Dome Theory

There don't seem to be many articles that deal with dome vs flat ports. I'll admit there are some, but the ones I've read tend to deal with generalities, and when I dive deeper to try to find some fundamentals with which to buoy my understanding, I flounder and sink into despair (ok... lousy puns, but you get the point!).

One general basic about dome ports is that the dome creates a virtual image that is closer to the lens than a distant subject, whose distance is dependent on the dome radius. Does anyone know of a good reference that goes through the optics/math? I've always wondered if this gives any particular advantage to lenes that can focus up close.

Another point I've read, and experienced, is that a dome does not focus an image properly for use with an uncorrected lens. For instance, if you put a wide angle lens in a dome, the center of the image might look fine, but the edges are out of focus. Apparently a correction lens is required for use with a dome, whose diopter is dependent again on the dome radius. It is optimally located at the center of the dome... In developing a deep water (i.e. 20,000 ft deep) system, with a small dome, this was clearly (there's those puns again) apparent. Working with the Nikon 24mm lens on a coolpix 990, the difference in edge to edge sharpness is quite obvious when a corrector lens is used.

This begs a question... how well does the 19mm lens work with scuba housing domes??? Anyone seeing this issue? I've never noticed it with a 950/24mm on a Ikelite housing.

Again... anyone know of good technical references on the subject?

Mark

Have you seen this site?

There have been several posts on dome ports in the past. A dome produces a virtual image. The camera lens must focus on this virtual image. The location of which depends upon the radius of the dome port. If the lens does not focus close enough to focus on the virtual image then you must put a diopter on the lens to allow closer focusing. Usually a + 3 or 4. A lens that focuses close enough won't need a diopter. I use the 17-35 and 18mm lenses which don't need a diopter with a 9" dome.

To get the maximum optical effect the lens should be the correct distance away from the dome so that the image is focused on the antierior nodal point of the lens. The quick and dirty way to determine this is to set your lenses aperature wide open. Look for your reflextion in the lens aperature. It should not move as you twist and turn the housing around, but should stay centered in the aperature.

Many people set the lens closer to the dome than the optimal distance. This allows them a closer focus, but causes poorer optics. This is often the cause of complaints about the sharpeness of dome ports. Properly set up they can be very clear. My 17-35 is as sharp as the Nikonos 15 mm lens when properly set up. That lenses front element is of course a dome, but with optics maximized for UW usage. :ph34r:

As a complete ignorant of UW photography, I wonder what is the true advantage of dome ports?

Take an example: ikelite offers a housing for the nikon CP5000 in either a dome port model or a flat port model. With the dome port you can use the nikon 19mm lens which you can't change on the fly, with the flat port you can purchase the Sea and Sea adapter and use sea and sea 16 or 20 mm lenses and macro kits. These lenses can be removed and changed underwater for shooting different styles. Inon also makes conversion lenses that fit a veriety of housings.

Why would someone chose a dome port option in this case? Wouldn't it be better to go the Sea and Sea (or Inon) route? Wouldn't this eliminate some of the problems with sharpness? Or is the quality better with the dome port and fixed lens.

Stupid newbie question,

Chris

The quality of photos taken with the Nikon 19mm lens/Domeport combination is much higher than with an add-on lens.

Cheers

James

James -

You over-simplify by that statement. Perhaps you intended to do so. I understand that wetlenses, theoretically, aren't as sharp as dry lenses. But would you say that "photos taken with dry lenses are higher quality?"

Personally, I find the flexibility of wetlenses really exciting, and the image quality I've gotten using my Titan wide and macro add-ons have been sharp, right out to the corners. Personally, I think the aversion to wetlenses is partly a relic from the film world, as they tended not to be sharp at the corners while shooting wide angle.

Kasey

I am consistently disappointed with the results I see posted which have been shot with the Inon WA lens. I can't speak to the others especially the Titan, as I have seen very few sample photos. From time to time I see a nice shot taken with an add-on lens (examples from Herbko, Jeff, NLAVD, etc), but most often not.

I have been tempted from time to time to post some of the poorer photos with a circle in the center (to show that about only 75% of the shot is in focus and the remaining 25% in the corners is horribly soft) but I didn't want to single out anyone's shots, as that would be rude.

Contact me via email or private message if you'd like to discuss further.

Cheers

James

I have no experience with add ons other than my own. I think they are very sharp from corner to corner. Here is an example - don't hesitate to critique.

another:

wetlens macro:

Those are gorgeous Kasey! Would you mind posting the retail prices that LMI sells the Titan and the two wetmate lenses for? Might be interesting for another discussion...:-)

Cheers

James

Yeah, and I'll throw a plug in here for my cheap and crappy INON wide angle lens, which this TURTLE thought was pretty cool, and for my Olympus external macro lens, which this BLENNY seemed to like . . .

Hey no fair Mike! I already told you guys I'm not going to post any of the "bad" shots...

Cheers

James

It was the beginning of the summer and I didn’t have my push button controls ready for my own housing’s so I decided to buy a housing.

I bought a housing from a well respected camera housing manufacture. I was not at all impressed with my results. It was for my Nikon 5000. I was using Nikon’s 19 mm wide converter with a dome port. The images were way to soft in the corners and I believe it was partly due to the lens being way to close to the inside of the dome.

Here is a clear example of the soft corners the camera and housing produced . The picture has been resized only.

Marc

Marc,

Which housing was this?

James -

I think the housing is $2699, and both lenses around $1400. Pricey, but flexible. I've been watching the forum in digital diver comparing the E-20 to the true D-slrs, and I'm appreciating more and more the range of subjects I can photograph while underwater. I would think the Titan D100 will also support these lenses. Imagine the titan wetlens on top of a D100 with 20mm lens, now we're talking super-wide angle!

Kasey