how to figure guide number on two flashes?

[ASA400]

So, I've got 2 YS-01 flash units I use with my Nauticam housing. I know how to figure f-stops with using one flash. How do you figure the f-stop when using two flashes?

 

[tursiops]

All other things being equal (strobe settings, distance of strobe to subject, etc), two strobes is twice as much light. That is one f-stop. Or, multiply your Guide Number by the square root of 2, i.e. 1.4. So a GN of 20 would become 28. At 5 ft the one-strobe f-stop would be f/4; with two strobes it would be f/5.6....one f-stop.

 

[ASA400]

Thanks!

 

[oly5050user]

All other things being equal (strobe settings, distance of strobe to subject, etc), two strobes is twice as much light. That is one f-stop. Or, multiply your Guide Number by the square root of 2, i.e. 1.4. So a GN of 20 would become 28. At 5 ft the one-strobe f-stop would be f/4; with two strobes it would be f/5.6....one f-stop.

Not so in my experience. It's not like 1 stobe puts out more light overpowering the other if both are equal. Use same guide number for both if each strobe set at same power level and distance from subject. Using 2 strobes does not give you more power, it only increases angle of coverage of your light source and eliminates shadows. If using strobes of different power settings or distance from subject go by guide number of more powerful / closer strobe. To figure out f stop divide strobe/subject distance into underwater guide number. That will give you a starting point for correct f stop and you can also bracket exposures from there as different conditions and subjects reflect light differently.

 

[tursiops]

Not so in my experience. It's not like 1 stobe puts out more light overpowering the other if both are equal. Use same guide number for both if each strobe set at same power level and distance from subject. Using 2 strobes does not give you more power, it only increases angle of coverage of your light source and eliminates shadows. If using strobes of different power settings or distance from subject go by guide number of more powerful / closer strobe. To figure out f stop divide strobe/subject distance into underwater guide number. That will give you a starting point for correct f stop and you can also bracket exposures from there as different conditions and subjects reflect light differently.

Yes, if the two strobes re not pointed at the same place, then you don't get more light at that place. One f-stop difference is the maximum extra light you get if both strobes are illuminating the same subject. If the strobes are just being used to broaden the field of coverage, then there is no extra light to compensate for. The difference is zero f-stops. My "all other things being equal" was meant to include this. Sorry if it wasn't clear. OP, you'll adjust from not-at-all to not-more-than one f-stop, depending on how you have the strobes aimed.

 

[ASA400]

okay, I understand. I'm used to using TTL with my flash (or flashes), but I've been told that the Sea & Sea flashes don't recognize TTL with my Olympus e-m5 camera.

 

[tursiops]

Nor do they with my M10!

 

[Searcaigh]

Shoot manual then and experiment

 

[JohnN]

Shoot manual then and experiment

And shoot in RAW so you have a better chance to fix the mistakes