I am interested in how works AI SERVO in terms of metering. As it tracks objects, when is the exposure set and how can I set it manually? Is it set automatically when the shutter button is pressed?

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    Have you read the "AI Servo AF for Moving Subjects" on page 67 of your camera's manual yet? If not, why not? If you have, which bit of it was not clear? (No, I'm not going to copy and paste it here - you are expected to do some basic research before posting questions on Stack Exchange).
    – Philip Kendall
    Apr 3 '14 at 8:02
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    As for how to set it manually, well, you use manual mode. That's what it's there for.
    – Philip Kendall
    Apr 3 '14 at 8:03
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    As the manual says, it is set when the picture is taken. It don't see what else there is to say; focus has nothing to do with exposure.
    – Philip Kendall
    Apr 3 '14 at 8:13
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    @AJHenderson It is important to know that metering is tied to the AF point you or the camera selects. When you press the Shutter button halfway down to focus, the camera simultaneously meters the light primarily at the selected AF point to calculate the exposure. The AF point may or may not be the point of critical metering. If it isn't, then be sure to read the topics on modifying exposure later in this chapter, which explain how to compensate exposure or decouple metering from the AF point. - This is what I found in a manual :)
    – Morpho
    Apr 3 '14 at 14:54
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    Evaluative metering is context sensitive. The readings from each zone in the meter are compared to a database included in the camera's firmware and the nearest match is used to determine the proper exposure for a scene. For example, if the top 1/3 of the frame is very light and the bottom 2/3 are uniformly darker, the camera will probably assume you want the exposure to be more correct for the landscape than for the sky. If the top 2/3 of the frame is very bright the camera might assume you would rather expose more correctly for the sky.
    – Michael C
    Apr 4 '14 at 2:50

You seem to mix up focus features with metering features. You manual section on AI focus is assuming that you are in auto metering mode. it will keep tracking metering as well as focus in that mode, ASSUMING you have auto metering on. The metering mode, however, is set independently. You have your auto and semi auto modes. full auto, P mode (you chose between permutations of T and A, and then you have the "fix one, auto the other, Av and Tv, and you have full manual. (lets ignore ISO for simplicity). On top of that you have the asterix to lock an auto exposure. Oh, and you also have exposure compensation if you dont like the automatic one given the metering mode you chose "spot", mean, evaluative. Oh crap, there goes the simplicity. But the point is, if you leave any kind of automatic adjustment, the final adjustment is made as you snap the photo. No matter what focus mode you chose. You can even follow the changes it makes to exposure while tracking the focus in the viewfinder. And you can lock it with asterix.


The answer to your question as written is:

  • The exposure is set at the point the picture is taken, or put another way, when the shutter button is fully depressed. From previous questions, you've said you're using a Canon 550D; this information is available on page 67 of the manual.
  • You can set the exposure manually by use of manual mode on your camera; if all you need is an offset from the camera's exposure, you can use exposure compensation.

None of this is specific to AI Servo focus mode though - it applies in every focus mode, even manual focus. The only exception is if you are using One Shot focus and evaluative metering, when half-pressing the shutter button gives you exposure lock.

  • From the comments, I suspect this isn't the question you're trying to get an answer to, but it is the answer to your question as written. If it's not what you're trying to get an answer to, maybe you could try to clarify your question somewhat.
    – Philip Kendall
    Apr 3 '14 at 19:40
  • I suppose you wanted to say: ''the shutter button is fully pressed.'' ?
    – Morpho
    Apr 3 '14 at 19:55
  • No, I meant "depressed", the past participle of "depress", used in the sense of "to press down" (meaning 1).
    – Philip Kendall
    Apr 3 '14 at 19:59
  • My question is while the exposure is set after I have fully depressed the button, if it is not correctly exposed, how do I control it?
    – Morpho
    Apr 3 '14 at 20:06
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    The same way you'd control the exposure in any other focus mode - either with exposure compensation, or by switching into fully manual mode (including manual ISO).
    – Philip Kendall
    Apr 3 '14 at 20:22

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