CN FORUMS ARCHIVE Telescope Reviews: Is Chinese BaK7 glass really Schott BaK4 glass!?

Actually, if what Mardi has found can be corroborated, this may not necessarily be good news. But, depending on other design parameters, it may not be a bad thing at all.

The confusion here lies in the fact that what the consumer thinks may be BaK4 prisms, which are better suited to fast focal ratio light cones of most binoculars, may in fact not be the forumla for glass as we normally refer to BaK4. If the formulated glass is indeed closer to BK7, then the resultant prism (if still made to the same size) would have the affect of causing considerable light loss within the prisms. But there is no indication the formulation is close to BK7.

Frankly, I have not seen any evidence of this in any Chinese binocular. So much more explanation is needed to understand what is going on here. This may in fact account for the new trend it what is being referred to as oversized prisms in many models, because if it were a glass formulated closer to BK7 it would require a larger sized prism to accomodate the light cone. The good side to that would be, if is is formulated closer to BK7, and it is made bigger, it's a better glass to use for prisms.

Read this From our Best Of links on BaK4 vs BK7

BaK4 (Barium crown) has no advantage over BK7 (Borosilicate) when used in in a situation where the incoming light beam angle is very narrow. However in wide field applications such as low power binoculars there are advantages - increasing the brightness of the edge of field while having no effect on the on-axis image. There would be no advantage to BaK4 over BK7 to any part of the image for instance in a Binoviewer because of the narrow beam angle wherein generally they are slow systems without wide fields.

The disadvantage of BaK4 is that it introduces more spherical aberration into the optical path than BK7. BaK4 also introduces more chromatic aberration than BK7. You may not notice this, particularly if the optics are poor to begin with, you may not be able to separately identify which component is causing the aberration. These aberrations can almost always be noticed in binoculars, but as stated above BK7 if made to the same size is simply the wrong choice for prisms in fast binoculars. Exceptions might be those f/11 BT80 Vixens, or opther f/8 and slower binoculars.

BK7 may be the clearest, most defect-free optical glass available for prisms. BaK4 is close, but not quite as good. BaK4 is ok for low power applications of a typical binocular, but not for high power low contrast situations seen in telescopes or binoviewers. For a high powered slow f# telescope, with a very narrow light beam, BK7 would be a better choice for a prism. Similar applications are found in binoviewers.

Unless you plan to use your binoviewer on an f/4 or f/5 telescope, BK7 would be the preferred glass for prisms in a binoviewer. So why is BaK4 glass normally found in binoculars. The need to handle a wider faster light cone.

Of course none of this explanation answers, What is the formulation of the Chinese glass currently being referred to as BaK4?, as noted in Mardi's post above. Hopefully we will find out more.

Although you should be careful not to trip here. Take note that Schott BaK4 is labeled Chinese equivalent BaK7, NOT BK7. So there is no confusion here between BaK4 (Barium crown) and BK7 (Borosilicate). There is a difference in the number from 4 to 7 (not to mention other differences). Then also note that Schott BK7 is labeled as the Chinese equivalent of K9.

Of course further confusion arises not only from "who's term" for Bak4 is being used, Schott or Chinese, but also from any typos in trying to relate the information. In any layman's (typical forum) references to these terms the extreme caution should be noted that more than half the time the letters are confused by the writer. The glass and the terms as we know them from Schott are BaK4 (Barium crown) and BK7 (Borosilicate). If you were to read a forum post and the term "BaK7" were typed, you would have no idea if you were reading a reference to a Chinese glass term or someone had mistyped BK7.

edz

Edited by EdZ (03/28/06 11:44 AM)