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I'm beginning to work on understanding and hopefully effectively using different metering modes in my photography. I have done some reading and understand the benefit of using spot and partial metering in portrait photography. I am using a Canon 500D for reference, but I imagine it would be the same for most DSLR cameras.

My question is this: When I set the camera to spot metering, find the location in the frame which is the "most important" and let the camera meter that spot, will the camera always set the exposure with the assumption that the location I metered has 18% luminance?

To this effect, if I'm shooting a portrait and focus on the forehead for example, I can assume the camera set the exposure as if it were neutral gray, and then increase the compensation for lighter skinned people, and decrease compensation for darker skinned people?

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Note that the actual number is probably more like 12%, not 18% –  Evan Krall Mar 25 '11 at 5:42
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@Evan Krall: That article is widely repeated, but @Jerry Coffin debunks it pretty convincingly here: photo.stackexchange.com/questions/1048/… –  mattdm Mar 25 '11 at 12:08
    
@mattdm Nice; I hadn't seen that answer before. –  Evan Krall Mar 25 '11 at 20:23
    
@Evan Krall: it's one of several which makes me which I could ★ answers. –  mattdm Mar 25 '11 at 20:28

3 Answers 3

up vote 6 down vote accepted

Yes, that's how it works.

This can be helpful when you want to ensure that some subjects are to be exposed correctly. For example, meter off a white shirt and open up 1.5-2 stops to ensure it's not blown out. Another classic example is a black leather jacket. Metering off it and then stopping down 1,5-2 stops will ensure it looks black without losing detail.

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This is probably a much better way of using spot metering, to pick the 'difficult' part of the photo and set your exposure compensation based on that so that it's detail is retained (or intentionally blown/black). You can use curves or simple general adjustments to bring the mid-tones up/down, but if your highlights are blown out or the shadows pure black/noise, then its too late. –  drfrogsplat Mar 24 '11 at 13:46
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By stops here do you mean Exposure adjusting, or aperture stops? Changing the aperture would seemingly change my composition, so I want to believe you mean the exposure adjustment. Is that correct? –  Jeremy B. Mar 24 '11 at 14:14
    
Changing EC (Exposure-Compensation) is what you want. How much EC you dial in depends on the dynamic-range of your camera, so you need to do a few tests first and then memorize one value for shadow metering and one for highlight metering. –  Itai Mar 24 '11 at 14:52
    
@Jeremy: Same thing. Go from f/5.6 to f/4; from 1/200 second to 1/100 second; go from ISO 100 to ISO 200; it's all the same in this respect; one stop. "Stop" is just a convenient shorthand, not necessarily a physical aperture stop. –  Staale S Mar 24 '11 at 14:54
    
What you are essentially describing is the Ansell Adams Zone System - Google that for plenty of helpful info. –  ElendilTheTall Mar 24 '11 at 15:02

In short, yes.

All metering modes on the 500D assume 18% luminance, the different modes just let you choose the area and optionally different weightings for different parts of the area.

This is true for most cameras... except that:

  1. As Itai and others point out in the comments below, some cameras have highlight and shadow spot metering modes (that are used somewhat like the method described by gerikson, though on these other cameras its perhaps a little more 'automatic'), and
  2. Apparently some of the newer DSLRs claim to be considering colour when metering (rather than assuming 18% luminance), however I'm not sure if the 500D is such a camera, nor whether they've allowed colour to make much difference. (I can't actually remember or find where I saw the colour metering info, but I'm pretty sure it was a DSLR released in the last year or two that claimed this)
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That is wrong. Olympus DSLRs and SLDs have had for a long time options to meter for other luminances than 18%. There is an option called Spot Highlight that meters closer to 90% (exact value not disclosed) and Spot Shadow that meters closer 5% (exact value not disclosed). Those are more practical to use because finding which area is a highlight or a deep shadow where details is required is easier than judging what falls around 18%. –  Itai Mar 24 '11 at 14:49
    
@Itai: I believe that particular feature predates digital in general by a few years. It goes back at least as far as the Minolta Maxxum/Dynax/Alpha 9000, which was originally released in 1985, if memory serves. I have to agree about its utility -- it was extremely useful, and I believe helped me produce a much higher percentage of well-exposed shots than anything since. –  Jerry Coffin Mar 24 '11 at 19:30
    
@itai - the Olympus OM-4 had that function (from about 1983), so it's certainly been around a while. Now I know what it does, it's a shame I wrecked the camera, so I can't experiment with it. –  Mike Mar 25 '11 at 2:26
    
@Itai, by "All metering" i was referring to the different metering modes on the 500D in the question. I'll clarify my answer to reflect this. –  drfrogsplat Mar 25 '11 at 5:10
    
+1 Much better! –  Itai Mar 25 '11 at 13:44

It's not actually that the object has 18% luminance, but that you want it to have 18% luminance in the exposure.

This is what cameras do when they set (or suggest, such as in Manual) the exposure setting to use.

If you point the spot meter at a person's face, you are telling the camera to expose so that the person's face and any areas in the scene equivalent in luminance are exposed to be 18% in the photograph.

When you are shooting manual and you meter the scene and it says you are one stop under exposed, this means that it's taken the reading and calculated that the exposure for the metered area would be under 18%.

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