New personal best

Yesterday I've set my new personal records in breath-hold diving. It's now 100 meters in Dynamic With Fins (DYN) and 3:40 in static breath hold (STA). So my personal record table is now

• DYN - 100 m (pool)
• Dynamic No Fins (DNF) - 50 m (pool)
• Static (STA) - 3:40 (pool)
• Constant Weight (CWT) - 18 m (open water)

PowerShot s95 and Dyron wide angle lenses field of view

Some time ago I've changed the camera I use to take pictures underwater from Olympus u1030 to Canon PowerShot s95. The main reason behind the change were the limitations of the u1030 (no custom white balance, smaller sensor, low video resolution), but I was also hoping to get a bit wider angle of view. However, knowing that 'bit wider' will most likely not be wide enough I also bought Dyron 24mm wide angle wet lens and adapter for the Canon housing (WP-DC38). Overall picture quality underwater was good and there was no vignetting. However, first dives showed this was not as wide as I hoped for and I should've probably gotten one of the wider ones or even one of their wet fisheye lenses. Not seeing much difference underwater I've decided to check whether this lens offered any field of view (FOV) improvement. So here's how this was done.

My 'lab' set consisted of a tub filled with water, a ribbon with marks 1cm apart and a measuring tape. I calculated FOV on the surface, underwater without wide angle (WA) lens and underwater with WA lens. To do so, I've placed the camera so that the center of it was 28 cm away from the ribbon and took shots in each setup:

In air

Underwater, with Dyron WA lens

Underwater, without WA lens

Now, I've counted the number of marks visible on each of the pictures and by using trigonometric functions calculated the α angle as the diagram below shows:

Angle of view calculation diagram

On the above diagram:
a - length of the ribbon visible
b - distance from the camera center to the ribbon
α - half of the angle of view

The horizontal angle of view is 2* arctan ((a/2)/b)

The results were:
angle of view in air: 2 * arctan(15 / 28) = 56.35 deg
angle of view in water: 2 * arctan(10.5 / 28) = 41.11 deg
angle of view in water with WA lens: 2 * arctan(13.5 / 28) = 51.48 deg

This is how the horizontal angle of view looks like for each of the above cases:

Uderwater horizontal field of view  for Canon s95 and Dyron WA lenses

DISCLAIMER: the figures below are more of a rough estimates than precise calculations due to the makeshift setup of the lab, but the general principle would be the same for proper measurements and I think they give a good feeling of how things look like with this lens.