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I just started experimenting with spot metering + exposure compensation. I'm wondering if it's possible to make adjustments during post processing to correct the metering. I'm pretty sure (correct me if I'm wrong) this is not the same as adjusting the exposure during post processing.

For example, I try to meter off of someone's face and find that it is a little washed out (overexposed). I then increase the shutter speed a bit and take the shot again, repeating the process until it looks like I got the metering right. Can I just adjust the exposure compensation after the fact? If so, what should I be looking for (in programs like Aperture and Lightroom)?

mattdm
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Tom
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3 Answers3

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If you're shooting RAW, you may have a little (or alot on some newer sensors) of wiggle room with your exposure. But, how your camera metered to measure the light to expose the scene in camera doesn't mean ANYTHING after you take the picture. Spot metering is just how the light meter in your camera thinks it needs to expose the current scene (and for spot in particular, just the specific, very small place you indicate). You can check the exposure of your scene after the shot by reading the histogram and your camera may have an option for blinking blown highlights (way over exposure and unlikely to be recovered).

Fixing this is just the "normal" exposure adjustment during post processing. While you may have some wiggle room when shooting RAW, its always better to get it right in camera. You can darken and lighten the exposure in post with Lightroom using the "Exposure" slider for the whole picture, or selectively in areas by using the "adjustment brush". Adjusting too much this way is likely to lead to ill results though, such as artifacts or loss of contrast.

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rfusca
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  • Wow, I feel silly. I definitely misunderstood what metering was for. For some reason (probably because of the way it was explained to me) I was under the impression that it affected other characteristics of the color in the photo. I was getting confused because it seems like the color of the "spot" changes the way the picture looks. For example, with gray I need to use a "proper exposure" whereas with black I need to stop down the exposure. I realize this just has to do with the amount of the light the camera is throwing at that spot. Is that right? I need to read more about histograms too. – Tom Jan 20 '11 at 13:59
  • Do you have any suggestions for how I can make this a better question for the community? It's a bit misleading right now since I didn't understand metering. – Tom Jan 20 '11 at 14:00
  • You're not wrong. Metering does indeed affect the "color" (both luminosity, and, to a lesser extent, saturation) of your picture, though perhaps less directly than you're thinking. Metering attempts to set the luminosity (brightness) to a "correct" (balanced) value: typically what's known as "18% gray. With spot metering, the camera only considers a tiny area, as opposed to averaging the entire picture. If you spot meter a black surface, the camera will increase the exposure so that it becomes middle-gray... possibly overexposing everything else. – Craig Walker Jan 20 '11 at 15:36
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    I wouldn't change the question, I think its useful as it sits, just maybe not as you intended. – rfusca Jan 20 '11 at 15:39
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    Craig is right, your camera is still doing the "right" thing, exposing for where you point it. It just considers only a very small area and exposes that to grey. As the amount of light entering affects your colors to a certain extent, it changes EVERYTHING. It sounds like you just need to learn how to spot meter in general. That may be a question worth asking on here separately. – rfusca Jan 20 '11 at 15:41
  • @rfusca, thanks. I did find your answer helpful (why I +1'ed it the other day)... but I feel like Craig's answer is more clear and closer to what I was looking for (even though it has less votes). I am going to accept that one. If I could accept a second one, I would accept yours as well because I think both answers do a good job of telling the whole story. – Tom Jan 22 '11 at 18:29
  • The Stack Exchange model has the upvote/accept split for a good reason; it allows for the difference between "a good, useful answer to the public" and "the answer that helps the asker the most". The latter is worth far less than the former rep-wise, so don't feel bad about accepting a non-highest answer. It's all good. :-) – Craig Walker Jan 22 '11 at 19:20
  • @Tom - No worries. They're both largely the same. – rfusca Jan 22 '11 at 20:44
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There's only one exposure level (ie: amount of light) per picture; spot metering just means that the "target" level is based off of a particular area rather than the average of the whole frame (or some fancier means such as matrix).

Another means of setting the exposure level is through your manual controls (shutter, aperture, ISO). If you're changing your shutter and getting a new (non-washed-out) exposure level, then it sounds like this is what you're actually doing -- not spot metering. (If you were changing your shutter and still getting a similar exposure, then you'd probably be in aperture priority mode, and the camera would be compensating; this could still be using spot metering.) Note that you can base your exposure off the results of a spot meter, but the minute you change your dials you're effectively in manual mode.

You can change your effective exposure in post-processing, and get similar results as changing it in-camera. Of course, the results may not be exactly the same: shutter speed will change your blur; aperture will change your depth of field, ISO will change your noise, post-processing could introduce all sorts of artifacts.

The nice thing about post-processing programs is that they will let you selectively adjust exposure. You could change just part of your image area, or you could change the contrast, or change the brightness of one particular color. And, of course, you could undo and redo your changes without taking a new picture.

Lastly: note that if you overexpose your shot in-camera too much, you'll "blow out" the brighter part of your exposure and lose detail. This is difficult-to-impossible to fix in post production. It's also particularly a problem with digital imaging; film works differently in this respect. Most higher-end digicams will have a "blinking highlight" function that will show you when & where you've done this.

Craig Walker
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  • I was using manual mode. From my understanding, spot metering is best with manual mode. When you use a priority mode, the camera is choosing the "correct" exposure for you. With spot metering, it helps you choose how much to over or under expose (something you can't do in a priority mode) so that you can choose how to expose a particular spot. If I used spot metering in a priority mode, I would always have to meter off of something gray in color. If I metered off black, the picture would be very bright because I can't compensate. Is that wrong? – Tom Jan 20 '11 at 14:07
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    I think we agree except for one point: I think the whole reason spot metering is useful is because you can "base your exposure off the results of a spot meter". Do you ever find spot metering useful in a priority mode? – Tom Jan 20 '11 at 14:09
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    It's definitely useful, because a priority mode and spot metering solve two different problems. Priority metering is most useful when there's large differences in brightness between the subject and the background. Priority modes are useful when you want to control motion blur (shutter) or depth of field (aperture). There's no reason you can't mix and match these scenarios. – Craig Walker Jan 20 '11 at 15:29
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    Lastly, you can certainly intentionally under or overexpose in any situation: manual, priority, spot metering, or otherwise. Exposure compensation is the tool to use in most modes; in manual it's left entirely under your control. – Craig Walker Jan 20 '11 at 15:30
  • Shoot, I can't edit any more. That should read "Spot metering is most useful when there's large differences in brightness" – Craig Walker Jan 20 '11 at 15:37
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    As I understand it, an exposure-mode like "manual mode" is not about making a meter reading it's about how you or the camera prioritise aperture, speed and ISO to control the exposure as a result of that meter reading. Metering-modes like spot-metering, centre-weighted metering, matrix-metering, etc only† control which areas in the viewfinder are used in the algorithm that arrives at a meter reading. Metering-modes and exposure-modes are somewhat orthogonal. †mainly – RedGrittyBrick Jan 20 '11 at 18:25
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    RedGrittyBrick, that's pretty much correct. The only addition I'd make to that is that you can take a metering (of any variety) in manual mode (or, for that matter, with a standalone light meter). Connecting the meter reading to the exposure control is then handled by you, rather than by the camera. The meter reading is a "suggestion" for what exposure settings you should pick, but it's really up to you. – Craig Walker Jan 20 '11 at 18:54
  • @Craig Walker: Thanks! The accepted answer goes to you because this is closer to what I was looking for. I finally get it. I still think that spot metering is harder to use in priority modes simply because doing exposure compensation in priority modes seems clunkier to me. However, I do understand that it makes sense to use spot metering in a priority mode. – Tom Jan 22 '11 at 18:28
  • I'd like to add that, in some sense, spot metering is like a contract with you and the camera. You are telling the camera that you "promise" to point at something grayish when the camera is tells you it is a proper exposure. When the camera tells you that you are a couple stops off from a correct exposure, you promise to point at something black or white. To do anything else would result in a technically incorrect exposure (even though it may actually look good). Does that make sense? It just changes the way you adjust the exposure meter. – Tom Jan 22 '11 at 18:33
  • I think it's something that, over time and practice, you'll come to understand more. I've been studying exposure (online and in-camera) for around 6 months now and it feels nearly second nature to me now. – Craig Walker Jan 22 '11 at 19:14
  • I like the contract metaphor. I'd tweak it one step further: instead of a contract, it's more like an understanding between friends. Both the camera and the photographer are doing the "best they can" to get the exposure right. When they work together it'll come out nice... but if one wants to go in their own direction strongly enough, the other has to just shrug its shoulders and say "oh well, whatever you want." :-P – Craig Walker Jan 22 '11 at 19:17
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No, that's not possible. The exposure compensation affects how the exposure is measured, so it's applied before the image is taken, not afterwards.

You can adjust the image to a certain degree to make it appear correctly exposed, but that means that you are losing something else. If the image is too burnt out, you lose details in bright areas. If it's too dark, you will get a lot of noise when making it lighter.

I'm pretty sure (correct me if I'm wrong) this is not the same as adjusting the exposure during post processing.

Sorry, you are wrong. Although the information about the exposure compensation is recorded in the meta data, it's not used in post processing. There is no separate setting for exposure compensation in the post processing, you use the regular setting for exposure (which of course doesn't actually change the exposure, but adjusts the image to get the same lightness as an exposure change would have given).

Guffa
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