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Photoshop Tutorials For Begginers Lesson 4: How To Read A Histogram
In this tutorial we're going to be looking at how to read a histogram. The histogram is a really important part of the Photoshop workflow. Many of the changes you will learn how to make in future lessons involve the histogram. So let's get started...
What is the histogram?
The histogram is a clever way of representing all the tonal information contained in a photograph. On the left are shadows, in the middle are midtones and to the right are highlights. The intensity of different tones is shown by the height of the graph at different points.
The most important thing to look for in the histogram is 'clipping'. Clipping refers to when the shadows have become pure black, or the highlights have become pure white. In other words, all the colour information has been lost from those parts of the photo.
By looking at the histogram, we can assess the exposure of an image and use it to guide any adjustments that could be made. Luckily, there are some great features in Photoshop for improving the tonal range of photographs, which we'll come on to in a minute.
Opening and viewing the histogram.
To open the histogram go the Windows menu and select Histogram from the list. It will appear as a floating window, which you can dock in the panels area (see lesson 1, Getting Started With Photoshop, if you don't know how to do this).
Click on the histogram icon in the panels are to open it up. Click on the top right symbol in the histogram panel to bring up an options menu. Choose 'Expanded View' instead of 'Compact View' from the list.
Then select RGB from the 'Channel' drop-down menu, instead of the default 'Colors' (this is not essential, but I think it is easier to work with RGB view).
Default Colors setting:
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Using the histogram with high contrast images
So, let's take the example of a high contrast image. Say you have a photograph where the subject is positioned against strong back lighting. The subject has been slightly underexposed, but the background is very bright, and there may be some clipping.
Take a look at the histogram, which will show lots of information in the shadows and highlights, with a comparatively sparce information in the midtones. We want to create a more balanced exposure, by lightening the shadows and darkening the highlights.
More information in shadows and highlights than midtones:
Firstly, hit Cmd/Ctrl + J to duplicate your background layer. Go to Image>Adjustments>Shadows/Highlights.
This is a really useful adjustment tool for dealing with excessive contrast. The Shadows slider works to lighten shadows and the Highlights slider works to darken highlights.
Make sure your histogram panel is open and visible. Drag both sliders in the Shadows/Highlights window to 0, so you can see the original image. Now move them both to the right, to darken highlights and lighten shadows. Look at all the information at the edges of the histogram (highlights and shadows) moving towards the centre (midtones).
Doing this can sometimes lead to a problem. A 'halo' effect appears around the edge of your subject:
Luckily, this is easily eliminated. Tick 'Show More Options' in the Shadows/Highlights window and reduce the 'radius' of the highlights by using the slider. You'll see the halo disappearing as you do this.
Using the histogram with low contrast images
Sometimes photos can be a little 'flat'. There is too much information in the midtones, and not enough in the shadows and highlights. So, open and image that you would like to have more contrast. Take a look at the histogram.
Now, go to Layer>New Adjustment Layer>Levels. Alternatively click on the half white, half black circle at the bottom of the Adjustments panel and choose Levels from the list of options.
Levels is a feature you will be using a lot in Photoshop! You'll see that it features an RGB histogram, just like the one we've been looking at. Directly below the graph are black, gray and white sliders, corresponding to shadows, midtones and highlights.
The black and white sliders are at the far edges. Anything that is positioned to the left of the black slider is pure black (clipped shadows) and anything the right of the white slider is pure white (clipped highlights). Drag both of these sliders inwards so that they meet the edges of the information shown on the graph.
Alhough there is some contrast, it is within the midtones area:
Obviously, as you re-define the white and black points, the tones in the rest of the image fall into line and are brought 'closer' to white/black accordingly (i.e. the contrast of the whole photo increases).
Occassionally it is even worth pushing the black and white points inside the edges of the graph. In other words, you turn some existing colour information into pure white/black, just to increase the contrast of the whole image.
Try this out. As you move the sliders, you can hold Option/Alt to display a heat map on your image of where shadows or highlights are being clipped.
What does a 'perfect' histogram look like?
There is no such thing as the perfect looking histogram. Most of the time it's really important to avoid clipping highlights and shaows. You can use the histogram in your camera, as well as the one in Photoshop, to check which images have clipping.
But, as we've seen, it can sometimes actually be worth clipping highlights and shadows to increase contrast in the rest of the image. Similarly, not all high contrast photos need to be 'corrected' with a Shadows/Highlights adjustment layer. Many photographs work best with high contrast, particularly black and white (see Converting Black and White Images In Photoshop for more).
One useful tip, though, is that it's best to 'expose to the right' when taking pictures with your camera. This means you should gather as much information on the right hand side of the histogram, the brighter areas, as possible. Err on the side of over-exposing (See this tutorial on exposure compensation for help).
Why? Because it's easy to add contrast to bright photos in Photoshop, but hard to regain detail in dark photos, without reducing quality a lot.
So that's lesson 4 finished of the Photoshop Tutorials For Beginners series! I hope you found it useful.
Photoshop Tutorials For Beginners