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Deep-sea Camera - Watt Seconds


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#1 JamesWood

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Posted 26 April 2006 - 01:32 PM

A fellow faculty member just showed up with a 30 year old Benthos deep-sea camera rated to a depth of 12,000 meters. The camera is in remarkably good shape and is very well built – LOL, it would have to be. I'd like to double check my exposure calculations with the strobes output. The strobe output is 50 watt seconds, does anyone know how to convert that into a guide number for 100 ASA/ISO film?

If anyone has worked with deep-sea camera's please contact me or post below.
Dr. James B. Wood
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#2 Poliwog

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Posted 27 April 2006 - 08:13 PM

Hi:

I did a quick check on www.digitaldiver.net strobe finder comparison database and found the Ikelite Sunstrobe 50 and DS50 models with a 50 watt/second rating have a GN of 17 (air) - 9 (u/w).

This may be a rather simplistic reply for the level of accuracy you may be demanding, as there are a number of variables affecting the power output of a flash. Water turbidity, reflector design, colour temperature, and flash tube have to be taken into account.

The above GNs should “get you in the ballpark” for exposure calculations. If you require more detailed information two interesting articles can be found at:

http://www.imagingin...pubId=3&id=1043

and

http://tomar.com/pdf/pg099.pdf

Otherwise, using a flashmeter in air and halving the resultant reading will probably be just as precise as anything mentioned above.

Hope this helps.
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#3 JamesWood

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Posted 28 April 2006 - 03:32 PM

Thanks - that should get me in the ball park! Are those numbers per meter or per foot? In anycase, we are going to have to shoot a number of test rolls with this beast befor we send her down to the depths and try for the big squid on 35 mm. A ball park is all we really need for starting.

I'm told the water is very clear at 1000 m and the strobe is daylight balanced (or what passed for daylight in the early 70's). The reflector needs to be replaced or polished.

Thanks again.
Dr. James B. Wood
Associate Director of the Waikiki Aquarium
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#4 Glasseye Snapper

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Posted 28 April 2006 - 04:25 PM

Hi James, those numbers are for meters. If you like to think with your feet the numbers are 56 (air) and 28 (underwater).

Another important variable is the coverage angle. The DS50 has a 70 degree angle and therefore concentrates its 50 watt-sec in a smaller cone giving similar light density as the more powerfull DS125. I don't know the definition of the "guide number" but I expect that it is independent of coverage angle. However, if you only know the watt-sec for your strobe it is something you should look at. I expect it will scale with the square of the sine of half the coverage angle.

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#5 JamesWood

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Posted 29 April 2006 - 03:51 PM

The 1970's manual has the equation

Z* = (1.6 times the square root of (ASA times Watt Seconds))/Aperature

They don't define Z*, it is on a log scale, I though it was distance from flash to subject. . . Maybe Z* is the amount of turbidity?

With 100 Watt seconds, 200 ASA and f11 they say you can correctly expose a subject 23 feet away – which seems like one heck of a lot of light to me! I get 14 feet when I try it. . . hmmm that is what is on the X axis. So Z* must be turbidity?

The actual strobe is 50 WS, not 100 and the film will be 100, not 200.

So, at f11 I should get a correct exposure at: 10.6 feet (wow!???) Seems to high for a 2 stop loss.

Guide number, which is independent of area of coverage as you mentioned is GN = Distance * Aperture (at a set ASA/ISO). It is an easy way to do exposures if you aren't on TTL. I'm just trying to get into the ball park here. . . apples and oragangs. . .

So that equation gives me a correct theoretical exposure at 2.5 feet, not 10.6 feet! Who knows how much the beam is focused by the big reflector. It looks like I'll have to shoot a series when we get it working and find out, at least I have a reasonable range to play with. Also, it looks like the 28 mm lens may be wider than the flash coverage.

Thanks!
Dr. James B. Wood
Associate Director of the Waikiki Aquarium
The Cephalopod Page

#6 bvanant

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Posted 01 June 2006 - 04:10 PM

According to a paper by Benny Gallaway and colleagues, they used a Benthos 372 camera, 28 mm lens, 200 watt second strobe to shoot Iso 200 film looking at a patch of ocean bed, 1.3 x 1.8 m from a 2 m height above the ground. No description of the reflector shape, but that should be accounted for in the strobe output, I think. You might give the Teledyne guys in MA a call, they could probably help you with the repolishing.
Bill

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