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Levels and histogram in Gimp

In this tutorial we will explain to you how to read and adjust exposure of a photo using levels histogram.

Italian Web master: Bad English, but good tutorials.

When you shoot a photo is not easy to understand if the exposure is good or not. There is only one way for check in a second if the exposure value is right: the Histogram. Any camera should have this option somewhere, read the manual if you can't find it. When you will find it, with the help of this tutorial, histogram will have no secrets for you.
I choose a photo that is quite underexposed, the subject is a rally Ford Focus, that has won the "Dolomiti Rally".

Rally car for level and histogram

Gimp has his own histogram, the characteristics are the same of your camera, so let's see what a histogram is:
Open you image in Gimp, then go to Color - Levels.

Colors Levels, Gimp's Histogram

The histogram is really useful, in fact is a graphic that shows the quantity of pixel for each level between black and white.
On the right there are dark colours, on the left you can see light colors.
In vertical you can see the quantity of pixels for each luminosity.
Is really important to understand that the histogram must be read horizontally, vertical segments are only for give you an idea of the quantity, but the lines can exceed without problems.

The problems comes when the lines touch the left or the right sides. If the graphic goes out on the left, your photo is too underexposed (dark) and you have lost informations about low lights, if the histogram exceed on the right, you will have a overexposed image (bright).
The best solution is when graphic fits perfectly on both sides without exceed.
Let's see some example with a description of the histogram:

underexposed

This is an underexposed photo, in fact the histogram is moved to the right side. We have no high lights (left side is blank). The contrast is low, the shadow under the car has no informations, in fact the histogram touches the left side.

overexposed

This photo is the opposite of the previous image. You can notice low contrast and no shadows, is overexposed too.

high contrast, full histogram
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This photo has an high contrast, in fact all the histogram is used. There are some problems on the high lights. If you focus on the background, you will notice that the house is completely overexposed.

Now you should know how histogram works, but how can you modify it for have better photos? Let's use another example:

Rabbit Histogram

This shot has low contrast, in fact the histogram is really short. Under the histogram graphic, you can see three small arrows. You can change the position of any one of the arrows, changing the exposition and gamma level.
In this example there is a lot of free space between both sides of the histogram, so I will tight it, next you can see the result:

Closing the arrow

Pay attention: If you tight too much, you will lose data.
If the left arrow is moved too much toward right, dark zones will lose detail.
If the right arrow is moved too much toward left, light zones will lose detail.

Remember: Choose higher contrast is a bad idea if you want to preserve "foggy" effects.

There is a third arrow in the middle. You can use that one for change gamma (darker or lighter) without lose data informations, really useful.

There is another thing to say about histogram: In some pictures there is an high contrast level, generally when the sun is high in the sky, or when you have a great difference between dark and light zones (a lamp in the night). In this case is normal that the histogram doesn't fix in the space, in fact it goes out, even in both sides.
There is no solution in traditional photography, an option could be HDR Photography, but in this website I talk about Gimp, so I will not speak about HDR.
Anyway in most of your images you will find great advantages using histogram adjustment.

Next you can see the first photo of this tutorial before and after levels adjustment.

Photo before and after levels adjustment
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