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How "far" does strobe light reach??


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#1 NWDiver

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Posted 06 March 2005 - 02:37 PM

(was part of another thread that died out and was not answered)

Thinking about getting a third strobe as a back up to my 2 S&S 90DXs. To try some 3 strobe set-ups for reefscapes, wrecks, schools, etc. What do you think of adding a more powerful strobe than the S&S 90DXs I have now? I'm thinking along the lines of an S&S350 or Nikon SB104. Question time....

I have been told typical strobes reach out about 4ft max. Is there a strobe that "throws light farther"? (understand spectrum of light changes as light from strobe weakens)

Now this is a rookie question, what exactly does the "Guide Number" translate to underwater?

I understand different strobes have different Kelvin/lumen ratings. Does it matter if you are mixing strobes with different ratings? Is it a disadvantage? Why?

#2 Giles

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Posted 06 March 2005 - 03:23 PM

At 0 degrees Kelvin (equivalent to -273° Celsius), the theoretical blackbody emits no light. As the blackbody warms, it begins emitting red light. As the blackbody continues to increase in temperature, light wavelengths become more yellow, then green, blue, and finally violet. A candle flame on the Kelvin scale has a rating of 1800 K. Sunlight at noon, with a Kelvin rating of 5500 K, is typically referred to as full spectrum because it contains a blend of all colors throughout the spectrum. Reddish light has a lower K rating and color temperature, while bluer light has a higher K rating and a higher color temperature.

In nature, as light enters and passes through the first 15 feet of water, the red and orange wavelengths are absorbed by the water, increasing the light's K rating, and giving the light a bluer appearance. As the light penetrates to 30 feet, the water absorbs the yellow spectrum. And as the light continues past 50 feet, the water filters the green wavelengths, leaving just the blue and violet wavelengths. This results in light with the highest Kelvin rating.

LiveAquaria.com

Very nicely explained on that page, but light does travel through water more than 4 ft just different parts of light make it further.

In theory from this you would want a lower K rating strobe to create a redder light which would warm up the underwater image. I don't think this will make it go any farther though. Is it possible to make light more powerfull than the sun ? i doubt it, but maybe it's not about power. From my understanding it is density of water that effects the absoprion of the light spectrum, does water get less dense when it is cold ? is that why colder waters are green as they have more yellow in them ?
So if you could dive in ice you may get good distance from a strobe :blink:
I'm just kinda of talking as i think here from what I know .. am i even close to being right with the cold water thing ?

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#3 craig

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Posted 06 March 2005 - 05:15 PM

Strobes can travel much further than 4 feet. It depends on how you define the problem though. If you are diving at night how far away from a subject do you need to be before you image will be black?

Guide numbers don't convert well to underwater because (a) light is absorbed by water, and (B) the radiation pattern of the strobe is different.

It is OK to combine strobes of different power but it isn't wise to combine strobes with much different Kelvin ratings. If you do so you will get color shifts from side to side and in shadows. A few hundred Kelvin may be OK.

Lower Kelvin strobes will not travel as far underwater because they have a greater portion of red light which dissipates more quickly.
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#4 Tom_Kline

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Posted 06 March 2005 - 05:30 PM

I have had good luck to about 10 feet with ISO 100 with one SB104 or one SS200. Shooting moving fish with a 50 mm f/2.8 on an RS. I shot wide open. My tendency with the RS 50 lens has been either to shoot it wide open or closed down all the way according to whether I was doing macro or whole fish shots. The quality of the 50 is quite good at f/2.8 too, probably due to it being a water contact lens.

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Lens focal lengths ranging from 8 to 200mm for UW use. Seacam housings and remote control gear. Seacam 60D, 150D, and 250D, Sea&Sea YS250, and Inon Z220 strobes.

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#5 NWDiver

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Posted 07 March 2005 - 06:47 AM

Ah, I am starting to see the light (counldnt resist). I did not think about the color shift. What if you had four in the set up say for WA and mixed the strobes, still a problem? So if I have the 2 S&S 90DX strobes, looking for a little more power, what strobes would you all suggest? (based in trying 3 strobes for macro and 4 for WA)

#6 richgarrett

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Posted 07 March 2005 - 08:14 AM

a strobe will put out light past the water visibility. I have seen times when I could see another divers strobe flash and I couldn't see them. BUT:

when shooting a subject, the strobe light has to travel twice the distance to the subject (to the subj and back). light intensity drops off rapidly in water depending on the water conditions.

so if you are shooting a nice red fish at a distance of 10 ft. your strobe light travels 10' to the fish and reflects back 10' to you. you will have lost nearly all the red in the fish and have a very disapointing photograph. plus you will have 20ft of water to absorb the strobes output and there will not be very much reflected light reaching your film/digital sensor.

underwater guide numbers are not accurate, but the principle does apply.
in air the GN = distance X fstop. a strobe with a GN of 16 will give the right exposure at
1' @ f16
2' @ f8
4' @ f4
8' @ f2

reflected difuse light (as opposed to light reflected off a mirrored surface) obeys the 1/d squared
law. something like the

distance from light reflected
subj to film
1' 1
2' 1/4
4' 1/16

this shows up in the GN chart since each fstop is 1/2 the amount of light as the next fstop.
i.e f16 is 1/2 of f11 and 1/4 of f8.


the guide number of a strobe underwater is reduced by many factors (water condition, refractive index of water/strobe surface, etc) and is usually about 1/2 the GN in air, OR LESS.

if you are diving in Palau with about 200' vis the UW GN would be about 1/2 the Air GN.
but on a summer day in Puget Sound with 15' vis it would be much less.

HOPE this wan't too technical with too much math. the bottom is for better photos get CLOSER to your subject!

rich

#7 craig

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Posted 07 March 2005 - 01:30 PM

If two strobes isn't enough power you are better off getting two more powerful strobes than you are adding a third and fourth strobe to them. The S&S 90's aren't very powerful strobes.
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#8 fdog

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Posted 07 March 2005 - 06:05 PM

In crystal-clear water, I've been able to kick up the shadow of the side of a wreck by a bit (enough so that it wasn't painfully black) that was about 40' away. Used a SS400.

Blue as could be, of course.

All the best, James

#9 NWDiver

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Posted 08 March 2005 - 07:28 AM

More great info. Done a good job keeping it simple. So once I get a handle on Guide Numbers, Distances, Color Spectrum, Lumens and reflective light, I can start on adding filters! Better figure out what color cyan is first :blink:

Just picked up an S&S350 so will start experimenting with it mixed with the 90DXs.