Jump to content


cabdiver

Member Since 26 Sep 2011
Offline Last Active Mar 10 2014 06:12 PM
-----

Topics I've Started

Understanding my Strobe

20 November 2011 - 10:59 AM

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.
Posted Image
First Shot by cabdiver, on Flickr
http://www.flickr.co...157628056020095


Posted Image

7th Shot by cabdiver, on Flickr
http://www.flickr.co...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

Posted Image
UW GN Test @ f/6.3 by cabdiver, on Flickr
http://www.flickr.co...157628055975439

Posted Image
UW GN Test @ f/5.6 by cabdiver, on Flickr
http://www.flickr.co...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
  • [indent=1](Defined as f-stop where highlights just started to clip)
Summary of GN Findings by Variable:

Following 3 GN values based on 1 strobe directly over housing 2ft from subject
  • 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 for 1 strobe placed 2 ft to side of housing with diffuser
  • 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:
Conclusions:
As with most things, there is nothing new or unknown in any of these test results.

1. Manufacturer’s GN specifications are based on a testing protocol that does not necessarily represent the type of conditions UW photographers encounter. Because of this, these specs are most useful as an indication of the relative power of their different strobe models.
2. This test’s measurement of a GN of 14 in air for the Sea & Sea 110α is consistent with that reported by Backscatter. [see link above]
3. Water decreases the GN – about 2/3 stop in this test – compared to air. [Duh!]
4. A diffuser reduces the 110α’s GN by about a stop. [Previously documented]
5. Housing-subject and strobe-housing distances are key factors to consider for good exposure of foreground at a selected f-stop/ISO combination.
[Note: shutter speed has negligible effect on foreground exposure because flash duration is so brief.]

Personal Ah-Hah Moment:
One of the first things we learn in UW photography is to move the strobes out to the sides of the housing to minimize backscatter. This results in a cone of light that just cuts in front of the subject and reduces the number of particles illuminated directly in front of the camera (backscatter).

The recommended starting point is to place the strobes out to the side of the housing so that the housing-subject and housing-strobe distances are about equal. What I didn’t appreciate until now was the impact this has on the light intensity at the subject. For a 2-foot subject-housing distance and an equivalent strobe-housing distance, the resulting strobe to subject distance is 2.83 feet. And while this difference isn’t large, it has a significant impact on the light intensity because of the “inverse-square law.” This law states that light intensity at the subject is inversely related to the light source’s distance from the subject. To illustrate, if you move the strobe from 1 to 2 feet, the light intensity is reduced to ¼ of what it was [1 divided by 2 squared].

So in the tests summarized above, when the strobe was moved from directly over the dome to the side – 2 feet away from the housing – the strobe-subject distance increased from 2 feet to 2.83 feet. The effect of this was to reduce the light intensity at the subject by two-fold or one f-stop.

{Note: The formula used to calculate the change in light intensity when subject-strobe distance changes from 2.0 feet to 2.83 feet is: 22 divided by 2.832 or 4/8 = ½.}

Implications for My System:
After realizing the effect of these seemingly small changes in strobe position, I decided to calculate the impact of different strobe-subject and strobe-housing distances for my system. The following table assumes an effective GN of 16, based on the use of 2 strobes with diffusers and a baseline ISO of 200 for a Nikon D300.

Posted Image
Table_for_Post-1 by cabdiver, on Flickr
http://www.flickr.co.../in/photostream

This table illustrates that the biggest bang for the buck results from simply moving closer – about a stop from 3 to 2 feet and two stops from 2 to 1 foot. Moving the strobes in closer to the housing provides the ability to increase the light intensity a bit more with each foot closer to the housing – about 1/3 of a stop/foot when the housing-subject distance is 3 feet and 2/3 of a stop when the H-S distance is 2 feet.

All of the data and calculations summarized above represent the worst-case scenario where the ambient light provides NO meaningful contribution to the foreground exposure. The conditions for the wide-angle photos taken during the workshop illustrate this well. I was shooting into the light at about 40 feet, in green water with only moderate visibility. In these conditions, the strobes needed to provide most of the light.

Concluding Comment:
In conclusion, the greatest value from this “data exploration” was personal. I followed Steve Williams’ advice and got in the pool and started to understand my strobes and camera as a system. I can only echo Steve’s recommendation to others, get in the pool or some other controlled condition and experience for yourself what happens with the light.

Try it – you may like it.

As a bookend, the following is my best effort at the wreck.

Posted Image
Wreck by cabdiver, on Flickr
http://www.flickr.co.../in/photostream

Trying to Link to Flickr

26 October 2011 - 05:28 PM

Let's see if I can successfully use Adam Hanlon's suggestion in Eric's post re: Posting photos from Flickr.

If you see a photo in this space, then we have 'lift off.'

If not, further suggestions would be appreciated.

Chris

I just tried the "preview post" and was more than pleased to see that it worked.

Many Thanks to Adam et. al.

Posted Image
DSC_6247 by cabdiver, on Flickr
http://www.flickr.co...ver/6284236591/

Strobe Test Needed?? Or Are Results Typical??

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