Reading Your Histogram
Frequently I am asked: “What should a proper histogram look like?”, and while I admit this topic is not exactly a party ice-breaker it is a topic of constant consideration for the attentive photographer.
There is no absolute answer to this question: A proper histogram is one that reflects the tonal values that you, as an artist and photographer, wish to have present in the image that you are seeking to create out of the situation that presents itself in front of your camera and lens at the time you are creating the image.
“Ok, so what should that look like?” you may ask. Well the best I can do is show you a few examples and discuss what I look for in a histogram:
You should never judge the proper exposure by looking at the image density of the preview of the image on the camera’s playback monitor. Your perception of the image density will change based upon the ambient light under which you are looking at the playback monitor. The only way to truly assess the image exposure is to view the histogram of the image, while you are also looking at the image subject before the camera. Judging the histogram for proper exposure content is a three-way conversation: you (what you want), your camera’s histogram, the subject’s tonal values. All three of these aspects must be considered when assessing a proper exposure.
Keep in mind that your camera can only display a JPEG histogram and therefore the actual histogram will change once you get it into the computer–if you are shooting in camera raw format. I find that a “proper” looking JPEG histogram in the camera usually needs about 2/3rds of a stop more exposure to give me a “proper” raw histogram in the computer.
Mid-tone subject and resulting histogram:
Mid-tone subjects are usually very easy to work with. They respond well to adjustments to brighten or darken the over-all look or feel of the image. Expanding or contracting contrast is also not a problem.
Highlight-tone subject and resulting histogram:
The key to handling a highlight toned subject is to capture in camera raw format, and not over expose too much. If I see the above resulting histogram on the back of my camera I am comfortable with bracketing another shot while adding 2/3rds more exposure.
Night or Twilight subject and resulting histogram:
Twilight and Night subjects are tricky. Notice that most of the data falls in the darkest third of the image. This is the region that contains the fewest tonal values making these images particularly difficult to print and result in rich colors and tonality. You must also be careful not to over-expose the highlights-notice that this histogram already has a small “tail” forming in the highlight area.
Split-tone subject and resulting histogram:
Beware of a histogram that looks like the one above. These histograms are the hardest to deal with. It is difficult to determine how much data lies outside the useable range on both the shadow and highlight end of the spectrum. For strategies on how to deal with this subject matter see my article “Turning a two-hump camel into a one-hump camel” November 2011.