The histogram - Photography Course - Lesson 25

This article is part of the online digital photography course.

The histogram is an essential tool in digital photography, but many people don't know what it is or don't give it the due importance. Its purpose is to understand if the exposure is correct, if we've lost detail in the areas with shadows or high lighting.

Many people just rely on the LCD screen of the camera, a big mistake, because it doesn't always give a reliable result cause of its backlighting (too weak or too strong) or the lighting of the environment, which, if too strong, can annoy our vision of the picture and then misguide us in the evaluation of the exposure of a photo.

The histogram can be checked during the shot from the LCD screen of the camera can be very useful, the post production software one is helpful too.

In this article I’ll use as example my Nikon D90, check the user manual of your camera to see how to visualize the histogram.

Just after having shot the photo I can see the image preview on the LCD screen.

As you can see, I can see the shot details, the graph above on the right is the histogram. In some cameras you can access a more detailed histogram, divided in the three RGB colors.

Now we need to understand how to read a composite histogram, it represents the amount of pixels for the different lightings, the image is correctly exposed when the areas where the graph begins and ends are not cut.

In the histogram on the horizontal axis we have the lighting divided in 256 levels, we start with 0 on the left, which represents the black, and continue till 255 on the right, which is white, halfway we have the gray and on the vertical axis we have the tones inside the image.

Here we have the underexposed version of the same shot, in this case the curve moves towards the left, but being not cut we have captured all the information on blacks, then in post-production with Photoshop we can correct the levels rebalancing it without losing any information on blacks.

In this overexposed version the histogram is shifted on the right, and cutting the curve, I burnt part of the whites, this way I’ve lost information that I won't be able to recover not even with Photoshop.

Now you can understand the histogram’s importance, watching it when you realize a picture you can immediately understand if the image is correctly exposed and you can recover it in post-production.

Histogram and Photoshop

If you use Windows to open the histogram you have to press Ctrl+L, if you use a MAC you can press Apple+L.

It opens the levels where you can visualize the histogram.

In the level window there are 3 cursors under the histogram, on for the black 0, one for the middle tones 1,00 and one for the white 255.

If I take the white cursor and drag it to the end of the curve I’ll expose correctly the high lights: I made my picture brighter.

But the middle tones are still dark, so I can take the middle tones cursor and drag it to the left and this way I’ll obtain a correctly exposed image, or rather I won't have burnt my whites, the black will not result closed, the image is then clear. If I close and reopen the levels graph I’ll see that it’s balanced.

By opening the overexposed image (the one where I’ve lost the whites information) and using the levels tool, the first thing that comes up is that the curve is cut.

Photoshop unfortunately can't make miracles, I can't recreate information that wasn't captured during the shot, the overexposed image will remain such as it is, that's why it’s important to check the histogram during the shot phase.

There are situations where you will never have a perfect histogram, so we have to introduce the concept of exposure latitude (dynamic gamma), that is the capacity of the sensor to record the information between blacks and whites, generally a camera sensor captures between 5 and 6 stops, it means that between the blackest point and the brightest one there are 5 apertures.

If the scene we are going to photograph has an high contrast, that means that contains both very bright areas and other areas in very dark shadows, the camera won't be able to capture all the information available, then the photographer will have to choose if he prefers to renounce to have information on the lights or on the shadows. Luckily technology fulfills our needs with HDR photography, of which I’ll speak soon.

A curiosity: our eyes, in ideal conditions, can discern only 200 different gray levels, the 256 available tones in a digital image are more than enough to show even the slightest tone variations.

Translation by Nina Kozul

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