The pincusion distortion related discusion above relates to shooting under water or the portion that is under water in the case of over-under shots.
Thanks Tom and Paul, but I'm affraid I didn't fully understand the problem. The effect of different magnifications for half-half-shots is clear.
But if I shoot with a 21mm lens and a flat port in the air shouldn't it be the same as using 2 or 3 stacked glas filters with the same lens. If I do this with high quality filters I get nearly no loss of IQ and still perfectly sharp corners. I have for example used a combination of ND filters and a polfilter (which consists of two glasses surfaces glued together) both on my Zeiss 21mm lens and on my Leica 21mm lens with perfect sharpness.
Why is this completely different with a flat port? Is the glas of the flat port much thicker than 2-3 filters?
Sorry for the many questions from my side, but before making a significant investment in this underwater equipment I try to gather as much information as possible to avoid being dissapointed afterwards.
I have been using the Seacam 150D with diffuser as the sole strobe when shooting the 10.5mm Fisheye Nikkor with some success for several years now. However, it is more than your budget. Not quite as long I have been using the Seacam 250D with the 8-15mm Canon Fisheye on FF also as sole strobe. I use a thin semi-transparent white plastic bag (from a grocery store) tucked into the strobe's neoprene cover with the 250D as a diffuser since Seacam did not make one. The diffuser helps fill in the corners with a fisheye lens.
For both lenses I place the strobe over the center of the camera and forward just enough to light up a fish right in front of the port - positoning is critical. Most of shots taken with these set ups are of salmonids in streams, BTW.
I dug up two pix - one shows the set up and one a resultant pic, uncropped. Note the 150D strobe - it is hard to see the diffuser. The pole is 2.5 to 3 m long. I had ankle weights on it to hold things down in the current. Full post involves slicing a bit off the bottom and more off the the top to make this more of a panorammic shot. However I wanted you to see that the strobe lit the stream bottom foreground. This shot was taken when the day is about 20 hours long near the middle of the day - true noon is about 2 pm here with daylight savings time. It was overcast - I used ISO 400, 1/200 f/8 and the background is far from being too light
Thanks for the new great info !
But somebody has experience with the flashes from seacam ?