How and Why I Use the Back-Button AF
In search of a good learning topic, I thought about the camera settings and shooting habits I’ve grown to like and use over time. One of the major ones is how I use the autofocus system and the various buttons on the back of my camera which control autofocus.
The AF-ON Back Button
I only learned how to fully use my Canon 7D’s additional autofocus lock button, labeled “AF-ON”, after reading the article Canon published called, “Back-Button Auto Focus Explained” (it’s well worth a read).
When I got started, I depended solely on the shutter button and its half-press. I would lock the metering and focus by holding the shutter half-way and then recompose my shot. Then I’d follow the same steps shot after shot, even if nothing changed. That wasted time and slowed the camera down. The constant re-metering and re-focusing really became obvious in burst mode. If you hope to reach the 7D’s blazing fast 8 frames per second, you need to lock things down.
The rear AF-ON thumb button lets you lock in your metering and focus point in advance of releasing the shutter. While holding it down, the camera will not try to re-meter or re-focus.
The Canon articles goes so far as to sugget removing the focus lock from the primary shutter button but I don’t recommend that. If you do that, you can forget about sharing your camera with anyone else as they won’t have a clue how to focus. All you need to do is hold down the “AF-ON” button to keep the lock. The shutter will never try to re-meter or re-lock while you hold it down.
As the Canon article points out, the advantages of using the back-button autofocus are:
- Easier to lock focus
- Easier timing of shots
- Less risk of focus errors with moving subjects
- Easier overriding of autofocus with full-time manual focus
- Easier macro and close-up focusing
See the article for the details and examples.
Tip for Canon T2i Owners
The Canon 7D and 60D both have a dedicated “AF-ON” button while the Canon T2i does not. Don’t despair however, as the auto exposure lock button (indicated with a start icon) can be customized with a custom function.
- 0: AF/AE lock
- 1: AE lock/AF
- 2: AF/AF lock, no AE lock
- 3: AE/AF, no AE lock
I would suggest #2 so you have autofocus lock on both the shutter and the AE lock (*) button. Just hold down the AE Lock hold the focus across multiple exposures.
Using a Single AF Point
The need lock focus really comes into play when you use a single autofocus point. Trust me, I love the 7D’s 19-point auto-focus and it’s AF zones, it’s a top-of-the-line system. However, no computer chip can guess what you want to focus on exactly.
When your depth of field is measured in millimetres, as it can be with a fast prime lens, there’s simply no room for error. You pretty much have to tell the camera exactly what you want in focus (e.g. closest eye of your subject). This definitely applies to macro photography.
The only way to precisely autofocus your camera is to place your camera in its single AF point mode or “spot AF” mode which is even more precise.
To enable this on the Canon 7D, press the AF Mode button first…
And then press the multi-function button (M-Fn) repeatedly to cycle through the modes until you see one point lit up. Once lit, use the joystick to select the point you want to use.
Alternatively, you can do the same thing by select the Quick Menu (Q) and editing the autofocus mode. You’ll still need to use the multi-function button to cycle through the AF modes.
Enable Spot AF
I typically use a single “Spot AF” point (appears as a filled-in square). It gives the camera an even smaller area to lock onto which can’t hurt given how I’m using it. This would not be the right setting if you’re tracking a moving subject in AI Servo mode but it’s great for locking in on someone’s eye.
On the 7D, the Spot AF option is not enabled by default. Edit custom function III: Autofocus/Drive, option 6 “Select AF area selec. mode” as shown below.
AF Point Based on Camera Orientation
Another nifty feature of the 7D is its ability to have a different autofocus setting based solely on the orientation of the camera. This feature must be enabled in a custom function menu once again. This is in the same custom function area but head over to option 12 “Orientation linked AF point”. Choose “1: Select different AF points” as shown below.
Now, when you rotate your camera into portrait orientation, you can go through the same AF point selection steps and it will remember it separately.
When I use the single AF point, I rarely use the center AF point which is why this matters to me. I use one or two up from center which is where my subject’s face will be. When I turn my camera, that point is on the left which is useless. Thanks to this feature I can have the selected AF point be high and centered in portrait mode automatically.
Bonus Hidden Feature: Setting Two AF Points Simultaneously
Warning, this is an advanced topic with a good number of steps. So many steps that I call it a hidden feature.
There are three buttons for your thumb on a Canon EOS 7D and 60D:
- Autofocus Point Selection / Zoom In
- Auto Exposure Lock (*) / Zoom Out
- Autofocus-On (AF-ON)
If you don’t have much use for a dedicated exposure lock button (highlighted above), you can have it be dedicated to a “registered AF point”. That’s yet another AF point which you can invoke at the push of a button. This isn’t just for single AF points, it applies to all the AF modes other than the 19-point AF.
Say for example you’re in a situation where you need to go between two subjects. You could set one AF point or zone on the left side while having a second “registered” point on the right. Using only the AF-ON and the AE Lock (*) buttons, you could get your camera to quickly hop between those two points. This is also useful to tracking moving subjects if you wanted to activate the left or right zones quickly based on how the subject might enter the frame.
Hidden Feature Steps
Here’s how to re-program the AE Lock button and register your first AF point:
- Press (Q) to enter the quick menu
- Select the Custom Controls menu (bottom-right)
- Scroll down to the AE Lock start icon (*) and press Set
- Use the wheel and select the “Metering and AF start” option
- Now press “Info” to get a submenu for AF Start point
- Use the wheel and select “Registered AF point”
- Hit Set to save your changes
These screens might help:
Now that the button can invoke a registered AF point, it’s time to register one. Turns out that instructions for doing this is buried deep in the manual. Refer to page 219 of the Canon 7D manual if you’re keen but here’s a summary of the steps:
- Press and release the AF Mode button
- Press the multi-function button repeatedly until you’re in single point AF, spot AF or AF point-expansion (i.e. not Zone AF or the full 19-point AF)
- Use the joystick to select the point you want
- To register the point, press and hold BOTH the AF Mode button and the light button
Wow, was that obscure enough? The camera will beep and the selected point will flash.
Now when you look through the viewfinder, you’ll see a very small dot representing the registered AF point. The standard AF point must not be the same point or won’t see it.
Time to try it out. If you press and hold the AE Lock (*) button, your camera will immediately switch to the registered spot. Let your thumb bounce between “AF-ON” and the AE Lock “(*)” button to quickly change focus points. Pretty nifty, eh?
Remember that this isn’t just for the various single point AF modes. If the registered point is in the right-hand zone while your left-hand zone is selected, you can jump between the two zones in a single button press.
One final note: if you want to de-register that AF point, follow the same steps as above but press and hold BOTH the AF Mode and ISO buttons. Again, not very obvious.
If that wasn’t enough, checkout these two additional Canon articles:
I hope you found this summary useful. Let me know if you use the “AF-ON” button as I do. Have you tried the registered AF points? Please share any options or techniques you find useful!