How to change a drawing contrast

When you take a photo of a traditional drawing, or when you scan it, it is possible that there is a noticeable loss of dark tones. There are probably many reasons for that, and it is possibly a feature of scanners and cameras not a defect. Regardless of why they end up losing the dark tones, it takes part of the beauty of most traditional pencil drawings. Luckily, there are ways to bring those darker tones back.

One of these ways being using an image editor to manipulate the contrast of the drawing. For this tutorial I will be using the GNU Not Unix Image Manipulation Program, also known as GIMP. Which is free and open source. However, it is important to note that manipulating contrast is one of the most basic features in photo manipulation software. You can use this tutorial to achieve the same result using Adobe Photoshop for example.
The first thing you need in order to change the contrast of a drawing, is a drawing to change the contrast of. For the purpose of this tutorial I will be using a pencil fan art of Lawliet, or L, from Death Note, made by the artist Meiiji from deviantart. Lawliet hair is pure black. As such losing dark tones has an huge impact on the digital version of the drawing.

Open the drawing on GIMP. You can click the menu File, then click Open, or use the hotkey ctrl+O, or just drag and drop it. After the low contrast drawing is open, click on the menu Colors, then click on Levels. Some people prefer using Brightness-Contrast instead. However, Brightness-Contrast is just an simpler version of Levels, in fact, the software converts what you do in Brightness-Contrast to Levels before doing any work.

Things are going to get a little mathematical so pay attention. First, Input Levels is the range of brightness being converted. If you place the black input mark in the middle, that is gray. Everything gray or darker will become black. If you place the white input mark in the middle, everything gray or lighter will become white.

That is, if you don't mess with the Output Levels. If you move the output black mark to gray, everything that should be black will be gray. And anything that should be lighter than black is going to be lighter than gray. Well, I hope you got the gist of it already.

Basically, we are going to do is a simple normalization. That is, we are going to get our darkest tone, which is a shade of gray, and turn it in pure black. The Histogram, which is the white rectangle inside the Output Levels, shows more or less how much of the image is made out of a certain tone. The leftmost black points are the darkest tones in the image. The rightmost black points are the brightest tones in the image.

You can tweak the contrast by dragging the marks, typing in the boxes, or by clicking the eyedropper icon and then clicking a point in the image. You could quickly click the darkest point in L hair to make sure it becomes pure black, but it's always a good idea to try to fine tune it. When doing tweaks make sure you leave the Preview check box ticked, that way you can see in real time the effects of changing the levels.

After this all you have to do is press OK and tada! It's done. To save as JPG in GIMP, click the menu File, and then click on Export.... Don't forget to type .jpg in the end of the filename. Here is the difference of the drawing of Lawliet made by Meiiji before and after changing the contrast.