One way to take root picture using a camera is just to hold your root system by the collar and take a picture. While this technique might be the fastest and the easiest one, it is also to easiest way to get a bad picture.
Here is a couple of things to think of before taking such image.
Large root systems are not suitable for such pictures. Because they have a lot of roots, they will probably be stuck together, and you will be unable the take any picture.
Choose a camera with a high-resolution. If the resolution if too low, the number of pixels contained in the roots will be too low to extract any information. In order to increase the resolution, it is also possible to take the picture closer to the roots, but his will need an adequate lens.
Choose a background with a color different from the root (usually black is working well). Also be careful not the choose a “shiny” background, as it can create noise in the last image which can impedes the later analysis.
When you take a picture with a camera, its scale (pixels/cm) will change if the distance between the object and the camera changes. It is therefore important that an object of a known size is present in the picture to scale your image during the image processing steps.