Let's say I try to take a picture of the following scene: a nice, blue river on the bottom and a solid, grey building in the middle. It is a sunny day and I am in a large distance, so both "objects" are in the same distance for me. When I focus on the river (80% of the picture contains the river now) — I hold the trigger so the picture remains focused on the river — and I move upwards on the building (still holding the trigger), the picture is generally lighter (or differently lighter) than when I do the same vice versa; that is, focus on the building first and then move the picture downwards on the river (then, the picture is darker).

What causes this issue and how can I intentionally control this behaiviour?

My camera is Olympus OMD EM10. I use autofocus.

  • 2
    What exposure mode and metering mode are you using? This can have an effect on the point at which exposure for a frame is locked in.
    – Michael C
    Apr 10, 2016 at 7:21
  • 1
    I use the Aperture priority and ESP metering system
    – user52107
    Apr 10, 2016 at 13:19

4 Answers 4


It sound like your camera is locking the exposure in addition to the focus when you perform a half shutter press. This would explain the overexposure when moving from a dark river to the bright sky: the camera is set to expose a dark scene properly, but it then gets pointed at a brighter scene, and subsequently over exposes. The converse is true as well, moving from a bright area of focus/exposure to an overall darker one will underexposed the image.

From what I can quickly tell on mobile, you can set one of your function buttons to AEL, which stands for auto exposure lock. Generally the way this button works is you point the camera at the object you want to properly expose, press and hold the AEL button, and then recompose to take the picture. Your manual or a Google search should provide specifics on how it works.

In your example, I would frame your final picture and lock the exposure, via AEL, so the overall image is properly exposed. Then focus on the river or building, and finally recompose to your original framing to take the photo.

In a situation like the one you describe though, generally virtually moving the focus point over the object of interest is the easiest option so that you can focus and expose in one step.

  • Most cameras will update the exposure setting each time the exposure lock button is pressed. So he only needs to focus first, recompose, then press the AEL button to reset the exposure and then take the picture.
    – Michael C
    Apr 10, 2016 at 7:24
  • 1
    The other easy way to do this is to set the half press of the shutter to not initiate AF. Use the back button for AF On. Focus by pressing the AF-On button. Then hold it down (One shot AF) or release it (Continuous/Servo AF), recompose the scene and press the shutter to take the photo. The camera will not refocus when you press the shutter button, it will only meter.
    – Michael C
    Apr 10, 2016 at 7:27
  • Choosing spot metering mode (if available) can help specifying the exact part of the image that the exposure should metered at.
    – null
    Apr 10, 2016 at 11:25
  • If the "exact part of the image" is of medium brightness in Zone 5.
    – Michael C
    Apr 10, 2016 at 16:16

This happens because (by default) a half-press on the shutter not only sets the auto-focus, but the auto-exposure as well. If you don't want to set the exposure or focus manually, you can set one of the function buttons to activate auto-exposure lock ("AEL"). With this setup, you would point the camera at the object you want to expose for first, push the AEL button, then use the half-shutter-press to auto-focus as you normally would, allowing you to recompose if you want to before releasing the shutter.

  • It doesn't always happen by default. In many cases cameras don't do it unless you are shooting in aperture priority exposure mode and using a specific form of metering such as Canon's Evaluative or Nikon's Matrix metering.
    – Michael C
    Apr 10, 2016 at 16:18
  • To clarify, "by default" on Olympus cameras, which the question is about, in the modes for which it applies (which the question appears to be about). There are actually options for altering the half-press behavior for those modes deep in the settings. Apr 10, 2016 at 19:19
  • The main question is more general and doesn't mention a particular camera line, much less model. The mention of the particular camera model in the last line of the explanatory body doesn't necessarily mean we should write an answer applicable to only that one model without making it very clear in the answer that it is for only that one model. The way we've chosen to structure photo.stackexchange is to write answers that are applicable to the maximum number of readers who have the same question: "Why does holding the focus and recomposing also affect the brightness of my photo?"
    – Michael C
    Apr 11, 2016 at 7:29

Your camera is locking both focusing and metering with shutter button half-press - focus lock is called AFL, metering lock is called AEL.

It will be most convinient for you to setup your camera to not lock metering with half-press.page from E-M10 manual

You do not need to use additional buttons for that at all despite other suggestions.

  • 3
    I don't think the other answers are suggesting that you have to use other buttons. They're saying the same thing the page you've duplicated here says: in the modes where half-press of the shutter does not lock exposure, you can use the button labeled AEL/AFL to get that functionality.
    – mattdm
    Apr 10, 2016 at 12:14
  • @mattdm: unfortunately, they do. OP asks about preserving needed exposure and picking focus somewhere else, two other answers suggest to frame the photo, lock the exposure using second button and focus afterwards. Too much of a hassle. Apr 10, 2016 at 12:44
  • 3
    Well, the question also asks "how can I intentionally control this behavior?". I agree that if the metering for the final scene gives the desired exposure, there's no need to make it a three-step process.
    – mattdm
    Apr 10, 2016 at 12:49
  • There's always more than one way to skin a cat. +1 for illustrating very well that it depends on the selected exposure modes whether the camera behaves this way or not. But efficient use of the back buttons is much quicker and fluid without requiring removing one's eye from the viewfinder, much less digging through menus, if the shooter desires to rapidly switch back and forth between both methods (locking exposure with focus vs. not locking exposure with focus) alternately during the same shoot.
    – Michael C
    Apr 10, 2016 at 16:24
  • @michael-clark: sure. I did not suggest taking away the ability to switch between behaviours - one may still lock exposure at the moment of focusing with holding AEL button (it may be sticky or not BTW). However, it is natural for me to suggest that question author will rarely need to meter in the position which he only uses for focusing and he will not switch this behavior back in any forseable future. Shutter button half-press neither locks the metering nor activates the focusing on my camera and I can't remember the moment when I set it up so. If I ever need either I will use backbuttons. Apr 10, 2016 at 16:48

Another trick, which avoids changing settings but is easier with a tripod is to autofocus, release, flick the switch on the lens to set manual focus, then recompose and shoot.

I sometimes do this, or occasionally change focus point, but I prefer a half press to lock the exposure as well as the focus (if the background is lighter than the subject it's often a white backdrop that will eventually be blown out and cropped or made transparent).

  • Removing AF activation from the shutter button and using a back button to focus instead is much simpler and efficient - and you don't risk prematurely wearing out the Auto/Manual switch on your expensive lenses.
    – Michael C
    Apr 10, 2016 at 16:26
  • @MichaelClark I still use my old 350D in work. It doesn't have buttons to spare (I think the function I'd lose is focus point selection) or expensive lenses. But I keep the same setting on my other camera to avoid confusion.
    – Chris H
    Apr 10, 2016 at 16:44
  • I think you can remap the AE-L button to function as an AF-ON button. At least on later Rebels you can.
    – Michael C
    Apr 10, 2016 at 16:50
  • Even on the extremely compact E-PL6 body, you can map several of the (relatively few) buttons to useful things like toggling between auto/manual focus, Olympus is pretty good about this. Also, AFAIK, all auto-focus Micro Four Thirds lenses are focus-by-wire, so don't tend to have the switch on the lens. Apr 10, 2016 at 19:32

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