Guide number for stobe(S) ?

[MikeB]

On land I shoot 95% in full manual mode. UW is a different story. But I'm leaving Friday afternoon for a trip and want to try manual mode (as much as my current set up will allow). I'm currently using an MX-10/YS-40 combo with a 2nd strobe, a YS-30. I will probably shoot half the shots with the built in 35mm lense, and half with a 20mm wide angle. I will also be raising the stakes by using the new Fuji Velvia 100F.

Here's my question.
How do you figure the combined guide number? Do I just add them together?
The deeper I go won't the guide number change?
I've noticed a little descrepency on the guide number for the YS-30. I've read it to be either 33 or 22 ?
Is the YS-40 considered to be 16 ?

Sorry for the tons of questions, I should've posted earlier I guess.

Thanks

 

[Dee]

I started out with a MX-10/YS40A and moved on to a MMIIex. I have to admit I never paid much attention to the guide numbers. I'm basically opposed to math in any form and it just took the fun out of it for me so I ignored the whole thing! I never had any problems with my exposures so I either lucked out or was onto something.

So I'll defer you to the pro's who I'm sure can tell you exactly what you need to know!

 

[TheAvatar]

I'm not a manual flash expert, but a few things I do know.

Being in water will reduce the above water GN. Going deeper in the water will not.

The stated GN can be incorrect. Take the flash to a local camera store and have them test it with a flash meter. This will give you the true above water GN.

I don't think the GN are added. Inverse square law. I am too lazy to check this math right now.

 

[James connell]

the general rule is 1/2 the surface guide number. remember most maufacturers fudge the numbers a bit. testing is the best way of determining the UW GN.

avatar is right about not adding the guide numbers - it's about 1/2 more power ( i think it's GN times the square root of 2 (1.414), but it's 5am here and i'm just pouring my first cup of coffee).

i'm trying to remeber the ys40 specs - 400iso f8 @ -8ft?- is a full power dump i think.
math is GN = (distance_to_subject x fstop) / <film speed>. that puts the GN @ 36 - seems hot to me....

remeber for the <film speed> part - iso100 is 1 and any other iso is the Number of stops away from iso100 not the iso number. for example iso 400 is 2 stops faster so it's 3, iso50 is 1 stop slower so it's 1/2.

 

[vicky]

True, most manufacturers lie or mislead about their strobe's GN (like in automobiles, who claim they'll do 100Km per liter, yeah, sure).
What you can do is to "waste" one dive and a roll of 100ASA film.
Get in the water, find a nice object where you can sit comfortable 1m (exactly) from it, without raising sand or killing corals. Be sure you measured the correct distance!
Now take photos with different apertures (allways at the same speed. Set it manually if needed to something like 1/60, 1/90 or whatever is OK with your gear). Take one at f/5.6 then f/8, f/11 and so on. Write down on a slate the exact order that you were shooting.
Develop the photos.
Look carefully at the results and look for the photo with the best exposure. Suppose it was the third photo, taken at f/11. Then your GN=distance*f/# and since the distance was 1m your GN will be 11!
Very simple, works great. You can do it with different distances as long as you measure them correctly and also using other film speeds (but you should correct the GN accordingly: 200ASA divide by 1.4, 400ASA divide by 2 ad so on).

You'll get the best results with slides.

 

[Dee]

TheAvatar once bubbled...
The stated GN can be incorrect. Take the flash to a local camera store and have them test it with a flash meter. This will give you the true above water GN.

No wonder the GN thing never worked for me!

 

[MikeB]

Thanks Vicky,James,Dee and AV. Sometime around ... 3:15 am I thought that's exactly what I'm going to do. Just do a test like I do different lenses above water. Quick and easy one time test and then get on with the diving. I even went and bought one of those fancy-smancy DiveRite wrist slates. I saw that and knew I HAD to have it.

More than likely I'll be in the market for a different set-up once I get back. Maybe digital, maybe a Canon SLR housing to go with my lenses.. I don't know.

Thanks again everyone.

 

[Dee]

MikeB once bubbled...
I even went and bought one of those fancy-smancy DiveRite wrist slates. I saw that and knew I HAD to have it.

Did you get the one with the 'pages' on it? You're gonna love it! I changed the straps and put a single 4" wide piece of elastic with a velcro closure that fit my arm better.

It's so handy to write down your settings and stuff when you're testing something or need to keep track of something. Don't forget the 'bask side' of each of those pages. With a little practice, writing at that angle is easy!

 

[Dee]

Mike...check your regular email.

 

[MikeB]

That's the one Dee. That slate is one of those things you see from time to time and say "Damn, why didn't I think of that?"