Help with guide numbers

[Hoya97]

Hi,
I recently attended a photographic seminar at Oceanfest in Ft Lauderdale and the speaker emphasised the value of knowing the guide number of the flash unit to obtain proper ballance in your photos. Okay. Not a problem.

I am using a Sea and Sea YS60 attached to a Sea and Sea Motor Marine II with the standard lens. According to a number of sites I've visited, they suggest that the YS60 has a 72ft guide number at ISO100. The questions I have should be simple to resolve.

Specifically, how do you adjust the guide number when using faster film (i.e. 200 or 400)?

Also, how do you adjust the guide number when using 1/2 power? I think the response of halving the guide number (36ft?) is too simple in this world of complex responses to simple problems.

Any help you could give to a guy who has too many over- and under-exposed photographs would be appreciated.

Regards

Lee

 

[gert7to3]

The guide number refers to the output of your flash for a specific ISO film speed. For a higher speed film you would multiply the guide number by a factor derived by dividing the new ISO speed by the original ISO. ie for 200 ISO film: 200/100=2. The new guide number is 2 X 72= 144. Note the guide number is for air, not underwater.

Use the guide number to determine the size of your aperture or "f" stop (the lens opening) on your camera. Divide the guide number by the distance from your strobe to the subject (ie: for a subject 10 feet away from your strobe, the f stop would be 72/10=7.2). Camera apertures usually run 1.4, 2, 2.8, 5.6, 8, 11, 16, 22. 7.2 is nearest to f:8, so this would be your aperture using ISO 100 speed film. For ISO 200 film: 142/10=14.2. "f" 16 is closest to 14.2

Of course if you were shooting a very light or dark subject, allowance would have to be made by opening or closing your aperture accordingly.

Since you have a 1/2 power setting on your strobe, you could use the ISO 100 speed f:8 aperture, for a subject 10 ft. away, with ISO 200 speed film.

The 1/2 stop power setting can be used for macro photography were the full setting would overexpose the subject and your lens could not be stopped down enough.

Isn't there a flash unit dial calculator on your strobe?

 

[Hoya97]

Thank you so very much for your reply. I goofed around with it today during a dive. Results were less than spactular, but there was improvement. You stated that 72 guide number was for land. Do you have a computation for water?
Again, thanks a lot - I really do appreciate it.
Regards,
Lee

 

[alcina]

I have to admit that I wouldn't have the slightest clue on the guide number for my strobe.

Are you shooting digital? Does your camera have a histogram feature? This feature is worth exploring as a tool in getting the exposures you want on a shot.

I am sure that guide numbers are important, however, from what I've seen and experienced there are many other factors that are "more" important to getting great results. YMMV.

 

[alcina]

OK...it has been brought to my attention by a stobe & lighting guru that the information so far here may not be entirely correct.

you factor by 1.4 (not 2) when doubling or halving GN's or increasing and/or decreasing strobe light output in one stop increments (from full power to 1/2 is = to a decrease of one stop of light).

Also, the most powerful underwater strobes available are not effective beyond 5-6 feet. To give an example that states an answer of 10 feet is very misleading, imho......

Hoya...there are some really useful tips and very knowledgeable people at www.digitaldiver.net - I'd repost your question there under Strobes & Lighting and get even more answers. HTH

 

[gert7to3]

Alcina,

The discussion was about guide numbers, specifically a GN of 72 at ISO 100 on a Sea & Sea YS-60 strobe. Interestingly, Sea & Sea says their GN is 22 for the YS 60 strobe. Hoya97 noted a GN of 72 and I did not check S&S's website before responding.

Hoya97 also stated that he has a Motor Marine II, which if I have the model right is a film camera.

The example of 10 feet was not for underwater purposes, but for ease of computation. I specifically stated this in my first paragraph. This is also stated on Sea & Sea's website describing their strobes, their guide numbers are for air.

The formula I referred to is a standard flash exposure formula (in air).

f number = guide number/flash-to-subject distance or: f=GN/D

If this formula is wrong, your guru should address the matter to Ansel Adams, Alfred Blaker, Charles Swedlund, and Eastman Kodak.

This of course changes underwater due to water density, spectral absorbtion and debris in the water. It also changes when one starts to do macro photography. However, this was not part of the discussion.

 

[gert7to3]

What are the exposure instructions for your camera and strobe? Is the Moto Marine II a TTL camera for flash photos or does it require you to calculate your apertures?

Which type of film are you using?

Are you bracketing your exposures, that is taking several shots of the same subject at different apertures?

Slide film has less exposure latitude than print film. It's around 3 stops for slide film and 5 stops for print film.

What is the ultimate use of the images you are shooting?

 

[alcina]

Yikes...don't shoot the messenger!

 

[bobf]

Yikes...don't shoot the messenger!

Sorry for asking Alcina to forward my message to the board. I was trying to offer a quick reply without registering. But here I am.

I did not question the guide number formula stated by gert7to3. I questioned how he derived at a guide number relating to ISO 200. IF the guide number for a given strobe were to be listed at 72 at ISO 100, then the same strobe would have a guide number listing at ISO 200 (air, feet) of:

72 times 1.4 (not 72 times 2) or 100.8.

Likewise, if a strobe's GN at full dump were 72 (ISO 100, air, feet), then at half power the strobe's GN would be 72 divided by 1.4 or around 52 (ISO 100, air, feet).

Like gert7to3 stated, the filtration power of water changes illuminating powers considerably.

BTW, Sea & Sea may list the GN at 22. But if they do, check to see if the listing is measured in meters (not feet).

Alcina,

thanks again for trying to pass this info on.....

regards,
b

 

[gert7to3]

Bobf,

Good point about GN's expressed in meters, wish it was made clear by S&S.
GN(m) 22 X 3.28= GN(ft) 72

I was not trying to get into converting guide numbers.

My main point was that for a given distance one would need to close down one stop of aperture, if the film being used was an ISO 200 speed film versus the ISO 100 speed film which the strobe was rated for.

Hoya97,

Sea and Sea has a wonderful book titled: The New Guide to Sea & Sea which covers your questions very well. They state that underwater, guide numbers should be about 1/2 the land value in clear tropical waters and 1/3 the land value in coastal or turbid waters. They have tables specifically covering your equipment. This information is probably in the instructions which came with your equipment. If you bought the equipment used, with no instructions provided, contact S&S or buy the book.