Histogram Basics for Photographers
© Ken Osborn 2012
Light meter? Isn’t that something like a sliderule? A few photographers still use light meters but most of us depend on the histogram to tell us if an image is over or under exposed. And some know about histograms but don’t use them because they rely on the camera to make all of the photographic decisions. Nothing wrong with that if you have no desire or need for creative control, but if you’d like to go the next step and not let the camera make all the decisions, then this little tutorial is for you.
A histogram plots light received by the camera’s sensor with light value the horizontal axis and number of pixels on the vertical axis as in the plot below.
Light values for 8 bit pixels range from 0 (no light) to 255 (blown out highlight). The next chart shows light value assignments and the relationship to the Adams Zone System numbers. This makes for a total of 256X256X256 color shades when the Red, Green, and Blue (RGB) channels are combined for full color. Our eyes can see more shades of color than that but it’s close enough and many color printers can’t do any better.
So how do you use a histogram for proper exposure?
Below is a properly exposed photo shot at sunset. There are no blown out highlights or blocked up shadows.
Abandoned Building at Sunset
Now compare to the badly overexposed image below.
Here is the same photo with the histogram superimposed.
Note that the histogram is pushed to the right with no space on the right side: the highlights are blown.
An underexposed version of the same scene.
Perhaps you want that nighttime look. That’s good and exactly why you want control over your shot. Before you look at the histogram below, what will the curve look like?
Of course you knew the curve would be pushed to the left with the shadows blocked, yes?
So what does a ‘proper’ histogram look like? If you don’t want blocked shadows or blown highlights, then you will want a histogram that has nothing extending to the edges, as in the next image.
So that’s it. Properly exposed, a photograph should have neither blocked shadows nor blown highlights. Shadow areas that are too dark have no detail and generally lack interest. Highlights that are blown also have no detail and are usually distracting to the central features of the image.
And now you know what happens to the histogram if the shadows are too dark or the highlights are too bright. Of course that is not to say that the artist’s intention was not have shadows without detail or highlights that dazzled the eyes. After all, photography is an art.