Many of you may have seen a graph display on your camera or camera app on your smart phone. It may have shown up while taking a picture or while browsing the pictures you’ve already taken. Most of you have probably only seen it by accident, playing with options or settings on the camera. If you’re not sure what I’m talking about, it looks like this:
This is called a histogram and it’s a visual representation of your photo’s exposure. The idea is that every screen is calibrated differently and has physical limitations on how it can reproduce an image. No two screens look alike and what you print likely won’t match the screen. The histogram gives you information to help you better judge a photo’s exposure, no matter what screen you are looking at.
In this graph, there is a lot of useful information to a photographer and reading a histogram is easier than it looks, though it might take some practice. Let’s look at this piece by piece:
Looking at the graph left-to-right, we examine the shades of black/white. The far left is black and is represented by 0. The far right is white and represented by 255. What this means is that there are 255 shades of gray between black and white, all represented on this graph. If we have peaks on the graph on the far left or far right, we know we have pure black or pure white showing up in the photo. Depending how large the peaks are, this can be warning signs of overexposure or underexposure. We need to examine the vertical side of the graph to get the rest of the picture.
Now that we know we’re looking at shades of gray, we can examine what the vertical side represents: How many pixels are a particular shade of gray. As the peaks grow, we can determine how much of the photo falls into a particular shade of gray. If our photo is mostly tall to the left, we know the photo is underexposed, or too dark. If the peaks are weighted more to the right, we know the photo is overexposed, or too bright. Flat ends, such as the far right in the image above, may also be a sign of overexposure or underexposure, but you may want to judge with your eyes as well as the look you’re going for may give you flat ends.
Putting it all together
A histogram can tell us how well a photo is exposed without having to look at the photo – in fact, it’s intended to assist us in determining exposure when monitors can’t be trusted. Knowing left/right = shades between black and white and the vertical side represents how many pixels are within those ranges, you can look at the histogram and see if you need to adjust your exposure in the camera or editing software.
The histogram doesn’t always only show brightness/exposure information. If your histogram also shows colored graphs or the option to view the RGB colors within the graph, then you get the added benefit of seeing color balance on the histogram also. The idea remains the same, left/right = shades of gray and vertical = the amount of pixels in that range. The difference is that the red graph represents shades of red, blue represents shades of blue, and green represents shades of green.
The histogram can also aid in using a technique many professionals use daily called Expose To The Right (ETTR). The idea only works if you shoot your photos in RAW. Exposing to the right means that you intentionally overexpose your photo so that the histogram is shifted as far to the right as possible while keeping few pixels at the furthest right point. Having a flat left side of the graph may be acceptable here. The reason for this is that most RAW editing software can recover overexposure better than underexposure and doing so gives shadows more details that otherwise wouldn’t have been captured.