or by being cued by the camera's sensor; both the latter options make continuously variable adjustments. Manual attenuation settings are usually steps in powers of 0.5 (full fstops) that commonly extend five to eight fstops deep , , To calculate how reductions in power levels affect guide numbers, see Effect of power settings, below. Shutter speeds do not factor into guide number calculations with electronic flash and, for the most part, have no effect on exposures. See Effect of shutter speeds, below. Guide numbers are not affected by scene reflectance. Guide numbers are a function of the illuminance and duration of a flash (a property called luminous exposure that have lux⋅seconds as their units of measure) arriving at a scene as measured by an incident-light meter (pictured at right), not the amount leaving the scene. This often seems counterintuitive to hobbyists who incorrectly assume that cameras' built-in reflected-light meters are a definitive measure of exposure. However, this principle underlies why using a camera with a through-the-lens meter to photograph a park bench surrounded by sunlit snow underexposes the image, making the bench appear nearly black and the snow as dark as grass and foliage. This is because reflected-light meters are calibrated for an average scene reflectance of 18 percent and can’t "know" when a scene has non-average reflectance. See also Gray card and Light meter. Guide number distances are always measured from the flash device to the subject; if the flash device is detached from the camera, the position of the camera is irrelevant. Furthermore, unless a flash device has an automatic zoom feature that follows the setting of a camera's zoom lens, guide numbers do not vary with the focal length of lenses. Note that manufacturers of flash devices may provide guide numbers ratings specified relative to ISO 200, which increases them by the square root of the difference, or a 41 percent increase relative to those given at ISO 100. See Effect of ISO sensitivity, below. When comparing or shopping for flash devices, it is important to ensure that the guide numbers are given in the same ISO sensitivity, are for the same coverage angle, and reduce to the same unit of distance (meters or feet). When these three variables have been normalized, guide numbers can serve as an relative measure of intrinsic illuminating energy rather than an inconstant metric for calculating exposures. Effect of power settings Most