Wide-angle behind flatport

I test fitted a 17-50mm Tamron lens behind a flatport designed for 60mm yesterday. ok, there is no room for zooming but for the time being I have a 2nd lens that I can use for semi-wide-angle (17mm x1.6 crop = 27.2mm) unit I saved up for a dome.

What kind of distortion can I expect, or is it neglibe at this no-so-wide angle?

Maarten,

behind a dome that would indeed be similar to a 27,3 lens, but behind a flat port, the glass/water interface will work like another 1,33 factor (just like your mask). That means that your lens will be more like a 35 mm.

the curve of a dome helps preserve the original field of view. check this wetpixel article for a starter on dome port theory.

http://wetpixel.com/i.php/full/dome-theory/

Gerard

Ok, thanks for the pointer

35mm compared to the macro is still a lot wider.. going to test it a bit in the pool next week.

I still have 3 months until the Cuttlefish breeding season, enough time to get hold of a nice dome.

You may have a problem with vignetting, since your 60mm port is not designed for a WA lens.

There is no sign of vignetting above water.. and since the lens is at 2mm behind the glass I don't expect it to show vignetting under water.

Maarten, behind a dome that would indeed be similar to a 27,3 lens, but behind a flat port, the glass/water interface will work like another 1,33 factor (just like your mask). That means that your lens will be more like a 35 mm. The curve of a dome helps preserve the original field of view. check this wetpixel article for a starter on dome port theory. http://wetpixel.com/i.php/full/dome-theory/

Gerard

Hi Gerard and anyone else,

I am as far as one can get from a mathematician myself, but is it 1.33 or 1.25 the change in the focal distance? I saw 1.25 on the Successful Underwater Photography book, but I would lean (intuitively) at the 1.33. Anyone?

Edited:

I guess I found the end of the confusion:

Excerpt from the book: Scientific Photography and Applied Imaging By Sidney F. Ray p512

For lenses placed closely to a flat port underwater:

"... the object distance decreases by 25%, magnification increases by 33%.

Consequently, with a flat port, a lens used underwater has it's EFL (effective focal length) increased by 33% approximately...

... In addition the port introduces chromatic aberration, astigmatism and distortion"

Nice uh? So a 105mm with 1:1 magnification gets 1:1.33 underwater?

You also have to worry about chromatic aberrations due to the prismatic effects of the flat port. Possibly other lenticular aberrations as well. These are more limited with a dome port. A macro lens doesn't this problem to a much greater degree because the lens only focuses on the center of the flat port.

I use the Tokina 35 mm macro lens (dx) behind a dome port. Seems to give better image quality than using a flat port. I know people who swear that even the 60 mm macro benefits from a dome port.