kelpscape

In vacuum (and approximately in air) the field of view has angular width in radians theta[air]=2 arctan(L/2 f) where f is the focal length and L is the width of the sensor, and theta is in radians. But, usually field of view is quoted for the diagonal, and in that case L=sqrt(w^2+h^2), where w is width and h is height. If the lens is directed through a flat port into water, then the field of view has angular width given by Snell's Law: theta[water] = 2 arcsin( sin( theta[air] /2)/1.333 ) where 1.333 is the index of refraction of water. For example: The D70 sensor is 23.7 by 15.6 mm, so it measures 28.37mm diagonally. Then, an 18mm lens has field of view in air of theta[D70,air] =2 arctan(28.37/2*18) = 1.335 radians Convert radians to degrees by multiplying by (180/pi) = 57.29: theta[D70,air] = 1.335 * 57.29 = 76 degrees :: is the field of view of a D70 with 18mm lens in air. A full-frame sensor is 36x24 mm, so 43.27mm diagonally. Then, a 27mm lens would have field of view theta[FF,air] =1.34 radians = 77 degrees :: is the field of view of a FF sensor with 18mm lens in air. This is almost the same as the 18mm lens for the D70. The ratio. (27 mm/18 mm) is the "Nikon factor" of 1.5, to account for the smaller sensor. In water, the D70 with a flat port would have field of view theta[D70,water] = 2 arcsin( sin( 1.335 /2) /1.333) = 0.965 radians = 55 deg :: FOV of a D70 w/ 18mm lens in water. With a little algebra, you can show that this is the same field of view as a 41mm lens with a full-frame sensor in air, or with your D70 in air with a 27mm lens. I hope this helps!

Does anyone have experience of taking the shade off a Subal DP-54B port? I am using a Nikon 8-15 mm fisheye lens with a 20mm extension, and the shade on the DP-54B appears in images, even at 15 mm. It looks like shade and mount were machined from a single piece (of delrin?) I suppose a well-equipped machine shop could cut it off -- but that seems like the last resort. Many thanks in advance! Carl

Greetings! I've been shooting underwater seriously since the Film Age (1999). I prefer my Nikon/Subal setup, but usually use an iPhone or Nikonos (film) for beach diving. I'm here to learn and to share, if I can be useful. Check out my Instagram feed: @kelpscape -- Carl

Hello Folks! I am from the California Coast, not far from Refugio, site of the recent oil spill. Underwater I shot film in a housed SLR, then switched to a housed DSLR, and now am messing with Nikonos as well. I look forward to learning from the group.

FWIW I got a new SB105 in Feb 2015, in exchange for an SB103 sent in late in 2014. (It came along with a Nikonos body and lens ordered from eBay). I heard from a 3rd party that the program was about to end. The Nikonos is cheap, light, and small enough for a surfy beach entry; and it still is with the SB105.