Rangefinder "flash-matic" exposure system.

Hello
I have a rangefinder camera that has the "flashmatic" system on it. With the Flashmatic system, you just set the guide # setting on the camera, and focusing it opens or closes the aperture to the proper size for correct exposure. I have a small flash unit with a guise # of 50, with ASA 100 film. So that would give me a guide # of 100 with 400 ASA film. The guide # ring on my rangefinder is in meters, they are, 7, 10, 14, 20, 28, 40, 56. So which setting would I use for a guide # of 100?

Thanks
Russ

 

The guide number on your flash is probably in feet. The GN on the camera is probably in meters, since it's the same color as the metric distance scale. 100 feet is just about 30 meters, so 28 is probably the closest to your flash's output. Bear in mind, the guide numbers on flashes tend to be optimistic. Try some shots at 28 and maybe a few at 20. Good luck!

 

Jim
I think that you are correct. But the guide # on the flash is for ASA 100 speed film. I'm burning 400 speed film. So maybe set the guide setting on the camera at 40?
Thanks

 

A GN of 40 meters is about 130 feet, so I'd expect some underexposure. By the way, what camera is it? Do you know how far it stops down? If it only goes to f/16 it may not allow you to focus closer than its smallest aperture allows when it's in "flashmatic" mode. At GN 40 that would be about 8 feet. If you're shooting negatives the film's tolerance for overexposure will cover you if the GN you select on the camera is a little too low.

 

Not sure whether this helps - I'll try to check into it more later...
My Olympus 35 RC included the PS 200 flash and uses the guide number system, although it's an approximation. The PS 200 flash has a guide number of 45 at 10 feet and ISO 100, or GN 14 at 3.048 meters, so the factor of 3 is usually close enough to convert feet/meters. It's pretty accurate.
I've checked the flash output with my flash meter, and the camera's distance based guide number system stops the aperture down appropriately. I've double checked it against my digicams too, to save wasting film.
Oddly, the 35 RC GN ring doesn't have a marker specifically to match the PS 200 flash, but the dot between GN 32 and 65 (in feet) and 10-20 (meters) seems to correspond with the PS 200 flash. The flash scale shows only ISOs 80-125 and 25-32, typical for that era. The camera GN scale goes to 130 in feet, 40 in meters, which means I can use my much more powerful Nikon flashes with it too.
At ISO 100, works out to f/4 at 11 feet/3.5 meters; f/5.6 at 8 feet/2.5 meters; etc. I just add another stop for ISO 200, two for ISO 400.
To be honest, I'm lazy. I rely on the auto thyristor flash/non-TTL auto flash with my Nikons for my older film cameras in most conditions. I use guide number flash only on those rare occasions when auto flash may be fooled: nighttime or very dark indoors; white clothing against dark backgrounds or vice versa. But Nikon's non-TTL auto flash with the SB-800 is so good it even works accurately in rooms lit only by candlelight.

 

Jim
I've attached a couple snaps of the camera. Lex, yes, this is the same system that our Oly RC rigs employ. It's the Vivitar ES rangefinder. I don't know why I cannot submit an image, like I did in the opening thread?


Thanks
Russ

 

I just shot my flash meter with the small flash. The flash meter is telling me that at 10 feet with ASA 400 film, to shoot at f/8 Does this help at all?
Thanks
Russ

 

With small flash attached.

 

That's a GN of 80 feet. Metric would be just under 25, so the 20 setting on the GN ring will probably give you the best results.

 

Albinar 50A flash.

 

Albinar 50 flash.

 

The Fstop, film speed, distance guide number relationship gets more confusing when the ASA/ISO conversion is added. ASA is a linear scale where each doubling of the number is one F stop. ISO is a logarithmic scale where each number is like a decibel (dB) so an increase of 3 (dB) is one F stop. The difference of 21 to 27 ISO is the equivalent of 100 to 400 ASA or 2 stops. The scale on the back of your flash has meters and feet converted fairly well (13 ft/4m). GN = distance × f-number. GN m = 4mXF4 or 16. The next check is 3mXF5.6 for a GN of 16.8 which is pretty close to the first GN. GN for ASA 100 in feet is 13ftXF4 = 52 GN. The next GN is 9ftXF5.6 = 50.4 still fairly close. For the guide number at 27 ISO multiply 4mXF8 = GN of 32.

 

One small correction. If your guide number is GN 50 at ISO 100 it'll be GN 200 at ISO 400. Not GN 100 as you stated in your original post.
Sounds like you've got it all worked out.
JD

 

This thread seems to be an object lesson in how to make a simple problem difficult in the space of a few posts.
There's about 3.3 feet to the metre; isn't that simple enough? Also most stated flash GNs are at least one stop overrated, so your GN of 100 (in ft) @ 400 ISO is probably more like 70 if you want a decent exposure. Divide 70 by 3.3 and you'll get somewhere close to the real GN of your flash in metres, Russ.
Joseph, the GN has to be multiplied by the square root of the ISO multiplier because f/stop numbers increment by root 2. The OP is correct that the GN doubles if you quadruple the ISO speed. It's not a simple linear relationship.
Furthermore: ASA and ISO are identical. The arithmetic ISO (International Standards Organisation) scale was adopted directly from the ASA (American Standards Authority) scale for film sensitivity. I.e. 100 ISO = 100 ASA and 400 ISO = 400 ASA, etc. There is a logarithmic version of ISO that follows the old German DIN standard, but nobody in their right mind uses it.

 

Rodeo Joe,
Of course you're right, had to take my shoes and socks off to figure it out as I ran out of fingers to count on. Too many years of TTL I guess.
JD