Thanks that was a good explanation! What are the actual exit diameters of the Inon UWL-100 variants? I have a S&S lens made for 24mm equivalent that may work then with the RX100.
Further to the optics discusion. What hapens to the effective aperture of the lens combination? I suppose it is the same as the original lens or is it atually a larger aperture (lower f-number)?
The exit aperture would be roughly the diameter of the rear element of the wet lens, at close distance. And what your camera's lens sees is its diameter and the distance to the rear of the wet lens.
The wet lens works almost like binoculars, which doesn't focus the subject into an image, but changes its magnification factor, and your eyes, acts like the camera & lens. The eyepiece lens of the binocular has to be bigger than the pupil or else you have problem seeing a full image. When using it at night, your iris dialates and the effective pupil diamter is larger, making it harder to see the whole image unobstructed. Also if you pull your eyes farther away from the eyepiece, the problem gets worse, and you see a smaller diamter of the image (as in vignetting).
As for light gathering capability, it should not negatively impact. But on some tests, they said it is 'dimmer'? I guess it is pretty easy to test.