angle underlying the ratings; this has only been partially realized. For the most part, manufacturers state guide numbers relative to a sensitivity of ISO 100. However, manufacturers sometimes rate guide numbers at ISO 200, which makes them 41% greater. Furthermore, the illumination angles underlying manufacturers' ratings vary greatly, which can make it particularly difficult to compare models. Understanding guide numbers Units of measure Throughout most of the world where the metric system (SI) is observed, guide numbers are expressed as a unitless numeric value like 34, even though they are technically a composite unit of measure that is a two-factor product: As such, guide numbers can be reduced either to distance in meters or to fstops depending on how one uses the guide number in a calculation. In the U.S. however, photographers typically measure distances in feet and require guide numbers scaled accordingly. To serve the U.S. market, manufacturers of flash devices typically provide foot-based guide numbers and add nomenclature such as feet, ft, or the foot symbol ( ′ ) to unambiguously denote that fact, e.g. Guide number: 92′. Another common practice when flash devices are marketed in the U.S. is to provide two guide numbers—which can be expressed in a variety of formats—so distances and fnumbers may be calculated using either feet or meters, e.g. Guide number: Though nomenclature conventions such as these can make guide numbers misleadingly appear to be length-based units of measure, they serve as notations to eliminate ambiguity as to which length-based system of measurement underlies the guide numbers. As with metric-based guide numbers, foot-based guide numbers are a two-factor unit of measure except the units are To convert a guide number given in meters to feet, divide by 0.3048. To convert a guide number given in feet to meters, multiply by 0.3048. Calculating with guide numbers It is easy to use guide numbers to calculate either an aperture or a flash-to-subject distance. Consider a flash device with a rating of 28 meters (which might be marked "92 feet", "28 DIN", "28/92", "92", etc.). Suppose a photographer has a flash device with a guide number of 44 (m) / 144 (ft), sets the camera's aperture to , and wants to know the required flash-to-subject distance; he merely divides the guide number by 4. Thus, a subject 11 meters or 36 feet away will be correctly illuminated = 11 m, and = 36 ft). For