I have a funny story about it:
You can check what your main eye is by (with both eyes open) placing your thumb in front of a distant target, then closing one eye after the other, you can check that you MAIN EYE is the one who does NOT see the target.
As I started Photography in 1999, my main eye was my left eye, and I had no problem using it with my first UW camera (Motormarine IIex) and my first SLR (Nikon F80s). The thing started when I went to Germany to buy my first housing (Hugyfot) the owner himself explained that in order to have the best possible vision of the whole viewfinder I had to use my RIGHT eye, and place my nose in a depression made for it on the housing so that the glass in my mask was "glued" with the housing finder...
Several times after that first lesson I had found myself having extremely difficulty in UWPhoto, to realize that I was using my left main eye... and when I switched to the right everything was so easier!!!
The training was such that actually my main eye changed to the left one now!!!
Some housings almost require a right main eye, others like most DSLR housings, dont make such a difference, this is so because the back controls of DSLRs, prevent better ergonomics for the face
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Nowadays, I really like to shoot right eyed, with the left open scanning the environment. I have perceived so many shoot opportunities this way, that with the other eye closed I would have left... this approach is facilitated by the fact that the left eye view when the right is in the viewfinder is much wider than the right eye view when the left is in.
Other benefits I can point that I like are:
- Focus perception is MUCH better in the (D)SLR viefinder.
- Speed: The black-out time is much less.
- With DSLR you will not loose the ability to check it after the click.
- Photo information does not appear in front of the image as in the point&soots LCD.