Understanding EOS autofocus: Centre AF points

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Portrait taken with 50mm lens at f1.4

This is the second in a series of articles about the Canon EOS autofocus system.

In my previous article I wrote about the difficulties of focusing at wide apertures using prime lenses. I’ve been taking a lot of portraits recently and as I own both a 50mm f1.4 and 85mm f1.8 lens the temptation is to use them at the widest aperture settings. This produces portraits with beautiful bokeh (the opening photo was taken with an aperture of f1.4). The only thing is, that the depth-of-field at these apertures is so narrow that focusing has to be very precise. It’s easy to tell when you look at the photo on a computer screen if the camera hasn’t focused precisely where it is meant to (in this case on the sitter’s eyes, or the eye closest to the camera).

Nine point AF array

On the camera I use for taking portraits (an EOS 5D Mark II) the most accurate and sensitive autofocus (AF) point is the one in the centre of the nine point AF array (see diagram above). In low light, or if contrast is low, the outer AF points struggle to focus accurately. The centre one though focuses accurately nearly all the time. This is good to know as it helps me take better photos. But why are some autofocus points better than others?

Cross-type AF sensors

AF point sensors

The answer is that the centre AF point on my EOS 5D Mark II is a cross-type AF point. The rest are single line sensors (the above diagram shows the difference). Some of the single line sensors run horizontally, the others run vertically.

Cross-type AF sensors are sensitive to lines that move both vertically and horizontally across the frame.

Single line sensors

Single line sensors are sensitive to lines that move across the frame in a single line. There are both vertical line sensitive sensors and horizontal line sensitive sensors.

All the AF sensors on my EOS 5D Mark II require a lens with a maximum aperture of f5.6 or wider to be mounted on the camera in order to work properly.

If I’m using a lens with a maximum aperture of f2.8 or wider, the centre AF point becomes a high-precision AF sensor. It is twice as accurate than the other AF sensors. This is very helpful when using prime lenses at wide apertures.

Note that if you use your camera in any of the fully automatic modes (Scene Intelligent Auto, Portrait, Landscape etc) you have no control over which AF point (or points) the camera uses to focus. You need to be using a Creative Zone mode (Program, Aperture priority, Shutter priority or Manual).

More cross-type AF sensors

Now, wouldn’t it be helpful if there was more than one cross-type AF sensor on my camera? Yes, it would, and some EOS cameras do have them.

There are currently 13 EOS cameras with the same nine point array of AF points as my EOS 5D Mark II (15 if you count the astrophotography cameras the EOS 20Da and 60Da).

Of these, the majority work the same way. The centre AF point is a cross-type AF point. It becomes a high-precision AF point if you use a lens with a maximum aperture of f2.8 or wider. The others are single-line sensors.

The models that have this AF point arrangement are the EOS 400D, 450D, 500D, 550D, 600D, 1100D, 20D, 30D, 5D and 5D Mark II.

The EOS 40D, 50D, 60D and 650D are different. Every AF point is a cross-type AF sensor, and the centre AF point becomes a high-precision AF point with lenses that have a maximum aperture of f2.8 or wider.

If you own one of these cameras, you should be able to use any of the outer AF points with much more confidence than I can on my EOS 5D Mark II.

The EOS 6D has an 11 point AF array. It has just one cross-type AF point (in the centre) and the centre AF point, somewhat curiously, is a vertical line only sensitive high-precision sensor if your lens has a maximum aperture of f2.8 or wider.

Other EOS cameras

Autofocus is more advanced on most other EOS cameras, such as the EOS 7D, 5D Mark III and One Series cameras. I will look at the advantages of these cameras in another article.

Conclusion

This knowledge comes in most useful if you are using a prime lens at a wide aperture to take photos. In this situation you need to know how to get the best out of your camera’s autofocus system.

It is also useful to know if you are taking photos in low light, or the contrast of the subject is so low that your camera struggles to autofocus. Switching to the centre AF point should help.

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