Underwater Guide number?

There are very different views on the gudie number values underwater: some use a factor of 2, some even 4(!) to estimate underwater GN from land values.

Which is correct for clean blue seawater or 50m vis pool? I suppose color temperature also has a significant effect here, the 4200-4800K strobes getting punished for their nice warm light...

E.g how do the inon z240's compare to the subtronic novas in terms of underwater working distance? (iso, f-stop being the same)

Coverage might be the cause of confusion here, "flat port" strobes, like the inons, squeeze their beam underwater, e.g. the 100°-s of the z240 becomes 70°, approximately doubling the power/surface ratio.

I guess a factor of 2 is sufficient for "squeezed angle" strobes, factors of 3+are for dome-port units, like the subtronics, seacam.

Hello Tamas,

Few years ago i worked with measuring GN on the land and underwater.

I used 3 different optical instruments: 2 professional flash meters for land foto/video (packed in specially made uw housings) and Ikelite underwater flashmeter.

Before submerging i measured strobes on the land. All those flashmeters confirmed GN at the centre of light beam for Z 240, D 2000, S 2000 strobes, according their specifications.

But i got few absolutely different results with GN underwater (factor was 1.8…3).

Some later one optical factory confirmed me that very difficult to make true underwater housing+optics for a land flash meter for precise measuring. It would be a real mastery to make such housing, and they don't know about such cases. Flashmeter is rather complicated optical instrument. Optics engineer looked carefully all my 3 flashmeters and confirmed, that they currently show underwater just digits, but not a real GN.

So, you understand, why manufacturers of strobes sometimes give different GN for underwater. Almost impossible to measure underwater GN by standard optical instruments. Results will be very different. Some manufacturers just calculate approximate underwater GN for strobe specification, don't measure it.

In reality underwater GN is not so important for photographers, most of manufacturers specify it as a reference value. Usually for uw strobes we rate only GN measured on land, this is a main digit.

Trouble is when only underwater GN is provided, like in case of the subtronic units (how they measured, that's an other question...)

Trouble is when only underwater GN is provided, like in case of the subtronic units (how they measured, that's an other question...)

I think, Subtronic can answer about land GN by e-mail. This is not a secret.

Also possible to rate GN approximately, if we know the strobe's power and tube shape. For example, if you mean a 160ws strobe with round tube, the GN=24 is expected.

E.g how do the inon z240's compare to the subtronic novas in terms of underwater working distance? (iso, f-stop being the same)

For 250ws Nova approximately GN=32 is expected. I think, it is close to real value.

For the same light intensity the working distance for Nova approximately 2 times more than Z-240. Because GN difference is about 1 stop.

Another words, if you shoot with Z-240 at 1m from object, possible to light the same at 2m using Nova.

Also, you can change an aperture for 1 stop to get the same result.

 

For the same light intensity the working distance for Nova approximately 2 times more than Z-240. Because GN difference is about 1 stop.

Another words, if you shoot with Z-240 at 1m from object, possible to light the same at 2m using Nova.

Also, you can change an aperture for 1 stop to get the same result.

I think you mean 1.4m with the Nova. 2m would be a factor of 4, or two f-stops.

I think you mean 1.4m with the Nova

Sorry, i misprinted. For 250ws Nova and Z-240 the GN ratio approximately is 32/24=1.3

So, if we shoot at 1m distance with Z-240, we can shoot at 1.3m with the Nova.

I think, Subtronic can answer about land GN by e-mail. This is not a secret.

Also possible to rate GN approximately, if we know the strobe's power and tube shape. For example, if you mean a 160ws strobe with round tube, the GN=24 is expected.

Thanks Pavel,

This indicates a land value of GN40 for the 270ws Nova, which is close to double the working distance, especially considering its warmer spectrum.