Flashbulb Guide Number Information
- Clear Flashbulbs are used for black and white film.
- Blue Flashbulbs are used for color film daylight balanced and also for Black and White film.
- All values are approximations, reflector size and finish, shape, room size, ceiling heights,wall and ceiling color, average subject, natural and ambient light, etc. all have an affect on your final exposure.
- You should perform a variety of test exposures under different conditions to familiarize yourself with the use of guide numbers and exposure values using flashbulbs.
Number in parenthesis under lamp type refers to reflector size and type:
(1) = 4-5 inch polished reflector
(2) = 6-7 1/2 inch polished reflector
(3) = large studio reflector
(4) = 3 inch polished reflector
(5) = 2 inch polished reflector
Guide Number = f: stop
Distance To Subject
To determine your lens opening choose a guide number based on your ASA film speed, and your chosen shutter speed. Divide that guide number by the distance to your subject. The result is the f: stop to set on your lens.