optics question

[Anthony Barreiro]

Hi folks. I just bought a new pair of 8x42 binoculars, BAK-4 porro-prisms, fully multicoated, to use both during the day and at night. Overall they seem very good, with bright images and no obvious chromatic aberration or coma. But one of the barrels shows very bright objects (Jupiter and Venus) with two distinct diffraction rings, even when the image is otherwise sharply focused. I've never before seen diffraction rings that can't be eliminated by careful focusing. Obviously I'm going to return the binoculars, still trying to decide whether to take my chances with a replacement or just ask for a refund. I'm curious what would cause this particular optical anomaly. Can somebody explain this? Thanks.

[BDanielMayfield]

Just a guess, but maybe one of the multi-coating steps was omitted on one of the prisms.

Bruce