofocus technology – to get something faster and more accurate you would have to buy the EOS 5D Mark III or a One Series camera, all of which are more expensive. The autofocus of the EOS 7D will take a little longer to master than that of other APS-C cameras, but it will help you obtain more accurately focused images if you use prime lenses at wide apertures or shoot moving subjects.