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Ambient Backscatter


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#1 bmorescuba

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Posted 27 January 2018 - 01:42 PM

Recently, I spent a couple days in central Florida. We only had time for a few dives - one in Crystal River, one in Alexandria Springs, and one in Blue Grotto (plus a manatee snorkel). The first time I've done any of those locations. It was also the first time I've been able to take my new em1mkII underwater and I was pretty psyched. Since the average depth of most of the dives was <10ft., I thought I would keep things simple and just shoot with ambient light. This is not something I've done very much - I almost always shoot with strobes.

 

I was surprised by the amount of backscatter in most of my photos, especially given the generally clear conditions. It sure looks like backscatter to me at least. I hadn't really been worrying about it, since I always associated backscatter with strobes. Can others comment on backscatter occurring in ambient light and tips to minimize it? There was certainly particulate in the water, but it appeared generally clear with good viz.

 

Thanks!

 

Here's an unretouched example:

 

 

 

P1130015.jpg


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#2 tursiops

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Posted 27 January 2018 - 02:45 PM

Some of that looks like near-surface bubbles (was it windy? were there people splashing around?), and some looks like "marine snow." It will mitigate a bit on cloudy, dull days, but I'm guessing from the shadows it was quite sunny. So you see "side-scatter" and "forward scatter" rather than just back-scatter. Solution? None I know of, except post-processing. A video can help you determine the source....easy to pick out bubbles vs sediment from their different dynamics.



#3 Tom_Kline

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Posted 27 January 2018 - 03:48 PM

I have the same problem as well when shooting salmon in streams using ambient light. There is a fair amount of drifting debris. Bubbles can be a problem (as just suggested) as well. Shooting with a fisheye lens at point blank range helps to minimize the problem. As well, it is best to avoid shooting into the light. Sometimes one does not much of a choice other than not shooting at all.

Check out this shot:

 

http://www.salmonogr...mon/i-9CHRBCD/A

 

(easier for me to find image on my site). Note the light blotches in the water column. I see a dark one too (left side), probably a twig. Not too bad backscatterwise at the spawners that were real close. I angled the camera to avoid shooting into the sun - this gave the reverse perspective from many of my other shots - but easy to see the eggs emerging from the female from this angle! A downside is that the housing cast a shadow on the bottom (shot taken less than 2 hours from true noon).


Thomas C. Kline, Jr., Ph. D.
Oceanography & Limnology
Currently used housed digital cameras: Canon EOS-1Ds MkIII, EOS-1D MkIV, and EOS-1DX; and Nikon D3X. More or less retired: Canon EOS-1Ds MkII; and Nikon D1X, D2X, and D2H.

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.

http://www.salmonography.com/

 


#4 tursiops

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Posted 27 January 2018 - 04:37 PM

Excellent point on trying to not shoot into the light, although it depends on the kind of scatterers. Here is an interesting plot of the angular "Mie" scattering of red light from a water droplet of size 10 micrometers. Zero angle corresponds to pure backscatter, which is what we get from our strobes. Each circle is a factor of 10 less than the next larger one. note that getting off-zero by only about 10-15 deg will drop the scattered intensity by a factor of well over 100, which is one reason we want to put our strobes off to the side! But there is still significant scattering in the forward direction, and 90 deg off zero is the least scattered intensity. the exact details of the plot depend on the relative wavelengths of the light and the particle/drop/bubble, but the general picture remains the same.

r10-Perp-Polar-LogA.gif



#5 bmorescuba

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Posted 27 January 2018 - 06:05 PM

I have the same problem as well when shooting salmon in streams using ambient light. There is a fair amount of drifting debris. Bubbles can be a problem (as just suggested) as well. Shooting with a fisheye lens at point blank range helps to minimize the problem. As well, it is best to avoid shooting into the light. Sometimes one does not much of a choice other than not shooting at all.

Check out this shot:

 

http://www.salmonogr...mon/i-9CHRBCD/A

 

(easier for me to find image on my site). Note the light blotches in the water column. I see a dark one too (left side), probably a twig. Not too bad backscatterwise at the spawners that were real close. I angled the camera to avoid shooting into the sun - this gave the reverse perspective from many of my other shots - but easy to see the eggs emerging from the female from this angle! A downside is that the housing cast a shadow on the bottom (shot taken less than 2 hours from true noon).

 

I kept the Panasonic 8mm fisheye on for all the shots this trip. But I definitely notice and agree with both things you pointed out - the closer I was to the main subject, the less "scatter" I noticed. And the less I was shooting into the sun, the less "scatter' as well. Here's an example. I was super close on this shot and there's much less scatter overall. But the most scatter is visible up near the sunlight. Thanks to both of you for your insights. I love your salmon work, by the way - it really shows a thorough devotion to an important topic.

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