The histogram is one of the most useful tools in the camera that helps to evaluate proper exposure and is also one of the least understood and used tools. If you want to become a better photographer, you need to understand how the histogram works and use it to nail exposures. It is the best way to make sure that you are properly exposing the frame you are photographing.
Note to remember: Nothing can evaluate exposure better than your eyes, but then there are situations when the light is harsh or very limited, that you need some help evaluating exposure and you can use the histogram as a guide.
So, what is a histogram? A histogram is a graphical representation of the tonal values in a photograph (between 0 and 255) and is an accurate guide to exposure.
And what does it represent? It is a curve that represents tones and pixels in the image. The x-axis represents the tonal variations from black to white; that is, blacks, shadows, midtones, highlights and whites. The y-axis represents the amount of pixels for a given tonal value or range.
Here is a representation of the information you find on the histogram. Included here is only the luminosity histogram, but it is good to look at the RGB colours which we will look at in a different article.
In order to have a good variation of all the tones, the histogram needs to be a curve (almost bell-shaped, depending or an even spread of medium humps) touching almost all areas but not the very ends (which are the blacks and whites). The ends would mean a loss of pixel information, like complete darkness or completely blown out highlights and this is called clipping.
Here are some examples to see how the histogram looks when there are whites and blacks in the frame:
Image by Erik Scheel
Image by Pete Linforth
Note: If your scene has predominantly a lot of black areas, then you should not worry about the histogram hitting the left end of the graph and similarly if there are a lot of whites in the frame, the histogram may hit the right end of the graph.
Also, for clarification and ease of understanding, included here is another example, where you can see how the histogram varies when the image is correctly exposed, underexposed and overexposed. Using a single image for comparison while demonstration always helps!
Image by Ambir Tolang
So how do you go about getting the exposure right?
- Keep the histogram active on the LCD screen when composing the image and that way you can watch in real time how changes in aperture, shutter speed, and iso affect your exposure (check the camera settings or camera's user manual on how to get the histogram on screen).
- If you are not using live view, keep it on when reviewing images to get an idea of the exposure, so you can correct it (do not always rely on what you see on the LCD screen while reviewing).
- Use the correct metering mode depending on the scene you are photographing.
- Always shoot raw, that way you get to pull out a little bit of the lost details (both in darks and whites) while post-processing, but, it is always good to get the exposure right in camera.
- Do not worry if the histogram hits the ends when you have blacks and whites in the scene. Also note that a little bit of clipping is ok and there are times when that is unavoidable in scenes that have strong reflections from water, glass, etc.
That's about it. It is really that simple and not complicated at all like many would think or refrain from using. If you have any other tips on using the histogram to get the exposure right, please share your thoughts in the comments section below.
To take your exposures a step further, you should consider going further than just histogram appraisal and looking at exposure in a more wolistic way. Take a look at the guide to Understanding Light here.