I have noticed a strange abberation with my Nikon HGL 8x20. Firstly I want to say it's not diffraction spikes- stars are pinpoints are many artificial light sources with 'covers' are as they should be.
The aberration I'm talking about is visible when I look at a street lantern, a type which has a 'naked' bulb visible in lower half, at night. When looking at it there is a narrow 'line' of light starting at the middle(from the bulb) and going both ways horizontally to the sides of FOV.
I find it strange because I only notice this problem with one type of street lantern, its not visible when I look at other light sources. Any comments about this, is it bins flaw?
Despite what you say that's a diffraction spike from the roof edge of the roof prism.
The reason you don't see it with stars is stars are a lot less bright than a light so the diffraction spikes are a lot brighter too.
I'm not sure why you don't see it with other lights but a very bright point or line source shows this effect best.
It depends on the sharpness of the roof edge and varies across bins. I've tested a few with a naked mini Maglight bulb at 10m and it varies quite a lot though the better bins are better (on average).
Check each barrel and the magnitude of the spike may vary and certainly will vary in direction (as roof prisms are usually installed with the roof at 45 degrees or so to the "vertical" of the bin). Try blinking back and forth between eyes to see the effect.
I concur. The roofs I've seen had spikes at about 45deg, but there is no reason why a roof prism could not be rotated about the optical axis to some other angle. Any roof will do this more or less. A bare streetlight bulb is a brutal test.
Ron