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Message: <blockquote data-quote="looksharp65" data-source="post: 3639177" data-attributes="member: 83771">Saturday, and off work so I have had the time to play some more with the prisms.In the test I used a Meostar 8x32 with a 140 m/1000 m FOV, i.e. a wide-angle binocular.In front of one of the objectives, a 2 prism diopter trial lens with the base laterally: At long distance: I watch a distant chimney, turn the binoculars slightly so it's barely outside of the FOV.Introducing the prism reveals the chimney again, in the most lateral part of the FOV only delivered by one of the barrels. The other barrel's FOV edge floats next to it as a slight indication of the transition to the compound/binocular FOV. A slight reduction of the magnification is perceived as recently described. There's a nasty colour fringing which may be due to the plain crown glass of the trial lens.At fairly close distance: The effect is huge, with a significantly reduced overlap of the FOV's and an almond-shaped central part representing the compound FOV. The total FOV is considerably larger than without the prism. It reminds a lot of using a porro at very close distance without adjusting the IPD.Even here, a slight reduction of the magnification is perceived.Removing the prism causes massive double-vision for several seconds, indicating a major convergence strain.Apparently, the 2 prism diopter power is too much for me. Reducing this to the "1" power mitigates the worst undesirable effects, but the increase of the FOV is insignificant at longer distance.In front of one of the objectives, a 1 prism diopter trial lens with the base medially At long distance:Tried with the "2" power but this required more divergence than I'm capable of, and my exophoria is roughly 20 prism diopters of which 3 are corrected in my glasses. Apparently, the vergence of the eyes needed to correct for a horisontal prism is multiplied with the factor of the magnification when placed in front of the objective.Even I, with my major exophoria, can barely avoid diplopia with the "1" power". A vast majority of people would not accept any divergence.At fairly close distance: Nothing spectacular to reportAt very close distance: A relaxed view with a perceived increase of magnification, similar to the view of a reverse-porro.Prism between eye and the ocular lens:Very minor effect with these weak prism powers. I can adapt to the effect regardless of prism base direction, and detect no advantage of either direction compared to not using a prism. People with binocularity problems would be best served with spectacles addressing these.Discussion:My objections seem to have been largely correct.  - The advantage of a slightly wider FOV at long distance is compromised by the lack of binocular FOV where the advantage appears: near the edge. With a wide angle binocular, turning the eyes so far sidewards is uncomfortable anyway. Base-out prisms do increase the total FOV at the expense of the compound FOV. - The extra convergence needed to adjust for the prism may be favourable for a minority of users when looking at a distance, a minority that is further reduced when it comes to close distance viewing. - The effect obtained by the prisms imitate that of using porro or reverse-porro binoculars.However, in the porro case, without the advantage of the wider spacing between the objectives (parallax) which enhances the 3D perception. A prism cannot yield a better 3D perception; it is not a magic gem that can see around corners. More parallax can at least give a glimpse.  - The base-out prism causes the so-called porro effect: the magnification seems weaker than that of a roof binocular with the same power.For me, case is closed. Current wide-angle designs, roofs as well as porros, do a better job than diverging FsOV in every respect. Regarding convergent FsOV: In theory, a roof design with base-in prisms on the objective side would yield a better 3D perception than a reverse-porro, but that's about it. //L</blockquote>

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