How to measure the image angle under water?

Hi everyone!

I developed a wide conversion lens for oly-3030 and PT005.

Here is a sample photo I took at Kushimoto in Japan.

It is detachable under water and I can take ultra-wide (weitwinkel) view like this!

I want to know its image angle. Does anyone know how to determin it under water?

The only way i can think to do this is on a flat sandy floor or pool... Place spike aligned with centre of lens on flim/sensor plane and using lines from spike mark off peripheral edges - peg, then measure angle between lines at spike.

Looking at the pic there must come a point when you trade off wide angle with distortion. This falls into the latter category, I believe, although it may be good for that 'different' type shot. It almost has that 3D look about it eh?

Mark

Thanks Cybergoldfish! I also think your method is the only way. But it is not so easy....

Thanks Mark. I just want to take "full-circle fisheye" photos under water with digital camera. Some photographer take full-circle fisheye photos with SLR camera, but it needs a big lens and a dome port. They does not change lenses under water. Therefore I started to develop full-circle fisheye conversion lens which is detachable underwater! Now I can take much wider photo with my conversion lens than INON's one.

Anyway, I will measure the image angle according Cybergoldfish's method in April or May and then I will report it here.

Actually all wa lenses bend the image as you see in this one to some degree. Usually, the design is such that a rectangle is cropped out of the middle of the circle by overlaying the circle over the CCD or film plane. Fortunately things we see underwater are seldom straight enough to tell.

Check out the bowed legs of the oil rig in the following:

This effect is minimized if the lines are close to being radials from the center of the lens:

As for measuring the view angle of the lens, I can't think of an easier way than what was mentioned. You might use the corners of a rectangular pool, at least they're already in place.