[Coral-List] Digital Photo-monitoring

[Dave Zawada]

GP,

It is highly unlikely you'll be able to replicate the quality and  
field-of-view of a Nikonos image taken with a 15-mm lens using a point- 
and-shoot camera.  The Nikonos camera uses 35-mm film which has an  
active imaging area of 24 x 36 mm.  Point-and-shoot cameras commonly  
use 1/3"-format CCD or CMOS chips which are 3.6 x 4.8 mm.  This  
significant difference in form-factor requires that light rays be more  
aggressively bent for a point-and-shoot camera to have the same field- 
of-view as a 35-mm camera at the same imaging distance.  For a 35-mm  
camera, a 15-mm lens is very wide angle, which corresponds to an even  
wider angle lens in a point-and-shoot format.  Hence, the barrel  
distortion in your images.  A telecentric lens or some custom optics  
may alleviate the problem, but would require a larger housing and  
greater expense.

The good news is that digital images are easily corrected for barrel  
distortion.  You will need to create a grid and take a picture of it  
from your typical imaging distance.  The basic idea is to establish a  
correspondence between the locations of grid points on the printed  
target and in the digital image.  With a little linear algebra, you  
can define a transformation matrix to remove the barrel distortion.   
You only have to create one transformation matrix per imaging distance.

For details, see

Zawada, D.G.
Image processing of underwater multispectral imagery
(2003) IEEE Journal of Oceanic Engineering, 28 (4), pp. 583-594.
doi: 10.1109/JOE.2003.819157

and

Green, William B., Jepsen, Paul L., Kreznar, John E., Ruiz, Reuben,  
Schwartz, Arnold A., Seidman, Joel B.
REMOVAL OF INSTRUMENT SIGNATURE FROM MARINER 9 TELEVISION IMAGES OF  
MARS.
(1975) Applied Optics, 14 (1), pp. 105-114.

Best,

Dave


On May 30, 2008, at 5:28 PM, G.P. Schmahl wrote:
> We have a long term monitoring program of coral communities at Stetson
> Bank in the northwestern Gulf of Mexico where we have photographed
> permanently marked locations annually since 1993.  Historically we  
> used
> a Nikonos camera with 15mm lens at a distance of one meter from the
> substrate.  We are now trying to replicate the same field of view  
> using
> a lightweight digital camera.  Has anyone had this or a similar issue,
> and can advise on appropriate equipment?
>
> The use of a housed digital SLR is problematic due to the large size  
> and
> weight.  (The camera is attached to a framer bar, dual strobes and
> camera mount, which is quite clumsy to swim with.)  I would prefer a
> housed high quality point and shoot camera.
>
> We have tested a Nikon Coolpix P5000 in an Ikelite housing with the
> wide-angle adapter.  The photos come close to matching the Nikonos  
> field
> of view (if you move it 20 cm farther away from the substrate), but
> there is noticeable edge distortion.  I can send a sample photo
> comparison if anyone is interested.
>
> Can anyone recommend a digital system that will replicate a Nikonos
> w/15mm lens?  I would be interested in digital SLR/lens combinations  
> as
> well if anyone has worked that out.  Thanks.  GP
> -- 
>
> George (G.P.) Schmahl
>
> Sanctuary Superintendent
>
> National Oceanic and Atmospheric Administration
>
>
> Flower Garden Banks National Marine Sanctuary
>
> 4700 Avenue U, Building 216
>
> Galveston, TX  77551
>
>
> (409) 621-5151 ext. 102
>
> (409) 621-1316 (fax)
>
> george.schmahl at noaa.gov
>
> http://flowergarden.noaa.gov
>
> _______________________________________________
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> Coral-List at coral.aoml.noaa.gov
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Dave Zawada
US Geological Survey
600 4th Street South
St. Petersburg, Florida 33701
p: 727.803.8747 x3132