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  • Paolo Ferraris

What is a Histogram in Photography?

Updated: Nov 1, 2019

Histogram photography definition. You will hear, read this term and see its shape almost ubiquitously in digital photography. A histogram is the representation of the distribution of numerical data.


In photography, a histogram is a graphical representation of the tonal distribution in a digital image. The horizontal axis of the graph represents the tonal variations, from black to white, from dark to bright, while the vertical axis represents the number of pixels in that particular tone. The histogram counts and displays the number of pixels for each tonal value.

Histogram in Lightroom

What is a histogram on camera. How do you read a histogram in photography?


Sections on the left side of the histogram represent the black and shadows areas of the image, the central part is called exposure in Lightroom because the slider that bears the same name affects this area of the histogram. Tonal values in this section are called mid-tones. The right sections represent the highlights and white areas of the image.


Images where dark tones are prevalent will have a histogram shape leaning toward the left side while an overall bright image will have most data points on the right side. Images with a smooth tonality gradation will have a flatter histogram. Images with a lot of contrast will have peaks toward the left and the right extremes. Images with no contrast will have a histogram shape resembling a hill or a mountain.


The histogram chart is now present in some form in almost all camera models currently sold. Let me start by saying that according to me, histograms on modern camera screens are not very useful.


How do you read a histogram in photography? What should a histogram look like?


Histograms tell us how tonalities are distributed in the image.


When we shoot this doesn't really concern us. Modern cameras' metering systems are quite accurate, the philosophy of the RAW file makes sure that the tonal distribution is homogeneous and appropriate to capture the widest dynamic range possible.


When you are shooting your eyes shouldn't be on the histogram. Focus on the scene in front of you and judge by what you see. If you work with a mirrorless camera you will be able to see through your sensor and assess visually if the image is too dark or too bright.


The histogram on our camera screen can be useful when some exposure settings are blatantly wrong. If exposure is obviously off, the image will lose information in the dark areas, and in this case, we say we are clipping the shadows or will lose information in the bright part. In that case, we say we are clipping the highlights.


Histograms are great to understand at a glance if we are making big exposure mistakes.


If we seriously overexpose the image you will notice how we lose tonal information in the highlights. Many pixels, some areas simply become burnt out, white. We also miss part of the shadows spectrum, meaning that dark tones become too bright and the tonal rendition of the image is no longer accurate.


The opposite happens when we underexpose. Parts of the image lose tonal information, they become pure black and most of the highlights and mid-tones are dragged toward the shadows area, where they don't belong.


When exposure is all wrong histograms get crushed and pushed toward the extremes.


What is histogram in image processing? What does the histogram mean in Lightroom?


While I disputed the usefulness of the histogram on camera, the histogram during the image editing phase in post-production is crucial, essential.


When you edit your image in front of your computer you will judge the effects of your work by looking at and evaluating the changes on your monitor. The problem is that monitors are not perfect and they are not all created equal. Some are brighter than others, some render contrasty scenes more accurately than others. Often the lighting of the environment where we perform image editing affects the judgement skills of our eyes. An image can look bright if we do post-production in a dimly lit office at night and appear dark if we do the same work during a sunny day by a large window.


In short, what we see can be deceptive, especially when we have to decide how bright or dark our image should ultimately be, especially when we have to decide about exposure changes and highlights or shadows recovery. Histograms, on the contrary, tell us about how bright or dark an image is regardless of the monitor we are working on.


What is clipping in Lightroom? What is white clipping? What is shadow clipping?


Another great feature of image editing software is the color overlay that appears when we check for shadows and highlights clipping.


In Lightroom, the saturated blue shows us the areas in the picture, the pixels that have lost tonal information in the darker side of the histogram. These pixels are pure black. The red color shows us the pixels that have lost tonal information in the brighter side of the histogram and have become pure white.


Now to have some black areas in the darkest parts of the image or some specular highlights where a direct source of light or reflections shine is normal. What is not normal and not acceptable is when these tone clipping areas start spreading. In that case the color overlay is very useful because it tells us clearly that we have a problem to fix. Exposure settings must then be changed and highlights or shadows must be recovered.


In my post-production workflow, I usually set the right exposure and then I apply the necessary recovery to bring back tonal information in the brightest and darkest areas in the picture.


Paolo's take:


Histograms are useful to have a second opinion on the exposure settings of our images, both in camera and on a monitor. Sometimes you can't just rely on your eyes alone.


Histograms are not a measure of the quality of your images either. According to the scenes you are photographing and according to your style histograms will have the most disparate shapes and erratic trends.


Histograms are great when you need to judge the brightness of the images you're working on in the post-production phase regardless of the monitor. They give you a true tonal distribution and tonal range reading that can help you make informed decisions on the ultimate exposure treatment you want to apply to your pictures and the amount of shadows and highlights recovery you need to get the best result.


When you're shooting, keep your eyes on the scene, trust your internal light meter and make sure you bring home a file with the widest tonal distribution you can achieve with your sensor.


Get to know your tool and if you discover it has a bias toward the shadows or the highlights shoot with the appropriate Exposure Value compensation to offset that bias.


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