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P3D Re: Aquarium photography


  • From: Tom Hubin <thubin@xxxxxxxxx>
  • Subject: P3D Re: Aquarium photography
  • Date: Fri, 1 Oct 1999 22:26:42 -0600

> For aquarium shots, I would recommend placing the strobe at the glass as
> you did and then wait for something substantial to swim near the glass.
> Keeping the strobe at approximately a 45 degree angle from the lens line
> will minimize the illumination of the background scatter.

You could also try polarization to minimize back scatter. Backscattered
light from bubbles and small particles is highly polarization sensitive.
Use a linearly polarized light source. Use a linear polarizer on your
camera. Try various orientations of each. The polarizers may be best
parallel to each other or perpendicular. 

Right and/or left hand circular polarizers may also be useful. If your
flash is polarized right hand circular then its reflection from the
glass at normal incidence is left hand circular. So a right hand
circular polarizer on your camera will eliminate the flash reflection.
That means that you should be able to use the same circular polarizing
material on flash and camera without knowing which it is. This same
characteristic may also eliminate the backscatter from particles in the
water.

Antiglare screens for your computer monitor are two layer circular
polarizers. The light in the room passes through the linear polarizer
layer first and is linearly polarized. Then it passes through the
quarter wave retarder layer and the linearly polarized light is
converted to circularly polarized light. The light may be right or left
circular depending on how the quarter wave retarder layer is oriented
with respect to the linear polarizer layer.  

The light hitting the screen is circularly polarized. When it hits your
glass screen and reflects the circular handedness is reversed. The
reflected light then passes through the quarter wave layer and is
converted from circularly polarized light to linearly polarized light.
But, the linear polarization is now perpendicular to the linear
polarizer layer. It gets absorbed in the linearly polarized layer. Thus,
reflection of the room's light sources never gets to your eye.

So the flash and camera should be on the "room" side of the filter and
the aquarium should be on the "video monitor" side of the filter.

Now a quarter wave retarder of this type does a pretty good job aver the
visible spectrum. But they are not expensive and do have some
limitations. You might find that it eliminates the back scatter and
reflection of some colors better than others. You might find that the
flash reflected from the glass or back scattered from the particles is
blue because mostly the reds and greens are absorbed by the filter.

If the method shows promise then it may be worth finding a higher
quality filter.

You can experiment at home with a flashlight and any window. You can use
a glass jar or tray with flat surfaces. With and without water. As far
as the particle simulation goes, that is tougher. Any particles you use
at home may have different optical characteristics than the ones in the
aquarium tank. Maybe you can get a sample from the aquarium. Tiny, even
microscopic, bubbles may be the bulk of what you are seeing. 

Let me know how it goes.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design