It occurred to me that a guide number definition might be interesting as part of this post.
Guide number definition: the guide number is an index to the light emitting power of a flash and is calculated by multiplying the flash distance and the f-stop used. The above mentioned Canon flash units contain the guide number in their name e.g. the 580 EX has a guide number (GN) of 58.
Guide numbers are usually expressed in metres for ISO100. Therefore, at ISO100 for the Canon Speedlight 580EX, if you shoot at f16 the light will light a subject at a distance of 3.625 metres or about 11.9 ft (58 divided by 16 = 3.625m). If you reduce or increase the f stop the distance increases or decreases respectively. Also, If you shoot at ISO200 the distance doubles; at ISO400 you can quadruple it; at ISO800 multiply it by 8 and at ISO1600 you can multiply the distance by 16.
To convert metres to feet and inches multiply by 3.28 e.g. 3.625mx3.28=11.9ft.
The MR14-EX is a specialised ring flash that fits around the barrel of Canons 50mm and 100mm macro lenses. The guide number is so low i.e. 14, because with macro photography camera to subject distances are much less.
I have uploaded a table of GNs and distances you might find useful:
If you find this table difficult to read click it to go to a larger version.
How to use the table: for example for the Speedlight 430EX the guide number is 141 and at f8 the subject to camera distance could be a maximum of 18ft (rounded to nearest foot) for "correct" exposure. Fortunately owing to through the lens flash metering you dont have to calculate this, the camera does it for you. But I thought it was interesting to see the numbers.
The 580EX includes a manual override for the power output so you can leave it on full power or dial in a fraction of full power down to 1/128th of full power. I dont have one of these flashes but I have a similar facility on my studio flash unit and I use this feature all the time.
I have checked the above info and I am pretty sure it is accurate but if not please correct me.
Steve
Last edited by steveprice on Thu Feb 15, 2007 10:22 pm, edited 1 time in total.