The Histogram in Your Digital Camera Explained in Plain English

I admit I am a bit of a camera nerd however I cannot stress the importance of the learning how to interpret the histogram on your digital SLR camera.

First of all, what is a histogram? This is the time to look in your camera manual and find out which controls you need to use to see it in action. Once you do this you should be able to see it after each exposure you take. Basically it is a graph that showing the distribution of the images brightness level and the brightness level of each primary color - red, green and blue.

Let's talk about the two different types of histograms:

1) Brightness display shows the exposure level distribution, overall brightness and gradation.
The horizontal axis indicates the brightness level (darker on the left and brighter on the right) while the vertical axis indicates how many pixels exist for each brightness level. The horizontal axis represents 256 shades of gray from pure black (0) on the left to pure white (256) on the right. So the darker the image the more spikes you will have on the left and the brighter the image the more spikes you will have on the right. If the spike is too far to the left you will not have any details in the shadows and if the spikes are too far to the right you may not have detail in the highlights. This is called clipping and when you lose detail, you lose pixels which cannot sometimes be recovered. This is why I check the histogram, at least the first couple images, to see if I need to adjust my exposure.

2) RGB display shows color saturation of red, green and blue and gradation.
The horizontal and vertical axis work the same way but relate to the density of the colors. So for example, the more pixels there are to the left, the darker and less prominent the color will be and the more pixels to the right the brighter and denser the color. If the histogram shows a lot of spikes on the left the respective color information could be lacking detail. If there are a lot of spikes on the right, the color information could be too saturated and lacking detail.

Don't worry if you don't understand yet. Here is the plain English

Your digital camera can only capture 256 shades of gray from pure black to pure white. Mid-gray (and all middle tones) are in the middle. Remember black is to the left and has a value of 0 and white is to the right and has a value of 256. Mid grays have a value of 128 or so.

You should only be concerned with the right and left sides for these are the areas we have control over. I rarely check the RGB histogram as I am mainly concerned about over-exposing or under-exposing an image.

So you should be checking the brightness level histogram for clipping on the right of left side. What is clipping? Great question. Clipping is when the spike extends beyond the horizontal axis on the left or right sides. If it extends beyond the left side you are under-exposing your image and will lose detail in the shadows. If it extends beyond the right side you will lost detail in the highlights. If you lose detail you have lost pixels that cannot be recovered. (you can shoot in raw and recover some lost pixels, but that's another article).

So what do you do if you notice the clipping. You want to make sure your subject is exposed properly. If you lose detail in the background (maybe it doesn't have any detail) you are okay. However if lose detail (pixels) in your subject you will need to make an adjustment with your exposure and open up an f-stop for a spike off the left side and close down an f-stop for a spike to the right. Here is an example:

You are shooting in AV mode and your aperture is set to f/8. You notice a spike off the left side, you would reset your aperture to f/5.6 to open up one stop. Re-shoot the image and check your histogram and readjust if necessary. If you notice the spike to the right you would reset your aperture to f/11.

I hope that all makes sense. You can experiment with different backgrounds and learn how to read your histogram on your own digital SLR camera.