I use the Bulzeye boosters on my 4200 Elites. They work very well but you must adjust the scope back as they make the eye relief shorter. Not a problem on my air rifles or SB though. The way the boosters determine magnification increase is a bit odd. From what I can tell it appears as though the 4x booster increases the sight image by around 4-7x. When looking through my 32x with the booster and then a 36x-40x I find it just a bit smaller than the 40x. The boosters magnify the whole sight picture not just the reticle. Once you use one a few times if you forget to put it on the next time you will notice the difference in size immediately.
You can call the gentleman that sells them and he can explain it better. It was several years ago that he and I spoke. The boosters are not only designed to help increase magnification but are also used to help correct eye issues with older shooters and Myopia ( a condition we get around 40 where the lens of the eye no longer bend and flex as they once did causing near focus issues) These are supposed to help with that problem.
I like mine and use them all the time except for HP where I just use a straight FX3 30x as I find more mag than that washes out to easy in heavy mirage.
They are also very clear and I notice little to no loss in clarity. The lens are made from the same materials used on Abrams tanks main gun sighting system. The guy that sells them helped design that tank sight from what I understand.
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