Medium Format Family

Register a free account now!

If you are registered, you get access to the members only section, can participate in the buy & sell second hand forum and last but not least you can reserve your preferred username before someone else takes it.

Guide Numbers


New Member
Hello All! This is my first post here. I have been a Hassy 503 user for over a year now, shooting mostly available light and studio strobe. I am currently shooting weddings and social events. I use a Metz 45 CT-5 and have had wonderful results. My concern is trying to understand Guide Numbers. I want to perfect fill in flash situations and would like some guidance in this area. I do not use the TTL cables. I currently have been "feeling out" the strenght of the flash in the various (6) auto settings it provides. Is it true that all I need do is set the auto setting to the aperture I wish to shoot at and the flash does the rest? When is the best time to use the GN formula? ( aperture=distance divided by GN)The GN on the 45 CT-5 is 148. Is this number only when I use 100ASA? I shoot mostly 160 ASA and 400 ASA. Any input is greatly appreciated. Thanks, Jorge



First, did you notice the title of this part of "553 ELX"?

Yes, in auto mode, all you do is select an aperture on the unit, keeping an eye on the range scale next to the aperture scale, and set the same aperture on the lens.
The sensor in the flash works the same as the TTL one inside the camera, except that it obviously does not meter TTL. ;-)
It works perfectly well.

The guide number changes with film speed, yes.
Use the dial on top of the unit to see what changing the speed does to the GN by looking at how the aperture-distance pairs change.
With a dial like that available, who wants/needs to do calculations?


Just multiply the GN for 100 ASA by 1.4 for a film of 200 ASA or divide by 1.4 for a film of 50 ASA.
Just like the lens aperture; each f stop is 1.4 times larger (or smaller) than the previous one.
e.g. f4 x 1.4 = f5.6. f11 divided by 1.4 = f8.
(1.4 being the square root of 2)