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Strobe Test Needed?? Or Are Results Typical??


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#1 cabdiver

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Posted 26 September 2011 - 01:43 PM

Group - During a recent trip, I was working on wide-angle and started at an arm's length away from a wreck and set the strobes 2 1/2 ft away from the dome (pointed straight ahead).

My first exposure was at 125/f8 with my strobes on full power (Sea&Sea 110a) with diffuser.

I was surprised that the photo was underexposed.

I got closer and brought the strobe's in to match the distance from the subject.

After a couple of tries, I found that a distance of 18 inches resulted in an acceptable exposure. Based on what I've read, this seemed "underpowered."

This led me to test my system at home with tripods and a tape measure.

I've found that 31 inches from the subject at 125/f8 is underexposed by about two stops. (Lightroom histogram indicates highlights slightly blown at f5.6)

So, what do you think?

Is this result typical? Does it indicate a problem with my particular strobes? Is there a more definitive test of the strobes power? I'll be in the pool in a couple of weeks - any suggestions for another test?

Thanks for your help.

Chris

#2 Steve Williams

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Posted 26 September 2011 - 03:05 PM

Hi Chris,
A couple of things to check first, were you shooting ISO 100? When you say 2 1/2 feet away from the dome, is that between strobes or truly 5 feet between the strobes?

What kind of water are we testing in? Tropical, good vis, UK-green, ???

Which lens are you testing with?

Cheers,
Steve

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#3 cabdiver

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Posted 27 September 2011 - 03:51 AM

Hi Chris,
A couple of things to check first, were you shooting ISO 100? When you say 2 1/2 feet away from the dome, is that between strobes or truly 5 feet between the strobes?

What kind of water are we testing in? Tropical, good vis, UK-green, ???

Which lens are you testing with?

Cheers,
Steve


Steve - I was shooting in the Keys with a Nikon D300 at ISO 200. The visibility was reasonable - about 60ft in slightly greenish water that morning.

The strobes were about 5 feet apart and pointed straight ahead. The lens was a Tokina 10-17.

I was using this dive as a way to get familiar with proper lighting for wide-angle (It was time to move past macro).

My pre-reading of advice on the forum suggested that full power at this distance (even with the diffusers) in these conditions should have been more than adequate. My surprise at the need to get so close (~18 inches) to get a proper exposure led me to set up the test in my entry way.

And then, I was really surprised to find that in the absence of water [my entry way is above the Ohio river's flood plane ~:) ] at 31 inches from the subject with the strobes set an equal distance on each side of the camera that the shot was at least one full f-stop underexposed.

That's why I'm interested in understanding what experience others have had with the shutter speed and f-stop needed for a wide angle shot in tropical waters with reasonably good visibility.

Thanks, Chris

#4 twinner

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Posted 27 September 2011 - 06:39 AM

Hi chris

Pull your strobes in closer. It sounds to me like your just not getting enough of the light on your subject. Can u post an image?

Edited by twinner, 27 September 2011 - 06:44 AM.

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#5 cabdiver

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Posted 27 September 2011 - 09:49 AM

Hi chris

Pull your strobes in closer. It sounds to me like your just not getting enough of the light on your subject. Can u post an image?


Yes, you are absolutely correct. I needed to get within about one and 1/2 feet from the subject with the strobes pulled in to get a reasonable exposure.

In your experience, are you able to get reasonable exposures when you're about 3 feet away from the subject?

If so, I'd be interested in the camera settings and strobes used.

Thanks, Chris

#6 Steve Williams

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Posted 27 September 2011 - 10:10 AM

Hi Chris,

Iím with Todd, Iím thinking you may want to pull the strobes in toward the housing. If I understand your setup youíre adding about a foot or more of light travel through the water that isnít necessary. My go to jump settings in bright sun, tropical water - f/8, 125th, @ISO 100 is the typical exposure for a nice blue background. I then use my strobes to light the foreground subject, ĺ power is where I start with the 110s or ľ or half power with the 250s.

Sounds like the strobes are working as designed; maybe a little more play is in order. If you can find a pool, itís amazing what you can learn just shooting the side of the pool and watching what happens to the light with different strobe positions. I like your first thought of about 18 inches away from the subject as a place to start. 3 feet is pushing it.

Have fun,

Steve

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#7 twinner

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Posted 27 September 2011 - 11:23 AM

Hi Chris,

Like Steve said closer is better. I can get well exposed images from 3 feet away but I'll shoot closer if I can still get the composition I want. I think your problem is with strobe positioning but without an image to look at I'm just guessing.
All of this takes time and practice. Especially wide angle. Remember your shutter is having no effect on your strobe exposure only the ambient light. For strobe exposures your only dealing with strobe power, aperture and distance for a set ISO.
Move your strobes around any play with your settings. If you can get to a pool like Steve suggested I would.
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