6'' vs 8'' dome

stillviking wrote:

Wow, I was totally wrong. Thanks!

So should we move the lens away from the dome glass as far as possible? Is this valid for fisheyes and rectilinear wideangles?

No. If you're looking for a rule that's hard, fast, and easy to follow, there isn't one.

For one thing, not all domes are created equal. In general, there are two types of domes - fisheye and wide-angle, although they might not be labeled as such. Fisheye domes are typically hemispherical, i.e. a complete, 180-degree half of a sphere, and an 8-inch dome will have a radius of 4 inches. Wide-angle domes generally use a shallower cut, i.e. an 8-inch dome may be a segment of a sphere with a radius of, say, six inches. This is advantageous for rectilinear wide-angle lenses as they need as large a radius of a dome as possible for better corner performance, and they don't need a complete 180 degree arc as their field of view doesn't go beyond 110-120 degrees or so.

On the camera side of things, remember that what matters for positioning is the lenses entrance pupil, not the front element. Where this pupil is located is dependent on lens construction, and may not be marked or disclosed by the manufacturer.

Your best option is to follow the manufacturer's guidelines, if they have any. Nauticam and other serious manufacturers have specialized testing rigs where the camera is mounted on rails above a test tank and moved incrementally back and forth until optimal positioning is achieved with a specific lens and dome; this finding is published in their port charts. If your housing manufacturer does not have a recommendation for a certain lens, but has it for other lenses that are broadly similar, you can work out something approximate based on differences in setups suggested for those lenses in Nauticam's port charts, as those tend to be the most comprehensive.