What is not often stated is that the refractive index of water varies by wavelength of light. This results in chromatic aberration which can be seen even in a swimming pool. Behind a flat port the chromatic aberration is a function of the angle of view measured from the optical axis, i.e., the half angle. It gets much worse as the angle increases. Therefore one can only use flat ports with near normal focal lengths and longer. One can get away with a flat port with a 35mm focal length on the 24x36mm format for example. The non-SLR Nikonos 35mm lens used a flat port - this is why it was amphibious and not underwater only (all the non-SLR Nikonos lenses with focal lengths <35mm were under water only).
The main purpose of the dome port is to restore the angle of view of the lens. This also results in the removal of the magnification (that you get with a flat port - see Bill's comment) and thus variability of magnification related to chromatic aberration. However, one needs to properly set up the dome port (correct amount of port extension). As well, one cannot avoid the field curvature of the virtual image. For example with a 105mm macro you would need to move the dome port some distance way from the front of the lens and you may lose some of the lenses macro capability. The resulting air space between the dome surface and lens could also be problematic in terms of rig buoyancy.
Also important is the distance to the dome port virtual image is quite close. Most 50 1.4 lenses cannot focus close enough, i.e. at minimum focus (e.g., 0.45m) the lens is focused beyond infinity when behind a dome. The solution is to use a close-up diopter lens. These may degrade the image to the point of uselessness - it varies by individual lens design. For example I found that when using one with a 24-70mm zoom (min. focus of 0.38m) the result with single lens diopter was unacceptable. OK however when used with a two-element achromatic diopter except that the thickness of the diopter caused additional vignetting.
Edited by Tom_Kline, 10 January 2017 - 02:22 PM.
Thomas C. Kline, Jr., Ph. D.
Oceanography & Limnology
Currently used housed digital cameras: Canon EOS-1Ds MkIII, EOS-1D MkIV, and EOS-1DX; and Nikon D3X. Those more or less retired: Canon EOS-1Ds MkII; and Nikon D1X, D2X, and D2H.
Lens focal lengths ranging from 8 to 200mm for UW use. Seacam housings and remote control gear. Seacam 60D, 150D, and 250D, Sea&Sea YS250, and Inon Z220 strobes.