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BK-7 or BaK-4 Prism (1 Viewer)

vop

Well-known member
I have read that the Opticron ES 80 GA has BK-7 prisms. Is that also true for the HR 80 GA ED, and other scopes at the "not-so-expensive" side of the scale? Have also seen much cheaper ones that boasts BaK-4 prisms...

I also have understood that BaK-4 prisms have a higher refraction index, and thus lets more light thru, but are more difficult=expensive to make.

So, is the kind of glass in the prism not the biggest issue in creating a bright and sharp picture?
 

henry link

Well-known member
BaK-4 is preferable to BK-7 as a prism glass only when the focal ratio of the telescope objective lens falls below about f/5. Binoculars are mostly around f/4 so BaK-4 is needed for their prisms to achieve total internal reflection at the edges of the fast f/4 light cone.

Virtually all spotting scopes have focal ratios above f/5 so there's no disadvantage in using BK-7. It achieves total internal reflection just as well as BaK-4. In some telescopes it's actually a slightly better choice because a prism made with BK-7 has a little less spherical overcorrection and chromatic aberration at blue/violet wavelengths than BaK-4.
 
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henry link

Well-known member
In most cases I don't know. The glass often isn't specified since it doesn't matter as long as the refractive index is high enough for total internal reflection at the focal ratio of the scope. Any claim of higher light throughput from using BaK-4 would be marketing piffle.
 
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gcole

Well-known member
Henry .... What about binoculars ? Leupold uses Bak-7 in the katmai 6x32 & they are sharp,bright & clear ..Gwen
 
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Kevin Purcell

Well-known member
I also have understood that BaK-4 prisms have a higher refraction index, and thus lets more light thru, but are more difficult=expensive to make.

It's no more difficult to make BK-7 prims than it is BaK4.

As Henry points out BK7 actually has some advantages for some optical uses as it has a higher Abbe number (just another way of saying what he said). That's why people make lenses from BK7 (perhaps more than any other glass) but (almost) never make lenses from BaK4 because you want to minimize CA.

The problem is marketing has made "BK7 == BAD; BaK4 == GOOD". But life is more complicated than that.

Any bin with BK7 prisms will show vignetting at the edge of the exit pupil. It will be worse for wider field bins. You can check by looking for the squared off exit pupils characteristic of BK7 prisms in bins. The squaring up shows the missing portions from the round exit pupil that don't get TIR through the prism. That's lost light compared to a BaK4 prismed bin.
 

henry link

Well-known member
Gwen,

Sharpness and clarity aren't affected by using BK-7 and only the outer edges of the exit pupil are shadowed. If your eye pupil is closed to a size that fits within the unshadowed diamond shape inside the exit pupil there is no loss of light at all with BK-7 prisms compared to BaK-4.

With any binocular or telescope you can easily see the extent of the problem (if there is one) by examining the the exit pupil. If there is no diamond shaped shadow, then the prism is achieving total internal reflection no matter what type of glass is used. If there is shadowing only the outer part of the exit pupil that appears dimmed is affected, not even necessarily a bad thing given the high levels of spherical aberration in binoculars. Vignetting the outer part of the objective light cone acts to reduce aberrations.

Henry

Edit: Just noticed Kevin's post. We agree about everything except that this kind of vignetting is vignetting of the axial exit pupil, so unlike vignetting from an undersized internal aperture, it doesn't dim the image any more at the field edge than the center. It dims the entire field equally.
 
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Kevin Purcell

Well-known member
Edit: Just noticed Kevin's post. We agree about everything except that this kind of vignetting is vignetting of the axial exit pupil, so unlike vignetting from an undersized internal aperture, it doesn't dim the image any more at the field edge than the center. It dims the entire field equally.

We do agree completely.

I fixed what I originally miswrote (about the dimming of the edge of field when I meant edge of the exit pupil) in an edit when you were writing your reply ;)
 

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