"freeze" the rotors of a helicopter, whereas a slower shutter speed will be sufficient to freeze a runner. A tripod may be used to avoid motion blur due to camera shake. This will stabilize the camera during the exposure. A tripod is recommended for exposure times more than about 1/15 seconds. There are additional techniques which, in conjunction with use of a tripod, ensure that the camera remains very still. These may employ use of a remote actuator, such as a cable release or infrared remote switch to activate the shutter, so as to avoid the movement normally caused when the shutter release button is pressed directly. The use of a "self timer" (a timed release mechanism that automatically trips the shutter release after an interval of time) can serve the same purpose. Most modern single-lens reflex camera (SLR) have a mirror lock-up feature that eliminates the small amount of shake produced by the mirror flipping up. Film grain resolution Black-and-white film has a "shiny" side and a "dull" side. The dull side is the emulsion, a gelatin that suspends an array of silver halide crystals. These crystals contain silver grains that determine how sensitive the film is to light exposure, and how fine or grainy the negative the print will look. Larger grains mean faster exposure but a grainier appearance; smaller grains are finer looking but take more exposure to activate. The graininess of film is represented by its ISO factor; generally a multiple of 10 or 100. Lower numbers produce finer grain but slower film, and vice versa. Contribution to noise (grain) Quantum efficiency Light comes in particles and the energy of a light-particle (the photon) is the frequency of the light times Planck's constant. A fundamental property of any photographic method is how it collects the light on its photographic plate or electronic detector. CCDs and other photodiodes Photodiodes are back-biased semiconductor diodes, in which an intrinsic layer with very few charge carriers prevents electric currents from flowing. Depending on the material, photons have enough energy to raise one electron from the upper full band to the lowest empty band. The electron and the "hole", or the empty space where it was, are then free to move in the electric field and carry current, which can be measured. The fraction of incident photons that produce carrier pairs depends largely on the semiconductor material. Photomultiplier tubes Photomultiplier tubes