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I can confirm that the WO binoviewers do split the light unevenly but in practice you don't notice the difference. You can also get a lot of scattered light if there is a bright object just out of the fov. In most use you probably wouldn't notice it but it has annoyed me during a grazing occultation, where you do tend to put as much moon as possible out of the fov so you can see the star clearly.
Another issue with the smaller binoviewers is that they only have 20mm aperture and there is significant vignetting if you want a wide true fov, especially in fast scopes (btw the path length is about 100mm). I want to be able to use them in the 25cm f/5 newt and so considered the Sieberts OCA with 1, 1.5 & 2 magnification. However with the lowest power I would have 0% fully illuminated field. So I'll have to go for the 1.3, 2 & 2.5 magnification set.
There is another issue and I'm going to start a new thread about it.
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