The most common uses of still photography in security work are in surveillance, evidence, and accident coverage. The most versatile camera system for such work is a 35mm camera with interchangeable lenses. The basic camera functions that control the picture-taking process are the lens opening or aperture, which controls the intensity of light that reaches the film and the depth of field; the shutter speed, which controls the time of exposure; and the focus. Color film gives the most accurate interpretation of a scene, and for situations that require especially sharp color photographs, color slide film may be the best choice. Normally photographs are taken at eye level. Various techniques can be used to ensure that the camera is level. The distance from the camera to the subject dictates the choice of camera lens. Generally, a normal lens is sufficient for photographing evidence, accidents, or exhibits. For versatility at low light levels, choose a camera lens with a large maximum aperture. Light meters can help measure the available light. Various modifications can be made to cameras to facilitate their use in low temperatures and at fire scenes. Since reloading a camera on location is risky and a waste of time, 36-exposure 135 film magazines or 220-size rolls of film should be used. Cameras and lenses must be protected at all times from smoke, dust, flying debris, and water. A skylight or clear glass filter screwed into the lens will not affect exposure and will protect the front of the lens. To protect the camera from the elements, it should be placed in a loosely fitting plastic bag, with the opening of the bag secured with rubber bands around the lens (the plastic bag should not cover the lens). A table of film types for different situations is provided.