As you shop around for your next piece of observing kit, you’ll come across the terms BaK-4, BK-7 and K9. These refer to the formulations of glass found in the prisms of spotting scopes, binoculars and monoculars.
BK-7 and K9 glass can be found in cheaper products, and refer to borosilicate formulations. While used across a wide range of brands, this glass is said to lead to problematic exit pupils — in particular the field of view isn’t perfectly round, which can leave the observer with poor clarity around the edges of their chosen target. We must stress that this isn’t true for all pieces of kit, so we advise giving your chosen binoculars a test before parting with your cash.
While a touch more expensive than BK-7 and K9 prisms, the BaK-4 — or barium crown glass — is preferable for binoculars. With this type of prism you’ll achieve a higher quality of image, particularly when combined with a well-made optical system.