For portrait photography, it is important to get both the skin exposure and tone right on a human subject. Here are some things I've learnt from reading up on the subject and personal experience. I'm sharing these very useful tips with my fellow photographers out there.
Skin Exposure
One thing to always keep in mind as a photographer is that the exposure meter in the camera is only a rough guide. The camera is not intelligent enough to know what scene it is pointed at and properly expose those. Furthermore, the light meter is really only good when there is ample lighting; it works poorly in low light.
Now comes the technical bit. The light meter is calibrated to correctly expose a matte surface at 18% gray scale. In order to correctly expose the subject, one needs to estimate how much brighter / darker the subject is compared to the 18% gray surface, and then set the exposure compensation accordingly. This comes with experience, or just trial-and-error approach.
Keeping the 18% gray exposure at the back of your mind is important when photographing human subjects with different skin tones. Here are some rough exposure compensation settings:
Fair skin: +1 EV
Brown skin: 0 EV (Trivia: Halle Berry's skin exposure value is a perfect 18% gray)
Dark skin: -1 EV
Skin Tone
After properly exposing the skin, the other bit that one needs to get right is the colour of the skin. The best way is to get it right from the start by using a 18% gray card to calibrate the camera for the particular lighting. Different cameras have their own method to do this, so find out how your particular model works.
Another way is to get out from Auto White Balance (AWB) and either manually adjust the colour temperature or use a preset scene. Then, put the 18% gray card under the same lighting as the subject and take a test shot. Use this test shot to calibrate the rest of your shots in post-editing.
In post-editing, you can also adjust the colour temperature and tinge until the subject's skin tone is "correct". However, the human eye is not always reliable when it comes to judging colour (or rather, how the brain processes information). Computer monitor settings, and ambient light changes can easily throw off your eye. I have found a quantitative way to determine the correct skin tone colour.
In Lightroom Develop Mode, hover the cursor over a representative patch of skin. Look at the RGB values for that spot. Correct skin tone should have these values:
Red should be roughly 15% above Blue, while Green should be right in between. The absolute values will depend on exposure of course, but the relative RGB values should be roughly like that for human skin tone. Simply adjust the colour temperature and tinge until you get those values. For challenging lighting situations, one may also need to adjust the individual colour channels.
LightRoom uses percentages (0 - 100%) while Photoshop uses absolute RGB values (0 - 255), so convert accordingly.
Hope these tips help!
P.S. Always shoot in RAW and use spot metering!
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