Do these friction twist onto the eyecups, or screw in and replace them?

Anyone use them? Interesting concept.

Both. Swarovski version screws in in-place of the eye-cup. Vortex and others are pressure fit. None are worth owning.

I have the Bushnell 2 1/2 Elite extender and if it fits the binocular good it is a tool to use,it will not replace a spotter of course. I know of one person that uses this same extender on his Nikon 82 Fieldscope when using the lower power eyepiece to up the power to be able to confirm what he is looking at.
http://www.birdforum.net/showthread.php?t=219014

I would not say it is not worth owning.

Have often wondered how a pair of doublers would work on a set of binos? Would a set of 8x50mm with a pair of doublers function like a set of 16x50mm big eyes for instance?

Hi Pete, I only had the two different models. I don't think it would work that good, at least when I tried it, it was fiddly at best.

Well the zeiss victory ( the one I would need) is 450 bucks. So for 900 I would just opt for a new set if high power glasses.

My thought was only in weight reduction for back country trips. With 8 power 400-800 yards is pretty far to see that little single eye guard making a deer legal. Friction fitting a 3x doubler (the zeiss verizon) would get you 24x which would make the difference.

However fiddling with the fit and alignment or if is just was stupid, then it's not worth it. Especially at 450 bucks.

They don't recommend using on both eyepieces, just one as a small spotting scope application...

It would be virtually impossible to get both of the doublers each aligned with the center axis. In other words collimation would suck. I've seen it tried with the slip over style, but even with a quality screw on unit, they are not going to match either.

I would also say that they are worth owning, but they won't really replace a spotter or a big eye. They will not take up much space if weight is a concern and if you aren't pushing the optical limits too much.

I no longer have mine but if I remember correctly the swaro was adjustable? That said the way it connects and secures to the binocular its always centered with the eyepiece? I think there would be a concern of having two of them connected that maybe it would cause excessive strain on the body of the binocular!?