Canon Back-Button Auto Focus

Last modified on September 28th, 2013

I did a photoshoot the other day for a few friends of mine. The shoot was in a big field, at around 4pm in the afternoon. There was lots of light obviously, since it was a sunny day, but the sun was high in the sky and the shadows were pretty harsh.

For the most part, I always take a flash outdoors with me now. A long time ago using a flash outside would have seemed silly, but I’ve photographed a few weddings now, and understand that fill flash is required in many outdoor situations to help balance the ambient light. So for this shoot, I brought along my speedlight.

While the photos turned out fine, I walked away from the session a bit frustrated with how my camera works, something I’ve done fairly often lately. I’ve retrained myself several times on my camera, trying each time to find a sweet spot that results in a nice balance between spontaneity and camera workload. Unfortunately both times I haven’t really been that happy with the results.

Focus/Recompose Technique

When I first picked up my Canon EOS 20D, the only technique I had known was the focus/recompose technique. Most digital SLRs have a series of focusing points that are user selectable via a little joystick or rotating dial on the back of the camera. For the focus/recompose technique, you basically set the focus point to be directly in the center and leave it there at all times.

When taking a photo, you basically put the focusing dot (which is in the center) on your subject and press the shutter button down half-way. In most digital SLRs, this will cause the camera to auto-focus. While still holding the button half-way down, you then recompose the image to the final form, often rotating slightly such that the subject (which is still in focus) is in a different location. At that point you press the shutter all the way down and the camera takes a photo with the focusing point now in an off-center location.

The main reason I love this technique is that it allows you to get deadly accurate focusing. For example, usually portraits look best when the eyes are the main focus point. Using this technique, you would set the center-point of the camera on the subject’s eyes, press the shutter half-way (which would focus), the recompose and take the shot.

The downside to this technique involves the metering process (the process which the camera uses to determine the settings for the shot). In most cases, you typically want to meter on your subject. For a person, you typically want their skin and hair to be properly exposed, and can usually sacrifice the sky or the background to achieve that result. Unfortunately though, most cameras meter only at the instant the photo is actually taken, something I didn’t realize until after I had ruined a pile of photos. So while you may have focused on your subject initially, the recompose step causes the center point of the camera to be in a different position, which affects the cameras decision making process with regards to exposure. In many cases, this will result in a photo that is either underexposed or overexposed.

To get around this problem, Canon put a little AE-Lock button (donated by a “*”) on their cameras. Basically you can press this button at any time and force the camera to lock the exposure onto the subject such that the exposure will remain fixed after the recompose step. To do this, most people point their camera at their subject and press the AE-Lock button. On my camera at least, this results in a little “*” being shown in the viewfinder, indicating that the exposure has now been locked.

What I don’t like about this process is that it’s pretty involved. Basically you have to do the following every time you take a shot:

  1. Orient your camera such that the center area is roughly over your subject
  2. Press the AE-Lock button to lock the exposure on your subject
  3. Place the center focus point on the area you want to be in focus
  4. Press the shudder half-way down to trigger the focusing cycle
  5. Recompose the shot
  6. Press the shutter completely to take the photo

While you eventually get used to doing all of these steps, it’s still a rather involved process. In an effort to reduce the amount of work involved in taking a decent photo, I decided to retrain myself to another method.

Manual Selection of Focus Points

In most digital SLRs, you can select the focus point that you’d like to use for a shot. Thankfully Canon included nine focus points on my camera: one in the center, four adjacent to those, and four on the diagonals. To pick a focus point (at least on my camera), there is a little joystick that I can simply push in the direction of the dot I want to select, and it’ll become active.

So instead of doing a full recompose now, I typically select the focus point that’s closest to my main subject. While I still do a slight recompose on each shot, it’s generally fairly quick since the focus point was roughly in the same location as the subject. The main benefit of this technique is due to the metering. Since the focus point I selected was close to the subject, and my recompose shouldn’t have moved it that far away from the subject, the focus point should be in roughly the correct position to take an accurate meter reading from. So in most cases, I simply press the shutter the rest of the way to take the photo.

A summary of the steps involved in this process are as follows:

  1. Manually select the focus point closest to the subject (this may or may not be involved – for me I can do this while simultaneously performing step #2)
  2. Press the shutter half-way down to cause the camera to focus
  3. Recompose the shot
  4. Press the shutter all the way down to meter the shot, and take the photo

As you can see, it’s less involved than the first process. Unfortunately though, it’s often less accurate, both in terms of focus (in some cameras, the center focus dot is more accurate than the side dots), and in terms of exposure (since the focus point is only roughly co-located with the subject).

Tonight I went looking on the Internet for some kind of tweaks I could make to this process to help improve it. While scanning around a few forums, I discovered a relatively old technique that seems to be making a re-emergence: back-button focus.

Back Button Focus

With back-button focus, you essentially re-map the features on your camera such that focusing and metering are completely decoupled. After reading how this works, I realized that several of my friend’s Nikon cameras already have this functionality (or at least, they have it enabled by default).

The only button really worth remapping to (on the Canon 40D) is the AE-Lock button. There’s a special AF button that was probably put on the Canon 40D especially for this purpose, but unfortunately they left it off the BG2 battery grip, so there’s no point using it. To remap focus to the AE-Lock button on the 40D, you have to change two settings:

  • Adjust Custom Function IV, setting #1 to read “AE lock/Metering + AF start”
  • Adjust Custom Function IV, setting #2 to read “Enable”

The combination of those two settings will remove focus from the shutter button completely, and permanently map it to the AE-Lock button.

A typical shooting scenario now involves the following steps

  1. Place the center focusing dot on the subject
  2. Press the AE-Lock button to focus on the subject
  3. Press the shutter half-way down to meter on the subject
  4. Recompose the shot
  5. Press the shutter all the way down to take the photograph

Because the camera is now focused on the subject, future shots can be recomposed without the additional step of focusing (assuming the adjustments are minor). In addition, as long as the shutter button isn’t released completely, the exposure will remain locked on the subject. So in many scenarios, you could simply get away with step #4 and step #5 on all subsequent shots, allowing for a bit more spontaneity and reduced workload.

I still have to play around with this setup to ultimately see how I like it. But I just went for a 10 minute walk to test it out, and I have a feeling I’m really going to enjoy using my camera like this.

Here’s another good article about back-button auto-focus, although they kind of gloss over the metering aspects of back-button auto focus.

Update #1 – While writing this article I was asking myself why Canon didn’t included a custom function that would automatically lock the exposure the moment the shutter was pressed half-way down. This would make the focus/recompose technique easier since most times the exposure would be locked onto the same area as the focus point (which in like 95% of my shots, is probably accurate). I found a little blurb on a site that I thought was interesting: “With evaluative metering and one-shot AF the camera will lock the exposure reading at the same time as the AF locks.” So basically, if you set the camera into one-shot mode, and use the evaluative metering mode, the exposure will lock at the same time the focus does. Might be useful.