The dome or the dome part of a wide angle lens, such as the Inon Dome Unit for UWL-H100 or the Fantasea BigEye will simply eliminate the magnification of the water thus giving the lenses behind them the actual field of view they would have above water – 25% wider. That means the Fantasea dome, for example, will have no fisheye effect and the Inon dome will increase the fisheye effect of the H100 lens.
A dome port for DSLR or mirrorless housings, as well as some compacts these days, has the same effect – it increases the wide angle range by 25% giving you the same field of view as above water.
True wide angle lenses such as the AOI UWL-09 or Inon UWL-95 increase the field of view of the lens behind them using optical elements. If not used with a dome the magnification effect of the water will reduce the field of view of these lenses by 25% which will make them far less effective. This is why most wide angle lenses are used with domes.
A fisheye lens is one with a very wide field of view, often 160-180 degrees (like the Tokina 10-17mm). These lenses create a unique fisheye effect above water.
You might think that you don’t want fisheye, as that looks weird and distorted above water but underwater the fisheye effect is much less significant, mainly as there are far less straight lines and many of the best underwater wide angle shots you have seen were taken with a fisheye lens.
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