BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply to thread

Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1310027" data-attributes="member: 68323">Thanks for the comment, Kimmo.Yes, both causes have crossed my mind. The sharper the roof edge the smaller the area to scatter light into a diffraction spike and the "wider and lower amplitude" the spike will be.I wonder if I boosted the star-image diffraction pattern if one would see two contributions both in the same direction (perpendicular to the roof edge): one (perhaps smaller?) from the lack of phase-compensation making the image ellipsoidal and the other an overlaid much wider spike from the roof edge. Sort of like a line with a bump in the middle.Another check for roof prism knife edge quality is to look down the objective end of the bin and see if you can see the knife edge of the roof either against a light background through the oculars (in transmission) or by shining a flashlight through the objective and looking for reflection from the edge.With this example set of bins:<ul> <li data-xf-list-type="ul">Winchester 8x32: Knife edge in one barrel clearly visible (the worst barrel for spike and other aberrations). Knife edge in other barrel more difficult to see but visible. Both easily seen in reflection.</li> <li data-xf-list-type="ul">Promaster 7x32: Knife edge just visible with some observation in transmission. Seen easily in reflection.</li> <li data-xf-list-type="ul">Vortex Diamondback: No knife edges seen under any circumstances.</li> <li data-xf-list-type="ul">Bushnell Discoverer 7x42: No knife edges seen under any circumstances.</li> <li data-xf-list-type="ul">Pentax WP 8x32: No knife edges seen under any circumstances.</li> <li data-xf-list-type="ul">Zeiss Conquest 8x30: No knife edges seen under any circumstances.</li> </ul>So the correlation seem(ed) clear to me: see knife edges == knife edge diffraction in image == no phase-compensation.Though of course the two non-PC are also early 2000s low end Chinese roof prisms and suffer from some other issues (the Winchester more than the Promaster) so they may just have not very good roof prisms.But I don't see the roof edges with any of the other bins but I do see some (much weaker) diffraction spikes."It's the PC coating". QED. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />OK, that's not a very strong argument but it's the best I can do with the sample I have. And I'm still very open to the "it's not so good roof edges". Perhaps a combination of the two effects?I tried the same test with the demo Monarch (first time around ... I already had this in mind) and couldn't see knife edges either in transmission or reflection.The unit to unit variation was another issue that I wondered about. Nice to see someone has seen this vary across across multiple units. It would also be nice to thing that the REI bin people could cherry pick a demo bin.And yes, regardless of where it comes from it's a useful thing to test especially if you have a choice amongst many "identical" units. Of course it's an extreme test (you wouldn't go looking at a large bright light deliberatley but the subtle effects are there and might cost an ID. Or maybe not. I'm still not too sure how large an effect this is for birding. After all there were many happy (deluded, ksbird might say) Zeiss Classic 7x42 users before 1988 and P* coating came in.More data would be useful. It's an easy test to do.</blockquote>

Verification