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The default configuration for all auto focus SLRs is to enable both the exposure meter and the autofocus system when the shutter release is half pressed. For many users, this is an entirely effective way to work the camera.

However, many serious and professional photographers describe and advocate using a technique that decouples the AF from the shutter release and puts it on its own button. This technique goes by a number of names, but frequently it’s called rear button focusing or two button focusing.

What is Rear Button/Two Button focusing?

Rear button, also called two button, focusing is way of configuring your camera that decouples the activation of the autofocus system from the shutter release.

In their default configuration, Canon cameras will do a number of things when the shutter release is either half pressed.

1. Activate the Autofocus System
2. Activate the Metering system
3. In one shot mode, when the AF system has locked on target, lock the metering value

When the camera is configured for rear button focusing, step 1 and 3 are skipped when the shutter release is half pressed. Instead the camera just makes a meter reading and is ready to trip the shutter.

Why Use Rear Button Focusing?

Naturally, the next question might be why wouldn’t someone want the AF system to trigger when the shutter release is half pressed?

In many, if not most, situations, having the AF start when the shutter release is being pressed is actually useful if not desired behavior. However, there are many situations where the default behavior isn’t desirable. For instance, on Canon cameras, if the camera cannot attain focus, fully pressing the shutter release won’t trip the shutter. Moreover, if you’ve already focused and the camera starts hunting for focus, it will lose the focus position you set.

For example, when shooting a landscape or cityscape at dusk, in many cases you’ll focus once, when you compose the scene, then shoot many images as the light changes with the setting sun. As the light fades, it becomes harder and harder for the camera’s AF system to focus. Yet in many cases, refocusing isn’t desired at all.

Of course, an astute observer would note that you can deal with this simply by switching the lens from AF to MF, and the camera will no longer try to focus the lens.

The other alternative is to use the rear button focusing method. Moreover, I would argue that this is superior in general because it produces a more consistent mode of operating the camera — never mind some of the added features that more modern Canon cameras support.

I’m a big proponent of consistency in operating my cameras. This is in no small part because I’ve been bitten and lost images in situations where I did something out of the ordinary. For example, even before I started using rear button focusing, I almost never used my lenses in MF mode. Consequently, in the rare occasions where I’d switch them to MF mode to lock focus, I would usually forget to reset the switch on my lens (it’s not hard if you’re shooting a hour long sunset, then rushing to pack up after dark).

With rear button focusing, I always have to hit the button to focus. If I don’t want the camera to start autofocusing, I simply don’t hit the button. This makes for a very consistent way to interact with my camera. The choice of whether to focus or not is a distinct decision I make on every frame.

Moreover, Canon’s more modern and higher end cameras, such as the 7D series, the 5D series since the mark II (including the 5Ds and 5Ds R), and the 1D and 1DX series, offer the ability to configure a second AF point and mode that can be accessed immediately using a second button. You can, for example, have one rear button set to a single AF point in one shot mode, and a second button set to use the full tracking grid in AI Servo mode.

One Short or AI Servo AF?

Once you’ve decided to use rear button focusing, the next big configuration question is whether to use one shot (single AF) of AI servo AF. This is a consideration because when using rear button focusing, you control when the AF system is running. Consequently, you can stop focusing when you want to as well.

One Shot AF makes a lot of sense when you have the AF system tied to the shutter release. For static subjects, you don’t want the AF system constantly running and adjusting focus. Instead you want the camera to focus, and then stop. One Shot accomplishes this.

However, with the AF being done by the rear button, stopping the AF system when you’ve achieved focus is as easy as letting your thumb off the AF-On button. If you need to switch to a moving subject, you just keep the button pressed.

On my cameras, I run AI Servo AF all the time. My reason for this is that it gives me a consistent and predictable camera configuration in all situations, as opposed to having to reconfigure things and remember what I configured.

For example, I can be shooting a static landscape, and transition to shooting a moving bear or birds flying overhead simply by holding down the rear focusing button.

Effect on Evaluative Metering

One potential consideration to keep in mind, is that running rear button focusing can in some very specific circumstances have an impact on the camera’s metering.

One factor modern cameras consider in their metering calculation is to weight the reading under the in focus AF point. The logic is that if you’re focusing on a backlit subject, the camera knows that the thing in focus should probably be well exposed and not the bright area behind it.

When using rear button focusing, the AF system does not provide any information to the camera at all when it’s not active. Consequently the camera doesn’t know what metering area is over the in focus subject, and therefore to weight it more heavily.

In some circumstances, namely strongly backlit subjects, this can result in slightly different metering values than one would see under some conditions using the default configuration.

In my experience, these differences are most pronounced (though not necessary problematic) when comparing rear button focusing to the default configuration of One Shot AF on the shutter release. That said, I’ve also never experienced any real problems with these differences either.

Moreover, this this is all extremely complex, as interaction between Canon’s AF and metering systems changes depending on at least the focusing mode (One Shot or AI Servo) and focusing patter (single point, or multi-point). Consequently, depending on the configurations you’re comparing, the differences in metered readings will vary.

That said, I want to underline one critical point here. While there can be differences in metered readings, in all the years I’ve used rear button focusing, the differences have never been significant enough to have any real impact on image quality. More often than not, they’re on the order of a 1/3-stop. Moreover, in my experience at least, the benefits of using rear button focusing, vastly out weight the potential of needing to fix up a small exposure error in post processing.

Advanced Configuration

Canon’s more modern (since circa 2012 or so) higher end cameras (7D, 5D, 5Ds, 1DX, and EOS R), have even more advanced rear button focusing capabilities than their predecessors (and most of the competition too). With these cameras, Canon included the ability to register an AF point for later use separate from the normal active AF point.

For example, you might register a point on the left side of the grid, so that you can jump there immediately when changing compositions.

These registered points can be accessed either by configuring a button, such as the DoF Preview button, to switch to the registered point, or by setting up a second rear button focusing button that enables the second point.

For example, I have my cameras configured so that the AF-On button activates the primary AF point, and the AE Lock ✱ button activates the AF system with the registered AF point.

Further, most recent models support registering an entire AF configuration, not just an active point.

For example, the AF system could be configured to One Shot and Single Point AF, while the registered point could be AI Servo and Automatic selection AF. In this configuration, the second mode could be used to quickly deal with moving subjects, while the primary mode is used for more static or at least more precise focusing.

Configuration for this advanced mode is covered in the section on Type-3B cameras.

Camera Configuration Model Table

Canon’s UI and configurations have evolved significantly since their early DSLRs form the early 2000s. However, while things have changed, there are many cameras across a verity of product tiers and lines that use this same interface. I’ve chosen to group things by an unofficial type designation that reflects where the configuration options are stored and what options are available. This is in no way official Canon nomenclature, and because this is specified around the configuration of this specific feature it’s also not directly compatible with the nomenclature that I use in my auto exposure bracketing article.

Type 1 Cameras

Type 1 cameras represent the earliest cameras from Canon that supported decoupling the AF activation from the shutter release. These cameras do not have a dedicated AF-On button. Instead they use the Exposure Lock button (✱) for this task.

Configuring these cameras is done through the Custom Functions menu. Early cameras, prior to 2007 or so, used flat custom function lists, and the custom function you need is just Custom Function 04 Shutter/AE Lock button.

Post 2007, Canon started organizing custom functions into specific sub groups (i.e. autofocus, operation, etc..). For these cameras, the custom function that controls rear button focusing is the first item in the fourth menu (C. Fn. IV: Operation/Others). However, in many of these cameras, and in their manuals, the custom function may still be numbered sequentially. So C. Fn. IV–1 may also be custom function 6, 7, 9, or 10.

In any event, the name of the function is still Shutter/AE Lock button.

Shutter/AE lock button options

There are 4 options numbered 0 to 3. Canon’s nomenclature is to use a slash (/) to separate the shutter release from the exposure lock button (i.e., shutter release function/exposure lock button function).

Option 0 AF/AE Lock: is the default. This is the standard operating mode where the shutter release does both AF and exposure metering. Note, most of these camera manuals, Canon will say “AF” for the option, but exposure metering included.

Option 1 AE lock/AF: In this mode, half pressing the shutter release will engage the AE Lock, and the AE lock button will do AF and metering. The difference between this mode and option 3 is that in this mode, the AE lock button will behave like the normal half press of the shutter release, and half pressing the shutter release will behave like normally pressing the AE lock button would, i.e., it will lock the metered exposure.

Option 2 AF/AF lock, no AE lock: AF Lock is another configuration option that Canon offers for operating it’s AF system. The AF lock stops the AF system when the camera is shooting in continuous mode. For example, hitting AF lock while panning would stop the AF system from trying to adjust focus to an intermittent obstacle.

If this option is chosen, the camera will behave similarly to how it would in the default configuration. However, the AF system will be deactivated when you hold the AE Lock (✱) button.

Option 3, AE/AF, no AE lock: This enables rear button focusing as described in this article. Half pressing the shutter release turns on the exposure meter but other wise doesn’t do anything else. Pressing the AE Lock button will start the AF system. When you release the AE lock button, the AF system will be turned off.

To enable rear button focusing as described in this article, set this custom function to option 3.

Type 2 Cameras

What I’m calling Type 2 cameras are primarily characterized by the addition of an AF-On button in addition to the AE Lock button. These cameras do not require you to give up the AE Lock functionality to enable rear button focusing.

Configuration for these cameras remains very similar to post 2007 manufacture Type 1 cameras, only there’s an additional custom function that can be set to switch the operation of the AF-On and AE Lock buttons.

Since all of these cameras post date the 2007 change over, they have custom functions broken down into sections. For these cameras, the options you’re looking for are C.Fn IV–1 Shutter button/AF-ON button and C.Fn IV–2 AF-On/AE lock button switch.

Configuration options for Shutter Button/AF-On button are vague similar to the options for type 1 cameras.

Shutter Button/AF-On button Configuration

Option 0 Metering + AF start: This is the default configuration for the camera. When this option is set, half pressing the shutter release or pressing the AF-On button cause the camera to behave the same way; the meter starts and the camera begins focusing.

Option 1 Metering + AF Start/AF stop: This mode behaves the same way as option 2 does on type 1 cameras. Half pressing the shutter release causes the camera to behave as it would normally. Pressing the AF-On button causes the AF system to stop running locking the focus position. Releasing the AF-On button causes the camera to resume focusing.

Option 2 Metering start/Metering + AF Start: This option enables rear-button focusing as I’ve described in this article. Half pressing the shutter starts the exposure meter but otherwise does nothing. Pressing the AF-On button causes the camera to meter and AF.

Option 3 AE Lock/Metering + AF Start: This mode behaves similarly to option 1 on type one cameras. The AF-On button replaces the normal functionality of half pressing the shutter release, and the shutter release assumes the functionality of the AE lock button.

Option 4 Metering + AF start/disable: Choosing this option disables the AF-On button on the rear of the camera.

AF-ON/AE lock button switch

There are only two options here, disable or enable. Enabling this causes the functionality of the AF-On and AE lock buttons to be reversed.

One example of a place where this is useful is when shooting a camera like the 40D, 50D with a battery grip that doesn’t include an AF-On button.

Type 3a Cameras

What I’m describing as type 3 cameras are cameras where Canon has moved from using a custom function to configure 2-button/rear button focusing to a control configuration menu.

There are two ways to access the control configuration screen on these cameras; via the Custom Functions menu, or via the quick control screen (the [Q] button).

Configuring these cameras is somewhat different than the Canon’s older models. Instead of setting the behavior of the shutter release and AF-On button in a combined setting, you have to set them for each of the controls independently. This offers a lot more control and flexibility.

In the custom control menu, the 2 entries you want to focus on are the entry for the shutter release and the entry for the AF-On button.

For the shutter release, you’ll have the option of selecting 1 of 3 options:

1. Metering and AF Start
2. Metering Start
3. AE Lock While pressed

For the AF-On button, there will be up to 7 options.

1. Metering and AF start
2. AF Stop
3. AE Lock/FE Lock
4. AE Lock
5. AE Lock while held
6. FE Lock
7. Off

For rear button focusing, you’ll want to set the shutter release to metering start, and the AF-On button to Metering and AF Start.

EOS M3, M5 and M6

The EOS M series is a bit of an oddity in this system I’ve created here. The menu system is much closer to that of the Type 3b cameras than the Type 3a cameras, but the actual configuration is much closer to the type 1 and 2 cameras.

Enabling rear button focusing can be accessed through the camera control menu which can be accessed either though the menu system on the C.Fn III-1 in the menu system. Or through the full screen quick control menu (press Info until you no longer see the live view image and instead see shooting information, then touch or select the custom control menu option).

However, like the earlier cameras, the shutter release and AE lock buttons are tied together in a single entry in the menu. Change this from AF/AE lock to AE/AF, no AE Lock to enable rear button focusing.

Type 3b Cameras

There are two factors which I used to distinguish between what I’m calling Type 3a and Type 3b cameras. Specifically these are yet another change to the meaning system, and the added capability of registering an AF point that differs from the point that’s selected in the normal display.

Like Type 3a cameras, these cameras are configured via a custom controls screen that’s either accessed via the quick control menu ([Q] button) or through a custom function page. The difference here is minimal, but Canon has changed the organization of custom functions into a series of separate screens instead of sub menus.

Like the Type 3a cameras, there are a number of options that can be selected for the shutter release, AF-On button, and potentially the exposure lock button.

To configure rear button focusing, in the custom controls menu set the shutter release to metering start, and the AF-On button to AF and Metering start.

That said, what really sets most these cameras apart is the advanced configuration that they allow.

Configuring Advanced Rear Button AF Point Options
(7D mk. II, EOS R, 5D mk. IV, 5Ds [R] 1DX, 1Dx Mk. II)

Some of these cameras, support saving an entire autofocus configuration, not just a point. This includes the AF start position (normal or registered AF point), AI Servo AF characteristics, AF mode (one shot or servo), and area selection mode (e.g. whole area, partial area).

By using these configuration options, you can quickly switch from a single point single shot AF setup to a full tracking AF setup using the entire AF grid by just moving your thumb between two adjacent buttons.

Additionally, the camera can still be configured to preform AF with the half press of the shutter release. In conjunction with these overrides, you can have 3 different AF configurations that can be immediately accessed while shooting.

These options are configured on a per button basis by pressing the Info button while configuring the buttons function. The detailed settings menu offers a number of options.

AF Start Position: Controls what AF point the camera uses when this button is pressed. If set to the first option (SEL), it will use the normal AF point. If set to the second option (HP) it will use the registered AF point.

AI Servo AF Characteristics: Allows the button to be configured to use a different AI Servo case than the currently selected one. For example, you could set a rear button to use Case 5 (for erratic subjective moving quickly in any direction), while the camera is normally set to case 2 (continue to track subjects, ignore possible obstacles).

AF operation: Allows you to select whether to force the rear button to use One Shot or Servo AF, or use the currently selected mode.

AF area selection mode: Allows you to override the AF area (e.g., single point or area), or use the currently selected mode.

Interested in more details about camera configuration? You might be interested in reading about auto exposure bracketing settings.