Determining exposure in today's digital world is easier than ever. Determining correct exposure is somewhat more elusive.
The definition of "correct" exposure is going to vary depending on multiple factors. A landscape artist, commercial photographer and a portrait photographer may all have different ideas about what a correctly exposed file might look like. Each could/should be correct for their specific application.
Portrait photographers tend to be interested in correct exposure for the face. This too can vary somewhat depending on if it is a scenic view or how important the environment is in the image. For the sake of simplicity, however, I would like to describe a technique that works very well for traditional portraiture.
This technique is called "Face Mask Histogram." This is by far the most consistent means I know to achieve repeatable results in portrait exposures. It is what I have used for years and it has remained a valuable technique.
In "Figure 1" you will see how a histogram is found in Adobe's Lightroom application. The histogram here indicates that there are very few areas of the image that are dark (note the left side of the histogram) and many areas that are light (note the right hand side of the histogram). There are also areas in the image that are over exposed. When parts of the histogram push up against the right or left side of the histogram it indicates clipping or under or overexposure. In this case on the right or highlight side indicates overexposure.
Traditionally a "correct" histogram or normally exposed file has no clipping. It is very important at this point to understand that histograms indicate the tonal values for the complete image. This particular image has a very broad range of tonality beyond what can normally be captured digitally. So knowing that there are some areas in this image that are too bright for the camera to handle, how can I determine correct exposure for this scene?
By using the cropping tool (this works for Lightroom, Photoshop and Bridge's Camera Raw programs), crop only to the area of the face that you are interested in. The histogram will indicated the tonal ranges in the area that you have selected. (Note Figure 3.)
The histogram in Figure 3 indicates that the face has a near perfect exposure. There is no clipping in either the shadow or the highlights and tonal distribution look exactly like what I have come to expect from properly exposed faces. This translates into excellent results when receiving prints from my lab.
Please note that in this case there is a very bright edge light on the child's face coming from the sun. I purposely did not include that in my crop. I wanted to know the values on the child's face and knew that the very bright edge light would impact the Face Mask Histogram.
This technique works equally well in the studio. Just remember to crop to the "mask" of the face.
Obviously most portrait photographers do not shoot tethered to computers and therefore cannot use the Face Mask Histogram technique out in the field or in studio. I use the technique upon returning to the studio as a means of double checking my in studio or on location exposure technique. I also use it when producing digital files for ordering. I will check the Face Mask Histogram of all my files to make sure that they will print the way that I expect.
Hopefully you find the Face Mask Histogram technique as useful as I have.