Gimp gives us a number of tools for controlling the tone and colour of our images, but the undisputed kings are Levels and Curves. The vast majority of our photo editing needs will be catered for by either (or both) of these tools so they are quite important. This is just a primer to introduce some concepts, I’ll write a post showing Levels and Curves in action soon.

In order to use both Levels and Curves effectively you will need to understand the image Histogram, Sounds complicated at first but it’s actually quite simple. the image Histogram is a graphical representation of the tones in our image, from pure black on the left, to pure white on the right. The height of the graph represents that number of pixels within the image for each of the values in between.

So why is this information important to us?. Because it tells us quite a bit about our photo.

 
Exposure

if we see the majority of the graph clumped up on the left hand side it suggested that our photo is too dark (underexposed). If the majority of the graph is clumped on the right if suggests that our photo is too bright (overexposed).

histab1

In addition it can also tell us something else, if we see the graph spill off the right hand side it tells us that sections of our photo are “blown out” (pure white with no detail or texture), conversely if our graph is spilling off the left it tells us that our shadows have been “clipped” (pure black with no detail or texture).

histcd

 
Tonal Range

The image Histogram can also tell us about the tonal range of our photo. If we see that our graph is clumped up somewhere in the middle and not reaching to either (or both) end of the range, then our photo has a compressed or limited tonal range (our photo only contains a subset of the possible tones). The vast majority of scenes will contain both black and white so this usually suggests a problem. Look at the histogram of any photos you have which are hazy and lack contrast, limited tonal range is probably the problem.

histe

 
Colour casts

There is more than one histogram or course, there is the Value histogram (Luminosity) which gives us basic brightness information. However there is also a seperate histogram for the Red, Green and Blue channels (we’ll skip the Alpha histogram for now). We know that pure white, pure black and neutral grey should contain equal amounts of red green and blue, so if one (or more) of the colour histograms looks significantly different from the others it may suggest a colour cast. Here is the Red, Green and Blue channels from an image with a strong blue colour cast.

histfgh

 
Hopefully you now have some idea of the usefulness of Histograms, your digital camera may well have a histogram function which can be viewed during playback, if so consider turning it on and bring the power of histograms out into the field. Note however that many cameras only show the Luminosity histogram, this is of limited use, you really need the colour histograms if you plan to use the cameras histogram as a light meter, that said even just the luminosity histogram can be informative, especially on those sunny days when you cannot see the LCD screen on your camera well.

Tip: Don’t bite off more than you can chew, concentrate on the Value histogram until you are comfortable with it, it’s probably the one you’ll use most anyway.

 
T.

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