Always use a ruler for a scale. Use of a coin or a pencil as a scale is inappropriate and should be avoided. It is also critical that the ruler be made of a non-reflective material. Using a metal ruler and then taking a picture using a flash could ruin the photograph. The best approach is to purchase a non-reflective ruler and keep it with your camera.
Footprints require special care when being photographed.
Due to the amount of traffic at a crime scene, it is sometimes difficult to tell whether footprints belong to a suspect or to a member of the crime scene unit. This is where a Polaroid camera can be useful. Taking a Polaroid picture of the footprint and then comparing it against the shoes of the officers working the crime scene can help determine if the footprint is evidence. It is important to note, however, that once it has been established that the footprint may belong to a suspect, a photograph of the footprint must be taken using a 35mm camera. When taking a picture of a footprint, do so from a 90-degree angle.