kelpscape

Any updates on this? On my D500, my Tokina 10-17 fisheye overexposes by about 2 stops at 10mm focal length, when I use matrix metering. It underexposes by about 0.5 stops if I switch to center-weighted metering. The discrepancies are less if I go to longer focal length. I'm guessing that the issue has to do with the interaction of camera and lens, since it changes with metering mode. Maybe chimping is the best answer.

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.