View Full Version : How would I calculate my speedlite setting if I have no flash meter?

28-Mar-2017, 11:02
Hey guys, I have two canon 580EX II speedlite and I'd like to try and use them with my 4x5 camera. I do not have a flash meter though. I've been told that it's possible to calculate the appropriate setting on the flash units rather than use a flash meter.

How would I go about doing that?

Bob Salomon
28-Mar-2017, 11:30
Isn't there a guide on the back of your flash units?

28-Mar-2017, 11:31
If the flash has a manual mode, find the Guide Number (in the manual). GN divided by distance (in the same units as the GN is given) equals the aperture setting. That's your starting point. The GN will be given in either feet or meters, often both, and generally assumes EI 100. The manual will state the assumptions.

If the flash has an Auto mode using a flash-mounted sensor you can use that.

Dan Fromm
28-Mar-2017, 11:31
I'm not acquainted with Canon flashes.

If they're like most of the others they have a manually selected full power setting. The user's manual will tell you the guide number for that setting. f/stop to use is guide number/flash-to-subject distance. You may have to adjust the GN for film speed. They're typically published for ISO 100. If you're using, say, ISO 400 film, which is two stops faster, calculate the f/stop for ISO 100 and then stop down two stops more.

If they're like some of the others they have several "auto" settings and a calculator dial that will tell you which aperture to set given film speed and the greatest distance at which the flash will give the correct output.

Read the manual. If you don't have the manual, get it and read it.

29-Mar-2017, 13:05
I imagine one could put ND filter gel over the 'eye' of one to get a ratio on desired..1:2. 1:3...1:4 even

29-Mar-2017, 13:29
The GN of this flash depends on the zoom setting of the reflector, so take that into account when using it in full manual mode.

My 580EX also has an auto exposure mode where it measures the light reflected back at it and shuts off the flash output once it believes the correct exposure has been reached (though in my case it is consistently off by two stops). You tell the flash the film ISO and the aperture used and it does the rest. I assume the 580EX II still supports that. It is very difficult to figure out how to put the flash into this mode (I think it's a custom function). Check your manual...

David Lobato
29-Mar-2017, 17:25
I used a small flash many times for flower closeups outdoors with 4x5 film. I used the flash in manual mode and calculated the f-stop by dividing the guide number by the flash distance. An adjustable power level is nice to cut back the flash intensity. A tape measure is all you need. This method also makes it very easy to determine fill flash ratios with an ambient exposure. I typically set the flash to be -1 or -2 stops less than the ambient light level. Btw, the tape measure also is handy for figuring exposure compensation for bellows extension.

On the other hand, low cost flash meters are worth the expense if you will be using flash much more often, when time is limited, and for situations involving more lighting complexity.

Kevin J. Kolosky
19-Jun-2017, 22:52
What you have to do is establish a guide number, and then string your lights.

A guide number is just an easy way to determine exposure. So for example, if you had a guide number of 80 and you were 10 feet away from your subject, you would use f8 as your exposure

How do you determine that if you don't have a meter? It will take a little bit of film. Go into a dark room. measure 10 feet from a subject. then expose film at your predetermined iso for a number of different f stops. so expose a sheet for f 5.6 and f8 and f11 and f 16 etc. then process normally. then make a contact print using the clear film base as a guide. when that turns black on the contact print, but no blacker, use that time to contact all of the negs you just made. choose the best looking one. if it was the one at f8 your guide number would be 80. if it was the one at f11 your guide number would be 110, etc. Now, take a long piece of string and make knots in it at various intervals. every foot would be good. when you use your light, measure distance to subject with the string, and then use your guide number and distance to determine your f stop.
This is how we did it in the old days and it worked pretty well.