GN number and camera settings/ Did I do my math right?

Here is the setup and then the math;<br>

I'll be using a pair of Bowen Esprits, a TLR and a flash to trigger

the strobes. Normally I'd use TTL with the flash so then I don't have

to test the lights too much when using the strobes but since the TLR

doesn't have TTL mode I'm a little at a loss. (BTW, the TLR won't be

the main camera, it's just for candids)<br><br>

The GN number on the strobes is 250<br>

I'll be using ISO 160 film<br><br>

 

250 x sqrt(160/100) = GN315<br><br>

 

gn 315 = f32 x 10(+/-)ft<br><br>

 

Is this correct?<br>

Since the camera has a leaf shutter that can go as high as 1/500th

and the flashes have a duration of 1/700th then I should be fine

setting the camera for f32 at 1/500 while staying 10 feet away from

the subjects?<br>

I'll have access to a light meter so I'm not relying on these

calculations but thought I'd entertain myself.

That sounds right - assuming 10ft from the strobes to the subject (camera-to-subject distance has nothing to do with it).

 

There's probably not much need to go to 1/500s - 1/125s is likely to do just as well.

This is correct, but there's an easier way (at least easier for me anyway) to adjust the guide number to a differant ISO. I divide the GN by 10 (since 10 makes the math easy to do in my head), treat that number as an aperture setting for 10' away, adjust that up or down depending on film speed, then multiply that by 10 again to get the new guide number. So for GN 250 @ ISO 100 going to ISO 160, 250/10 = 25, f/25 + 2/3'rds stop (the differance between 100 and 160) is f/32, 32x10 = GN 320, which is equivilant to a GN of 315 (the differance is neglegable, and it's rounded off when shooting anyway, this way rounds earlier in the process). The only thing is, you said you're using two strobes, which changes things a bit, you have to start dealing with additive light. The easiest way is to just meter the thing, and that's what's done most of the time. There are ways of calclulating it, which isn't too hard, but it's kind of a long explanation, so I'll skip it unless you'd like me to go over it.

 

Peter

If you have the formula for adding the second light, can you add that? I'll just work with it for a while and see what I come up with.

assuming that the two lights come from the same direction:

 

fstop=sqrt(GN1^2/d1^2+GN2^2/d2^2)

 

Where GN1 and GN2 are the GNs of the lights, and d1 and d2 the distances from each light to the subject.

 

If the lights come from different directions, the exposure will be different in each direction, and you need to multiply GN^2/d^2 by the cosine of the angle between the normal to the surface and the direction to the light.

 

That is, if I remember correctly what I learnt years ago in computer imagery classes.