pentax 8x40 pcf wp ii

[eid]

I recently bought a pair of Pentax 8x40 pcf wp ii and am a bit concerned whether this pair are up to scratch. I've never owned a pair of bins before so I'm not sure if the following is a problem or is to be expected.

The first night I had them, I looked at the moon. The view was very sharp, but there was some CA; a thin blue line on the edge of one side, and yellow on the opposite. At the time, I figured this was probably normal.

However, on a hiking trip yesterday I was looking at some birds in a tree against a grey cloudy sky. There was a lot of CA on the branches of the tree which I found very ugly. A friend was with me with a pair of £20 bins which showed no CA at all, though they were very soft.

After a bit of playing around, I noticed that if I looked through one eyepiece at a time, the CA reduced quite a bit. Then I noticed it also reduced through both eyes if I closed the eyepieces together more, but this was a bit uncomfortable on my nose. My nose isn't particularly wide.

I want to try this on the moon and see if it goes there too. Hopefully tonight.

So, is this all to be expected or a sign of misaligned optics? I have no experience with bins at all really so forgive me if these questions are a bit dumb.

[AlBoning]

I have those too. And I've seen the same CA you report but only in daylight. The CA was very near the edge of field and the next time I tried to find it I had to work to get it to show up. My guess is if you've got day lit sky in the field or you are looking at the Moon you'll see it but other wise not. That is to say it's always there but takes a lot of light to become readily visible.

In reading the binocular comparison reports by Ed Zarenski that can be found at Cloudy Nights he mentioned that many of the binoculars he compared to this (and to the 10x50 which I have also) internally vignette the field (baffle too large, prism too small, various reasons) but not the Pentax, which would tend to mask the CA making it impossible to see. This may explain why one can see CA in some binoculars but not others. So one could look at it as a good thing that they are getting the full 40 mm of aperture and not something less. I regard these as exceptionally good binoculars for the price.

I've noticed the same CA you describe for the Moon when observing with my 102 mm f/11 achromat. I've also noticed that it varies from one night to the next, going from extensive to almost non-existant, which I take to be due to variation in the amount of atmospheric disperision.

[eid]

Hi Al,

It's true that this only shows up when I have the sky as background, and not always even then. I'm not sure exactly what caused it to be so prominent this particular time, but it was very bad and unbearable. As I said, squeezing the eyepieces together so that they become uncomfortable on my sinuses partially fixes it, but that's just not acceptable.

I noticed tonight that I cant quite resolve stars into one image. I'm wondering now if the optics are misaligned and that is causing these problems.

I'm going to return them to the vendor and try another pair.

Thanks for your response.

eid

[eid]

Is it normal to get some yellow and blue fringing on tv aerials against a cloudy sky? This is throughout the view btw, not just around the edges.