Archived posting to the Leica Users Group, 2002/07/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]- --On Friday, July 12, 2002 3:23 PM -0700 Ted Grant <tedgrant@shaw.ca> wrote: > Hi folks, > > This is right off the wall ancient need to learn it over again. > > A chap down the street called and asked me how to use the guide number on > the flash he has? > > Well OK I thought I knew.... however that was 45 years ago when the flash > of the day had guide numbers. Even then I had trouble figuring that stuff > out, then I discovered photo life was much easier when you didn't use > flash and go available light. ;-) > > Besides any flash I used over the years has always been something > automatic no brainer thing. So please help me with a very simple way to > explain this guide number and how to make it work with whatever flash and > camera is he has. > > thanks guys and gals. > ted As others have noted, the guide number is always referenced to the film speed -- the most commonly quoted in advertisements is the guide number for ISO 100. However, if using faster (or slower) film, the guide number must be adjusted for the film speed according to the manufacturer's recommendations. It is not always linear. Another complicating factor which used to be more prevelant than it is now - -- manufacturers inflating the guide number for competitive reasons, so that the manufacturer's own recommendations had to be adjusted by the user based on practical experience. This isn't so true anymore. The Vivitar 283, for example, is right on the money when used in manual mode. Finally Ted, when using flashbulbs in your Speed Graphic, you need to adjust the guide number for the shutter speed since the rise and decay speed of the bulb affects the exposure. For example, the guide number of clear Number 5 bulbs at ASA 100 (they didn't know from ISO then) is 240 at 1/25 or slower but drops to 190 at 1/100. (This is for the polished 5" reflector). :-) Last, but not least, simply divide guide number by subject distance (normally in feet) to get aperture. - -- Rolfe Tessem rolfe@ldp.com Lucky Duck Productions, Inc. - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html