Understanding my Strobe

CAUTION: This is long.

Why Post This?

Several weeks ago, I submitted my first post to the Wetpixel forums after returning from a September UW photography workshop (Thank you Reef Photo & Video). One of my goals was to “better understand manual lighting for wide-angle.” The plan was to experiment with different strobe power settings at different subject distances before attempting to take “real” photos of wide-angle scenes.

Because my first “experimental” photos were so underexposed; I thought there might be something wrong with my strobes and sought some advice before deciding what to do. A comment from Steve Williams was most helpful – “try getting in the pool and seeing the effect of the strobes on the wall.” This led to a series of tests of my strobe’s output (guide number) and then a session in the pool. [so, you see we really have Steve to thank (or blame) for what follows. ]

I decided to share this because there may be others on the forum like myself who are trying to figure out how best to use their strobes.

And for those who would like to find how this story ends, scroll to the bottom where the findings and conclusions are bulleted.

What Happened?:

For the first shot, I settled about 3 feet from an isolated part of a wreck, metered the water mid-way between the sunball and wreck, set the camera at 1/125th and f8 with both strobes (with diffusers) at full power and positioned about 3 feet from the center of the dome port. When reviewing the shot, I could barely see the wreck. I was really surprised and took another shot after checking that the strobes were on and firing – same result.

It was time to move to plan B. I gave up on the idea of doing a series of shots first at 3 ft, then 2 and 1 with different strobe power settings at each distance. It was time to just try to get a well-exposed photo. I did what we all do – got closer, moved the strobes in closer to match the subject distance and then took more shots until I was in the ballpark. After that I moved on to work on composition.

Initial Photos:

The first shot at 3 feet and final result at about 2 feet are shown below for information. My best wide-angle shot of this dive is provided at the end of this post as a bookend.

First Shot by cabdiver, on Flickr

http://www.flickr.com/photos/cabdiver/6365...157628056020095

7th Shot by cabdiver, on Flickr

http://www.flickr.com/photos/cabdiver/6365...157628056020095

What follows is a brief outline of what was done, key findings and conclusions.

Tests done:

• A series of strobe tests on land and UW
  
• Isolated variables: land, water, strobe-housing distance and diffuser
  
• Determined Guide Number in feet at ISO 100
  

Equipment Used:

Nikon D300; Subal ND30 Housing; 8 inch Dome Port; 2 Sea & Sea 110α strobes; Tokina 10-17 mm; nylon rope with knots – 1 ft apart; and a DGK Color Chart (laminated for waterproofing)

Important Caveats:

• Results can only be viewed relative to each other in context of the variables tested and cannot be judged versus manufacturer specs or directly extrapolated to other Sea & Sea strobes.

• Results also depend on the sensitivity of the camera’s sensor and cannot be extrapolated (from a D300 to a D7000, for example).

Example Photos:

The pool photo at f/5.6 defined the UW GN for strobes with diffuser – see below

UW GN Test @ f/6.3 by cabdiver, on Flickr

http://www.flickr.com/photos/cabdiver/6365...157628055975439

UW GN Test @ f/5.6 by cabdiver, on Flickr

http://www.flickr.com/photos/cabdiver/6365...157628055975439

Guide Number (GN) Definition Used:

GN = aperture (properly exposed at ISO 100) X distance from strobe in feet

GN Measurement Technique:

• Baseline exposure taken to assure test measured strobe power
  

• Shot taken in darkened room and indoor pool resulted in black photo

  
  

• All photos taken at distance of 2 feet from subject
  
• Increased aperture until photo was overexposed
  
• Properly exposed photo determined from Lightroom 3 histograms
  
• (Defined as f-stop where highlights just started to clip)
  
  

• Baseline
  (Air/No Diffuser) ⇒ Manufacturer – 22 and “This Test” – 14
  
  
• GN findings similar to that reported by Backscatter
  
  
• [link –http://www.backscatter.com/learn/article/article.php?ID=40]
  
  
• Air/Water
  (no diffuser) ⇒ Air – 14 and Water – 11 [2/3 Stop]
  
  
• No Diffuser/Diffuser
  (Water) ⇒ No Diffuser – 11 and Diffuser – 8 [1 stop]
  
  

• GN calculated
  assuming no change
  in strobe-subject distance – 5.6 [1 stop]
  
  
• GN calculated
  for actual strobe-subject distance
  – 8 [zero difference]
  
  
• Calculated based on geometry of triangle [c = square root (a2 + b2)]
  
  
• See Conclusions Below:
  
  

Good stuff Christian. Glad to see it's making sense to you. Did you play with shooting at 5.6 or so as a baseline? Given the conditions you were in it might be a good option. In the best case I like to have the ability to dial in more or less power on the strobe dial versus shooting at full dump. Faster recycle times, and allows you to vary the light across the image.

More great reasons to practise!

Cheers,

Steve

This was a terrific write-up. I've been looking for something like this for a while.

Thanks for taking the time to write it up and post it!

--Mark

cabdiver:

Exceptional post. As an engineer, I appreciate your methodology and the manner in which you displayed and discussed your results.

Thanks for sharing.

Harry

thanks for sharing it Cabdiver, I am using a different camera & strobes set up but can definitely extrapolate some concepts.

Great work ;-)