Ok photographers. I'm going to try to tackle a topic that is fairly easy to explain in person. I've done it a dozen times. But somehow in the course of prepping for this post I realized that it's easier to explain in person than in writing. But I'm taking my best shot!

The histogram. It's a graph that shows up when you hit "info" on your camera while viewing an image. It shows up when you're editing in Lightroom. It's everywhere! Many of you probably already know how to read one. But the more I've talked with photographers lately, the more I've realized how many of you don't. And that doesn't mean that you're failing at being a photographer! You don't have to know how to read a histogram to be a good photographer. But it CAN help immensely with getting your exposure correct in-camera and with proper post-production.

Let's take an image with simple lighting as a starting point. This image was shot in the shade - there is no backlighting, windows, or objects that are brighter than the subjects in the background.
how to read a histogram
This is the histogram for this image:
histogram in Lightroom
Imagine that it is divided into 5 pieces. Below is a graphic that shows which sections are controlled by which sliders in Lightroom. The leftmost section represents the blacks in the image, then the mid-tones of the image are represented by "shadows" (here it spikes due to the groom's tux), what Lightroom confusingly calls "exposure", and "highlights". Then the brightest part of the image is represented by the "whites".
5 histogram zones in Lightroom software
The most important thing to pay attention to when looking at a histogram is the sections representing the blacks and whites. In a shot like the one above, with no windows or backlighting, we want the histogram to taper off like this one does at the right and left bottom edges of the graph. This means there are some true whites and true blacks in the photo and the shot is properly exposed.
a histogram for a properly exposed image
The "Exposure" slider mostly impacts the mid-tones, but it also shifts the whole graph right or left slightly. Play with it a little and note how the histogram shifts. Then take a moment to play with the other sliders and note how they shift the area of the histogram and the tones that they are responsible for.
Lightroom sliders
Let's look at the same image and let's say it was under-exposed out of camera:
how to improve an under-exposed image in Lightroom
This is what the histogram for this image looks like. Note there is very little information in the "whites" section:
How to use your histogram in Lightroom to properly edit an underexposed image
To remedy this, slide the "exposure" slider to the right until the curve tapers into the right edge of the graph. You can then play with the "shadows" slider to adjust the mid-tones to taste.
Exposure slider in Lightroom
When shooting, you always want to expose for the highlights. Because of this, there are occasions where you will have images out of camera with no true blacks in them. This happens often to me when I'm shooting with backlighting or shooting a light colored object close up. Here's an example:
how to read a histogram
Note the histogram for this image has no info in the blacks section:
a histogram with no info in the blacks area
To remedy this, slide the "blacks" slider to the left until the curve tapers into the left edge of the graph.
adjusting the shadow tones of an image in Lightroom
More often than not, the subjects I shoot are not the brightest part of the image. There is a window behind them, or a light, or -- in the instance of backlighting -- their hair-light is a highlight that I don't care about properly exposing. So I purposefully over-expose according to what my camera meter is telling me. Here's an example:
properly exposing and editing an image using the histogram
Note below that the histogram is running off the right side of the graph instead of tapering down into the corner. In this case, that's ok. What it's telling us is there is detail in the highlights that we are losing - you have to imagine that the curve of the graph continues off the right side. But that information is not included in the image. In this case, we don't want to properly expose what is outside the window, so we're ok with it.

Now, note that the graph for the above photo also runs off the left side. That means that we are losing information in the shadow areas of the image. And you can see from looking at the photo that there is no detail in some of the black clothing - they are just big blocks of black. That's what the histogram is telling us too. There are very few instances where we don't want detail in the shadow areas of prints. But oftentimes, because we are exposing in-camera for the highlights, the blacks can end up being "stopped-up" like in this image. We can remedy this by sliding the blacks slider to the right:
How to adjust an image in Lightroom for proper exposure
Here is the resulting image and improved histogram. You can see that the curve now tapers into the bottom left corner, so we are no longer losing detail in the shadow areas. Yay!
Why a histogram is important to photo editing
I've noticed that a lot of newer photographers, in the name of having "contrasty" images, have very stopped-up blacks. You can greatly improve the quality of your images by paying attention to your histograms and making sure you have detail in your shadow areas.

Once I get my blacks and exposure adjusted for an image, I oftentimes use the "shadows" slider to adjust the mid-tones to taste. The "shadows" slider is our friend because it doesn't impact the brightest whites or blacks as much as sliding the exposure or blacks sliders do. So once you get the edges of your histogram looking good, use the "shadows" slider to lighten or darken the overall look of the image.

I hope this helps demystify the histogram for you! Remember, you ideally want to shoot to properly expose your highlights in camera then all you should have to do in post is pay attention to your blacks and mid-tones.

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Why a histogram is important for proper photo editing and how to read one
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