Friday, March 23, 2012

What Can You See In The Waters Of Lake Murray?

Absolutely nothing.

The problem with tidepool filming is that your timing is almost never right. You need the tide to be out far enough to give you access to the deeper pools and you can't have had high surf or rains beforehand because of the sediment it stirs up. I figured a lake would have none of these problems, so off I went to Lake Murray to film.

The reeds seemed to have the most promise to find little fish or other creatures and the water looked clear. I had no problem clearly seeing the bottom. The camera wasn't so lucky. The focal length on my PlaySport is 3', so I chose a distance from the reeds of about 5' to make sure everything would be in focus. Instead, I just got a bunch of muddy water.

The video is only a minute and there's an HD version that will make the mud really pop on your screen. If you're into that sort of thing.



tim eisele said...

Huh. The water didn't look that muddy until after it went under. Was there just enough mud to mess with the autofocus, so that when there were actual objects present it couldn't focus on them? Maybe it would be possible to lock the focus at a particular distance to cure that.

I thought at first that it was going to be a good hydrophone, at least, but then the sound cut out. Does the microphone stop working when it completely fills with water?

K T Cat said...

Tim, I thought the same thing. The sediment is coarse sand, save for the floating particulates, apparently. The sand just sat there when I placed the camera. My next experiment will be to place it in a running stream. At least the water motion will be interesting.

As for the microphone, it does fine. There just no noise in the lake.

tim eisele said...

Something that just occurred to me about the focal length: lenses depend on the difference in refractive index between the lens and the medium - and water's refractive index is a lot closer to that of glass than air's is. There's still air behind the lens, so it will half-work, but in water the lens is going to have a significantly longer focal length because the light won't be bent as much as it should coming into the lens[1].

So if the camera can normally focus on stuff 3 feet away, under water its closest focus might be increased to as much as 6-8 feet. So five feet away wouldn't be far enough, and in that case your camera was just too close to the reeds to focus!

[1] My brother is crazily nearsighted (20/200 vision), but he says that when he's swimming and opens his eyes underwater, his vision is almost 20/20.

tim eisele said...

Then again, that depends on whether the lens is actually in contact with the water, or whether there is a flat piece of protective glass in front of the lens. If there's a flat glass window, with air between it and the lens, then it should have the same focal length underwater as in air.

tim eisele said...

OK, not as bad as it might be, but the manual says that the minimum focus distance underwater is 4.4 feet, as compared to 3.3 feet in air.

K T Cat said...

Yeah, it's not the focal length that's the issue, it's the suspended particles in the water.