Getting Toned Up
A colleague of mine has been taking digital photographs for a while, and whilst his images are usually of a high standard to begin with, he’s beginning to use Adobe Photoshop to help bring out the absolute best in his photography.
The only crux is he’s never really used it and has asked for a bit of help.
To that end, I’ve decided to include a few tutorials in my blog to give him a bit of a head start into learning the dark art that is Photoshop!
The first tutorial I was planning to write up was how to use the level’s tool to make those little tweaks to the image brightness values that can really improve an image. However, as my colleague didn’t quite understand what we were doing and what the histogram represented, I thought it best to actually start with explaining what the histogram actually is…
According to Wikipedia, an image histogram is:
An image histogram is type of histogram which acts as a graphical representation of the tonal distribution in a digital image. It plots the number of pixels for each tonal value.
By looking at the histogram for a specific image a viewer will be able to judge the entire tonal distribution at a glance.
However, considering I feel that actually understanding image histograms is probably the most important concept to become familiar with whilst working with digital photographs, I think we best expand on that definition a little more!
(I truly believe that understanding how histograms work will not only improve your ability to manipulate images within Photoshop to greater affect, but will also improve your photography skills in general. It did for me!)
As Wikipedia states, a histogram displays tonal distribution within a digit image. With this data, you can easily check if an image has been properly exposed, if it was correctly lighted, and (more importantly from a Photoshop perspective) what adjustments will work best to produce a better final image.
A common misconception is that there is an “ideal” histogram which all images should try and mimic. This isn’t true; histograms should merely be representative of the tonal range within the scene being photographed and what the photographer is trying to convey.
Whilst there are a few different types of histograms (RGB, CMYK, etc), they all work in a relatively similar way and all produce the same basic graph-like display as shown below:
For the purpose of this tutorial, we will only be looking at an RGB histogram.
So, How do they work?
Every pixel within a digital image has been produced by some combination of the three primary colours red, green and blue (RGB). For “8-Bit” images, each of these colours is given a value between 0 and 255 which represents how bright that colour should be. For example, if red and blue were set to 0 and green to 255, then the pixel would display a bright green colour. If all three were set to 128, then the pixel would display a medium grey colour.
An RGB histogram is built by scanning through the brightness value of these three colours for each pixel within the image and records how many are at each level from 0 to 255. These results are then used to create the graph.
Below, I’ve added a few comments to the graph shown within Photoshop to help better explain how to read the graph:
Looking at this example, we can see that the vast majority of the pixels within the image have low brightness values. This area where the majority pixels can be found is called the “Tonal Range”.
Tonal range can differ greatly between images, so building an intuition for how these numbers map to actual pixels is often critical – both before and after the image is taken. Below is an image that hopefully gives you some idea of how the histogram ties up to the actual image it is created from:
This image of a creek has very little highlights and deep shadows, but lots of mid-tones as shown by high pixel count curve, in the off-centre of the histogram.
Another example is this shot of Anfield, which has hardly any mid-tones or highlights, but is dominated by heavy shadows:
However, lighting is not normally as extreme as the above image. A properly exposed image, with even lighting will usually produce a histogram with peaks in the centre, and gradually dies off into the shadows and highlights, similar to the following:
Most digital camera will have no trouble automatically reproducing which has a histogram similar to the one shown in the above image.
Ok – so that’s it for now. Try viewing the histograms for images you’ve already taken and try to build up the “knack” of guestimating what your image’s histograms may look like, as this will be helpful when we move onto the next section!
(Tip: To view your image histogram in Photoshop, first open an image, then whilst holding the CTRL key, press L. Or, select “levels” from the “Image->Adjustments” menu.)
In the next post, I’ll explain what high and low key images are and why they’re important in digital photography…
http://www.blog.ianmellor.co.uk/2009/07/28/the-highs-and-lows/