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I'm confused on how AE-lock works. I was advised and also tried this. Walk into a room point at what I want to capture, lock the exposure with AE-lock. Re-compose and take the shot. In theory the shot should have the same exposure as when I locked it. I don't see this.

For example, I went into a dark room and found some light. I metered the light and my Shutter was at 1/200. To see if it was right I metered the dark spot and it was 1/60. I AE-Locked the 1/200 from the light, locked it, re-composed and the shot came out dark. My camera focused again while I pressed the shutter all the way down and took whatever shot it wanted. I was in Av mode with a fixed ISO.

What happened?

Related questions

What's the difference between holding a half-pressed shutter button and pressing the AE lock?

What are typical situations where you should use auto-exposure lock (AE-L)?

This question is more about: Why didn't AE-lock keep the exposure that I metered from and focused again before pressing the shutter all the way down?

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  • What shutter speed did the camera actually use for the shot?
    – Michael C
    Sep 4 at 10:39
  • @MichaelC I’ll reproduce when I get home. Or look at the photo in question
    – JcbJoe
    Sep 4 at 17:29
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The AE-Lock function simply locks the metered exposure. Cameras generally have two modes of operation for this feature: Press-and-Hold or Press-to-Toggle. Depending on your camera, either one could be the default but this can be configured on most but not all models.

First, be sure you actually used AE-Lock. Sometimes you can override the button to do something completely different. You should notice in the status line that the exposure-values stop changing. Many cameras will show a AE-L icon. Canon uses an asterisk for this. If you were in Aperture-Priority the shutter-speed should not change after activating AE-L but it probably did before.

If you press the AE-L button and the exposure does not change while you recompose, then you did it. You have used the AE-L function. If after looking at your photo, you notice the resulting image to be too dark or too bright, it simply means the metered exposure was incorrect. It is perfectly normal that this could happen. Consider the Metering Mode selected. Depending on the mode it may consider the whole or part of the scene. It is easiest to understand with Spot metering. The spot should be exposed to be a mid-tone gray. After you recompose, that spot which is now in a different location will be mid-tone grey but that guarantees nothing about the rest of the scene. With Multi-Segment metering it gets more complicated but the same principle applies.

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  • So I have a canon 80D I turned off back button focus cause I really didn’t know what I was doing. So I focus and shoot with the shutter button. I metered the light in the dark room. Did the asterisk re-composed then took the shot and it refocused before firing. Did I do something wrong or was I suppose to keep holding the asterisk?
    – JcbJoe
    Sep 4 at 2:58
  • On Canon cameras the default is to toggle, so as long as you see the asterisk, the exposure is locked. The metered shutter-speed should stop changing which confirms this. If you still see it changing, then you are in the mode that you need to hold it. It's been a while since I had an 80D in my hands, so I don't remember where this is configured. Again, confirm by checking that exposure no longer changes.
    – Itai
    Sep 4 at 3:17
  • @JcbJoe The AE Lock button on Canon cameras (by default) only locks exposure, it does not lock focus as well. If focus changes when you point the camera in a different direction, you're using 'AI Servo AF' or (perish the thought) 'AI Focus AF' which will shift from 'One Shot' to 'AI Servo' if the camera detects a change in subject distance after having locked focus. To lock focus when recomposing, simply use 'One Shot AF' and hold the shutter halfway (if a half press initiates autofocus) or hold the AF-ON button (or whatever, if any, button has been remapped to be the 'AF-ON' button).
    – Michael C
    Sep 4 at 10:37
  • Don't forget that by default, in 'Evaluative Metering' mode with Av Exposure mode AE does not lock on the shutter half press and metering remains active until the shutter button is fully pressed. In such a case, you must press the AE-Lock button to lock in metering before recomposing if you want to preserve the initial exposure calculation.
    – Michael C
    Sep 4 at 11:00

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