Lesson 5: Histogram — Focal Point

Chelsea London © 2019. Fujifilm X-T10 | 56.0 mm | ƒ/1.2 | 1/250 | ISO 2000 - Histogram showing a mostly even exposure with some leaning to the shadows.

Introduction to the histogram.

As discussed, exposure is one of the most important controls of the final image. We have discussed how to modify exposure, but not how to review it. This is the role of a very powerful tool: the histogram.

As a rule of thumb, the LCD screen should never be trusted to evaluate exposure. It is not designed to produce an accurate rendition of the image. How bright your photo appears will depend on a variety of factors, including the ambient light levels, and the brightness setting you applied to the screen. The camera’s JPEG preview has also been applied to your raw image. For this reason, you might think you have the right exposure when out shooting, only to find out the screen misled you when you get back to your computer.

A histogram, on the other hand, is a more “scientific” way of evaluating exposure. It will always be available and identical on all devices, whether the LCD screen of your camera, or your fancy calibrated computer monitor. All digital cameras offer post-capture histograms – often in one of the “image details” modes (check your manual). A large number of mirrorless cameras also have “live histogram”, a very useful feature showing what the histogram would be if you took the photo at that instant.

The histogram is a visual graph that shows the distribution of brightness levels in a photo. It represents the range of tones from dark to light, with shadows on the left, midtones in the middle, and highlights on the right. The height of each part of the graph indicates the frequency or amount of pixels at that particular brightness level. A well-balanced histogram ensures that the photo has a good mix of shadows, midtones, and highlights, helping us assess and adjust the exposure to capture a properly exposed image without losing important details in the shadows or highlights.

Chelsea London © 2017. Fujifilm X-T10 | 56.0 mm | ƒ/3.61 | 1/160s | ISO 800

Above you see a very dark image and it’s associated histogram. Notice how all the data is shifted far to the left, with almost nothing on the middle and the right side. Also notice that the highlights from the street lights are too small to be noticeable in the histogram.

Chelsea London © 2017. Fujifilm X-T10 | 56.0 mm | ƒ/3.61 | 1/160s | ISO 800

Here’s a fairly bright image and its corresponding histogram. Notice how its shifted to the right. The white triangle on the top right indicates that some of the highlights have been “blown out,” or are entirely unrecoverable. You will also notice a slight bump on the left which is due to the dark blob on the left of the frame.

Chelsea London © 2017 Fujifilm X-T10 | 27.0 mm | ƒ/14 | 1/125s | ISO 200

Here’s a more neutral image and its histogram. You’ll see a mostly even distribution across the highlights, midtones, and shadows - despite the shot being made directly into the setting sun.

Chelsea London © 2018 Fujifilm X-T10 | 35.0 mm | ƒ/2.8 | 1/320s | ISO 5000

Finally we have an image and corresponding histogram where the both the highlights and the shadows are quite strong. Notice how the histogram has strong lines on the left and right, and the midtones (or the middle of the histogram) is basically empty. The highlights from the window have been blown out, as demonstrated by the peak all the way to the right of the histogram.

The histogram makes it very easy to visualize how modifying camera settings changes your exposure. All you are doing is shifting the entire histogram to the right (increasing your exposure) or to the left (decreasing your exposure). If you push it too far and hit the edges, something interesting happens: the histogram “crashes” and the shadows or highlights are clipped. This means that the information is lost forever, and this is something you will usually want to avoid at all costs. An “ideal histogram” is relatively easy to define. It is a bell curve covering the whole width and finishing exactly at the edges, with no lost details. This also happens to be what the light meter in your camera will try to produce. From this ideal exposure you will then have incredibly large amounts of latitude in editing your raw file, to craft it into the image you visualized.

Chelsea London © 2019 Fujifilm XT-1 | 35.0 mm | ƒ/1.4 | 1/1000s | ISO 500 = Histogram showing peaks in specific color channels.

What about color?

There are several more advanced points that can be discussed:

So far, we only talked about brightness, not about colors. Color information is coded in three channels (Red, Green and Blue, also known as RGB) and some cameras show individual histograms for each channel. This is useful information in one situation: when you have a very brightly coloured object, it is possible to blow out the corresponding channel (go so far to the right that information is lost) without it showing in the main histogram.

For raw shooters, you should be aware that the displayed histogram is the one from the JPEG preview file, not the one from your actual raw data. This means that you can sometimes recover more information than you think.

Due to the way information is stored in digital cameras, there are more details in highlights than in shadows. If you plan on using significant post-processing, you can try to shift your histogram to the right as far as you can without getting pure white, then shift it back left in post-processing. This is known as the “expose to the right” technique, and is worth your time to try out. You may find it suits your shooting style.