Domeport curvature and focal length or angle of view?

[carsten_wolff]

Hi and apologies for the cryptic question. I'm merely contemplating

using the glass from a 15mm domeport for, say, a 38mm Biogon, or a

Grandagon in that focal length ballpark in MF. Does anyone know if the

curvature of the port would be totally wrong?

[rob_barker]

You're only talking about an angle of view that would be seen by a 22mm- ish 35mm equivalent lens, so not that wide by underwater standards. A domeport designed for a 15mm lens on a 35mm system should easily enable the lens to focus.

 

The problem is likely to be in finding a port physically wide enough to fit a mf lens, assuming you're contemplating a homemeade housing - you will, most likely, need to get a dedicated MF housing system and they cost big bucks.

 

There are several very good reasons why MF is hardly used underwater and this is one of them! The limited number of exposures per dive is also a disadvantage.

 

There is also no real quality argument, for underwater work at any rate, for using MF when the latest 10MP plus DSLRs can be easily housed and offer so much in terms of capacity, quality and feedback.

[carsten_wolff]

You're of course right for 6x6 and most applications. Have since made up my mind and am gonna bite the bullet and try a 75 or 90mm lens in 6x17cm instead; Previously I had rented an SWC in a Gates housing and was very pleased with the pics I got, hence my initial inquiry. Anyway, my intended 6x17 will be set up for Scheimflug and Hinge rule due to the otherwise somewhat sad DOF situation in that format. If I then did anything digital with the transparencies I'd have ~ 100 MP equivalent @ 2500dpi, right? Anyway, some friends and colleagues want largish murals of underwater scenes and I'm not into pan-stitching, as I have other plans/needs for image manipulation during the actual exposure. I'm not planning on scanning them anyway either. I'm in the somewhat lucky position that I don't have to spend a lot on it (got all the bits lying about and access to a CAD-run marine workshop) so if it doesn't work out, no drama. I've been using that format a lot on land and think I can make it work underwater(insert nervous twitch...;)) So I'm currently mocking up a rough prototype for tests in da pool to see whether that domeport will work.

[rob_barker]

Blimey - this is esoteric stuff!

 

I'm sure you're well aware you won't get any extra depth of field by using lens movements (less in fact) but the concept of placing the plane of sharp focus along a horizantal-ish axis is very exciting.

 

You will need to be well above or well below your subject (i.e get a reasonable "J" in Merklinger's terms), unless you use a ridiculous tilt angle which would limit your depth of field. The concept of a reef scene sharp from near vision to infinity in a horizantal plane, with sharpness fading toward the top and bottom of the frame, is quite enticing. You could potentially get some really innovative stuff.

 

Of course, writing about this is much easier than doing it - we look forward to the results! ;-)

[rob_barker]

Thinking about it some more, if you used a dome port you'd have to tilt that too...

[carsten_wolff]

Yep, I've come up with a good system: I'm just rotating the film back in relation to the lens to avoid more issues with the lens to domeport design. I've finished the lens/port and I now have a mock-up ready for tests in the pool for scaling of the focal plane, as the camera won't have a focussing screen. If it all gets too complicated in the end(i.e. too much guesswork u/w) I might still use it for close-ups with a frame....

I should be able to post scanned pics in the next few months.