Measuring pixel / area relative brightness

Question

I'm going though Michael Freeman's books on exposure. I'll take his example of a light skin portrait to illustrate my question, although there is nothing specific to that kind of pictures. The author states:

The average brightness for this kind of Caucasian skin needs to be between 60% and 70%, around 1 stop brighter than an average reading.

Being familiar with the way metering works, my understanding is that at exposure time, metering for the skin and correcting at around 1 stop over the metered value will achieve this result.

Now, let's assume that I'm a newbie who still frequently messes his exposure (hum). I have a badly exposed portrait in Lightroom, and need to fix the exposure level appropriately: how, preferably in Lightroom, can I measure the relative brightness of the skin to know I've reached 60% - 70%? Said otherwise: can I measure a point to know where in the histogram it falls?

PS: I know people will be tempted to answer that I should simply know when the skin is correctly exposed, but truth is I really don't :-( So I would really like to be helped by actual measurement.

Answer 1

Skin tones vary wildly, so going by the numbers can only get you into the ballpark at best. At some point, you are going to have to learn to eyeball things (where "eyeball" is understood to be a subjective process carried out on a calibrated monitor so that you have at least a reasonable expectation that what you see on the screen is living in the same universe as what you're doing to the file).

Lightroom (and Lr isn't alone in this) is somewhat crippled in this regard because it only has RGB numbers to look at. But that should be enough to get you into the right range, at least in terms of exposure. (Actually getting the skin colour right when there is no colour reference in the photo is a lot easier when you can deal with standard CMYK -- which depends on the selected output device -- or LAB -- device-independent -- values.) In general, though, you are looking for a colour in a non-specular (that is, not "shiny") broad highlight area that has a red value somewhere in the 180-220 range (depending on complexion), with green and blue both being significantly lower (140-ish to about 180, again depending on complexion). Foreheads and chins are usually good places to look for such highlights; the cheek can have some makeup weirdness going on that makes the normal RGB values untrustworthy. (Keep in mind that you can always go to B&W temporarily to take colour out of the equation.)

But seriously, get your monitor to a place where you can trust it, then you can trust your eyes. A beach bum in late summer should not have the same skin values as a light-averse fashion model in midwinter even when they're shot under identical lighting. As an experienced shooter, you'd meter them differently (with a reflected light meter, such as the one in your camera). If you try to go by the numbers, you lose a lot of nuance.

Answer 2

I found my answer: Lightroom Tone Curve has a small button on the upper left corner which allows adjusting by dragging up and down using the mouse. When used, this tool shows two figures: original and adjusted brightness %.

Here, the area under my pointer has an original brightness of 62% adjusted to 64% (as shown by the curve shape).

Answer 3

To check how exposure values correspond to the changes made through a curve (or any other tool), you can use simulated Kodak Q13 The original step wedge looks like this: The step between the neighboring patches is 1/3 EV. The values from Kodak documentation are The RGB values for gamma 1.8 and gamma 2.2 can be found here, and they are also present in the "UserComment" exif field of the dng file from above, to keep things together.

To get the correct readings from the simulated Q13 dng, set the white balance to "As shot" upon opening the file. Now you can apply a curve and see how the exposure values shift.

At default conversion settings the "mid-point" on a normal image should be rendered close to proverbial 18%, represented as patch M. Patch M reads 98(RGB 0..255, gamma 1.8) / 116(RGB 0..255, gamma 2.2) / 38(RGB 0..100, gamma 1.8) / 46(RGB 0..100, gamma 2.2) If the controls are moved so that brightness corresponds to 1 stop higher exposure, which is, starting from patch M, is patch #4, the readings should be close to 143(RGB 0..255, gamma 1.8) / 159(RGB 0..255, gamma 2.2) / 56(RGB 0..100, gamma 1.8) / 62(RGB 0..100, gamma 2.2)