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Photography over water

Tom O'Reilly

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We are flying surveys over the ocean using a fixed-wing UAV with Sony RX1RII camera pointed at nadir. We want to minimize sun glint and sky reflection in our imaging. Following NASA guidelines we often (not always) fly survey lines directly toward or away from the Sun, and we generally fly when solar elevation is between 30 and 45 degrees above the horizon. Even with these constraints our images often show significant sky reflection, regardless of whether clouds are present or not. The camera does have a circular polarizing filter, but it's not clear how to set the filter angle before the flight. Skylight and glint doesn't seem to depend on just solar geometry - waves, swell and ripples can contribute too. How best to adjust the polarizer filter for these factors ahead of time? Difficult because we don't have the same overhead scene of the sea surface that the UAV has during flight. Any recommendations are welcome!
 
We are flying surveys over the ocean using a fixed-wing UAV with Sony RX1RII camera pointed at nadir. We want to minimize sun glint and sky reflection in our imaging. Following NASA guidelines we often (not always) fly survey lines directly toward or away from the Sun, and we generally fly when solar elevation is between 30 and 45 degrees above the horizon. Even with these constraints our images often show significant sky reflection, regardless of whether clouds are present or not. The camera does have a circular polarizing filter, but it's not clear how to set the filter angle before the flight. Skylight and glint doesn't seem to depend on just solar geometry - waves, swell and ripples can contribute too. How best to adjust the polarizer filter for these factors ahead of time? Difficult because we don't have the same overhead scene of the sea surface that the UAV has during flight. Any recommendations are welcome!

First off WELCOME to the forum!!

I just want to give my 2-cents in regards to using the CPL and your expectations of it. I've been "doing" photography for many years and for handheld cameras a CPL can help in many landscape situations. The problem is, the proper use of a CPL is to rotate it on the lens until you get the desired effect. SWEET! But, this is 100% dependent on the exact location/angle of the CPL in relation to your subject AND light. I've never understood how people can effectively use a CPL when the camera and the subject matter are so DYNAMIC with UAS operations. Just a few degrees of difference makes the CPL ineffective and possibly even detrimental to the image desired.

Imagine having a MANUAL focus camera and in order to focus it, you have to manually adjust the focus before you take off and then try to hope/pray that setting will work for your subject matter throughout the dynamic flight.

IMHO, I think CPL are more of a gimmick in regards to UAS when you really get down to it. While they absolutely CAN help in a very specific and static shot I don't have any faith that it can do any more than that for a moving UAS.
 
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Thank you BigAl07. We've been flying surveys over the ocean for several months now. We're looking for jellies, fish, plankton and other biota near the surface - hence the effort to minimize specular glint and sky reflection. Until recently we didn't try very hard to fly closely along the solar azimuth line. But conversation with the NASA airborne science group convinced us we should be careful about that (even though cross-wind crabbing can point us away from that line). We have a CP filter on the camera but have made no effort whatsoever to optimize its rotation angle. By sheer dumb luck all the angles very occasionally line up and almost completely remove glitter and sky reflection! As you say there are many factors that affect that optimal rotation angle. But now that bearing to the Sun will be small, and we've better characterized aircraft pitch (usually a few degrees nose up), we've run some ground tests and determined the optimal filter rotation angle for that UAV-Sun orientation. Yes, lots of moving variables - but at least now we're stacking the deck in our favor! ;-) We will hopefully try this filter rotation angle over the ocean next week...
 
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