knut_sverre_horn Posted July 12, 2000 Share Posted July 12, 2000 Water magnifies - but how much? Is there a factor that can be applied to the focal length (or the angle of view - or both) that tells me what e.g. a 20 mm lens will correspond to when used underwater? (Yes, I HAVE surched the archives, and I was a bit surprised that this information wasn't readily available.) TIA Link to comment Share on other sites More sharing options...
ryan_taylor1 Posted July 12, 2000 Share Posted July 12, 2000 I'm not aware of any factors for direct conversion lens focal lengths underwater. However, water typically magnifies an image by approximately 1/3. A 20 mm lens underwater is probably going to be equivilent to a lens somewhere in the 28 mm range on land. This is one of the reasons the Nikkor 15 mm lens is so popular (besides the fact that it is incredibly sharp). Link to comment Share on other sites More sharing options...
knut_sverre_horn Posted July 12, 2000 Author Share Posted July 12, 2000 "I HAVE surched the archives" Hey, who did the rewriting? I don't make typos like that! Link to comment Share on other sites More sharing options...
david_hempenstall Posted July 12, 2000 Share Posted July 12, 2000 Knut-Sverre, water makes things appear 25% closer, so the resulting magnification cuts the 'angle of view'. HOWEVER, underwater when using a housed wide angle (such as a 20mm nikkor), we have a dome port on the front of the housing. this dome port provides quite a few optical needs, and one of the benefits is that the lens retains its angle of view. therefore, a 20mm retains 94', the exact same coverage as the Nikonos 15mm. dome ports do however create a 'virtual image' very close to the lens, requiring it to focus on this point. diopters or large diameter domes fix this (the image is approximately twice the diameter of the dome away from the lens). the magnification issue does aid us when shooting macro/fish portraits as the resulting increase in magnification gives us just a little more working room (a fantastic thing UW seeing we have to be so close just to get rid of as much water between subject and lens). when shooting skittish blennies a 105mm micro behind a flat port gives you a nice distance. the 200mm just gets you out to far except for when the water is VERY clear, and you have VERY big strobes. fun and safe shooting/diving, dave Link to comment Share on other sites More sharing options...
john99 Posted July 12, 2000 Share Posted July 12, 2000 Tell us what equipment you're using. I go down with at Ikelite, and a 28-80 zoom or 80 macro. I use a dome port with the ikelite, which imploys the aforementioned virtual image. Angle of view is consistent, but a close up +4 diopter is needed. Link to comment Share on other sites More sharing options...
charles_glatzer Posted July 12, 2000 Share Posted July 12, 2000 David-Is 100% correct on all accounts. The best book written on UW Photography is The Manual of Underwater Photography / Heinz-Giert De Couet & Andrew Green / Verlag Christa Hemmen / ISBN# 3-925919-02-3 Check Helix Charles Glatzer UW Photography Adjunct Associate Professor Long Island University Link to comment Share on other sites More sharing options...
david_hempenstall Posted July 13, 2000 Share Posted July 13, 2000 i would reverse the compliment to say that charles has hit the nail on the head by recommending the andrew green book. all sorts of techo information, even building your own fill strobes and the like. some great thoughts and comments on what underwater photography is. to best illustrate the concept coverage angle, i would compare different gear. my aquatica4 (nikon F4s) has a massive dome port (8inch diameter), and as such the lenses use don't need diopters fitted simply because the virtual image falls outside the minimum focusing distance. with a subal/ikelite etc that have small domes the diopter is needed. with the large 'fisheye domes' the need is negated (there isn't any filter thread on the fisheyes anyway). the nikonos 15mm has the diopter already installed, so to speak. this is covered much more thoroughly in the book, and also describes why my aquatica system isn't the best (a 'standard' domne for the wides). as a side question for charles, what is the professor stuff in your sugnature? do you guys actually teach U/W photography in the states as a subject? maybe i'm in the wrong country.... dave Link to comment Share on other sites More sharing options...
charles_glatzer Posted July 13, 2000 Share Posted July 13, 2000 David,Yes-The University has an excellent Marine Science program and UW photo is a natural extension. I teach both Introductory and Advanced classes. You can check out my Bio and work http://www.Naturephotographers.net/cg.html Chas Link to comment Share on other sites More sharing options...
jim_greenfield Posted July 14, 2000 Share Posted July 14, 2000 David is right about the Subal but not Ikelite. Although only a small diameter dome, no supplementary dioptre lens is needed with a 20mm prime lens and 94 degrees angle of view is retained. I have used a nikkor 20mm for years behind this dome and it is very sharp. I think it is partly due to the fact that the min. focus of the lens is 12ins and this is precisely where the dome creates its apparent image. With Nikon 28mm and all zooms, then a supplementary is needed - probably +3 or+4 depending which lens. The Ikelite catalogue advises on this. Link to comment Share on other sites More sharing options...
david_hempenstall Posted July 15, 2000 Share Posted July 15, 2000 oops, i stand corrected on the ikelite issue. seems i didn't look up the specs before putting finger to keyboard. thanks for the right info!! regardsdave Link to comment Share on other sites More sharing options...
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