Histograms lie, unless…
When working with portraits and just photos with people in general, how can we tell if the image is exposed properly? More importantly, how can we tell if the FACES are exposed properly? Of course we could look at the handy dandy histogram on the back of the camera or even in our favorite image editing program. The problem is, the histogram shows the exposure of the entire image. And if the image has a large high key background, the histogram will be biased towards the right and vice verse for low key images.
Here the histogram for the image shows a huge spike in the shadow region, mostly due to the low key background and dark hair. Looking at this I would tend to think this image is underexposed and may make unnecessary changes to the exposure in Photoshop.
Here is where the “Facemask Histogram” comes in handy. What is that you ask? Simple. Just grab your elliptical marquee tool in Photoshop and be sure feathering is very low or off entirely and draw an oval around the face as shown. (How fitting the photo is of Will Crockett, the very person I learned this technique from. Gee, I hope he doesn’t mind.) Then simply open the histogram window and take a look. The histogram should have the vast majority of the peaks towards the middle and upper mid-range and should drop off sharply towards the highlight and shadows as shown. Yes it is OK if you select some hair and under the chin. Just avoid going into clothing or the background.
If the histogram extends to the right all the way to the highlights, you may have blown out details and unless you are shooting teenagers, lost detail is probably bad news. The histogram may extend to the left more since the pupils of the eyes can be very dark and dark hair may slightly throw it off. But if the bulk of the histogram is to the left, chances are the face is under exposed.
We can see by this example that the histogram is just what we are looking for and can proceed to color correction. But that is for another blog entry.
For a full step by step, see the complete archived article at ShootSmarter.com You may need to register for his site to view the link. Facemask Histograms for Exposure Control