The way I shoot is this:

  • turn the camera so that the subject is in the middle of the viewfinder, where my center point is,
  • press the shutter half way, the camera now focuses the lens on the subject and changes exposure parameters (e.g. shutter speed),
  • I recompose, holding the shutter half way (turn the camera so that the composition looks right to me),
  • take a picture (press the shutter as much as possible).

How else can you take a picture? I've heard of the AE-L/AF-L button and while I think I know what it does, I don't know how and when to use it. Also, why would you want to decouple focusing and exposure metering? When I want something focused, I also want it to be properly exposed. Are there other use cases?

I've recently read this:

The AE-L/AF-L button is placed similarly as on the D90 – further away from the rear rotating dial. I wish it was closer or there was a dedicated AF-ON button like on the pro-level bodies, because I usually move the focusing action from the camera shutter to a dedicated button (for focusing and recomposing shots).

So what is he talking about and what are some other ways to shoot and when should you use it?

  • \$\begingroup\$ Most cameras let you set the focus point. I imagine yours does that, too... so you wouldn't have to use the center point for focus, but one in the right 1/3 or left 1/3, or wherever your subject is. I find this is usually more cumbersome than helpful, though, as I don't always know where my subject will be until I'm ready to take a shot--and then it's easier to use the center point rather than reconfigure the camera for a different one. \$\endgroup\$
    – Flimzy
    Apr 23, 2012 at 9:09
  • \$\begingroup\$ Also, non-center points are worse (than the center one) for focusing. Or so I read online. So I'm gonna stick to the center point. \$\endgroup\$
    – duality_
    Apr 23, 2012 at 9:22
  • \$\begingroup\$ That depends on the camera model you have. For most sub-pro camera models, that's probably true. I believe the Canon 7D, for instance, has the "super awesome good cool neato" auto-focus on several (but not all) of it's focus points (the center included). The cheaper Canon cameras (like mine) have even fewer (or just the center one). So yes; safe bet is to stick with the center one. \$\endgroup\$
    – Flimzy
    Apr 23, 2012 at 9:28
  • 3
    \$\begingroup\$ @duality_: It's true that the center point is most sensitive ("best") and others are less sensitive ("worse"), but that doesn't mean you shouldn't use them. Experience tells me I can often use any AF point I want and get the focus just as I want; experience also tells me when I'm likely to have trouble and need to use the center point for a higher success rate. Don't be afraid to use all of the points to understand how well (or poorly) they work for you. \$\endgroup\$ Apr 24, 2012 at 2:10
  • \$\begingroup\$ @DanWolfgang you're right, use the right tool at the right time. \$\endgroup\$
    – duality_
    Apr 24, 2012 at 9:20

3 Answers 3


You lock exposure when:

  1. Something very bright or dark close to the subject is throwing the metering off - you can get close so that the bright spot is out of frame (or at least far enough from the center so it doesn't effect metering), lock exposure, move back into position, focus and shoot

  2. The subject itself is very dark or bright - you can lock exposure on a gray card or another medium brightness object than remove it and shoot the subject.

  3. You don't want to expose for the subject - for example for sunset photos you may want to expose for the background and add flash to light the subject

  4. You can't use the center focus point but you are still metering for the center - if you have a very shallow depth of field (for example, with a 50mm f/1.8 wide open at close range) recomposing will throw your subject out of focus

Personally, I prefer to switch to manual mode in those cases instead of using exposure lock.

If you find yourself in those situations often it makes perfect sense to have focus and exposure on different buttons (that's what "back button focus" or AF-ON is for) since if you already meter and focus separately why should you bother with always metering then focusing and keeping buttons half pressed.

  • \$\begingroup\$ I'm with you: switching to manual mode always seems easier than using the exposure lock. \$\endgroup\$ Apr 24, 2012 at 2:12

The technique you use called Focus and Recompose, as @MikeW said, is most common because it leaves autofocus fully functional and is very quick to use.

With the AF-On feature, autofocus by the shutter-release is disabled which means you must consciously decide where and when to focus each and every time. It is more deliberate but under pressure, there are more chances you will miss a shot.

The focus-point selection method, usually involving the 4-way controler or joystick (on cameras with one) is slowest because you have to move the focus from point-to-point until it is at the desired location. This technique though is most useful when shooting from a tripod where it much less practical to move the camera so that the center-point is where you want the focus to be.

  • \$\begingroup\$ I might be misreading your second paragraph, but at least with the Canon bodies I use you can still allow shutter half press to begin AF, even if the AF-ON button is enabled. If the AF-ON button is already pressed and being held down when the shutter button is half pressed, the AF-ON button takes precedence. (Assuming the focus mode is 'One shot' the focus already locked will be held and the half press of the shutter will not cause the camera to re-focus. Assuming the focus mode is 'Servo' the camera will continue to track whatever was under the focus point when the AF-ON button was pressed.) \$\endgroup\$
    – Michael C
    Jan 8, 2014 at 0:10

What you are doing is called "focus and recompose". It's probably the most common way of locking focus in use.

There are two ways to use the AE-L/AF-L button on a D90. For both you need to adjust custom setting f4 in your menus.

The first way is to set it to "AE/AF lock" or "AF lock only". Once you have focused in the normal way, using a half presss of the shutter, if you then hold in the AE-L/AF-L button it will lock focus. If you selected "AF lock only" then if you continue to half-press the shutter release, it will continue to meter and adjust exposure, but focus will be locked.

The second way is to set the menu option to AF-ON. This disables focus when you half press your shutter, but instead the camera will focus only when you press the AE-L/AF-L button. Takes some getting used to, but I find it easier when I separate focus from the shutter release.

A final way to focus is to frame your image, then select the focus indicator in your viewfinder you want the autofocus system to use.

I don't think the above lend themselves to particular situations, they all accomplish the same thing - you just need to find what seems easiest or most natural to you and use one of hte methods. I use the AF-ON option, but it takes getting used to. You would use one of these methods anytime you have time to carefully set focus - so in fast sports for instance you may have to let the camera detect motion and choose focus, to get your shot.

  • \$\begingroup\$ This is what I usually read online, but I don't know in what situations do people use these other styles. \$\endgroup\$
    – duality_
    Apr 23, 2012 at 9:30
  • \$\begingroup\$ I would use one of the two methods all the time, for virtually every shot. Anytime the subject isn't moving too fast, like birds in flight or fast sports, where I want to nail the focus, then compose my image. \$\endgroup\$
    – MikeW
    Apr 23, 2012 at 20:27

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