Prism Glass

Hi folks

can somebody advise what is the technical difference between BAK4 and BAK7 glass. I am loking at binoviewer specs.

Any info and/or advice is appreciated.

clear skies

Johnno

I've was advised, when looking at binoculars, that BAK4 is superior to BAK7. I was told that more light can pass through bak4 than bak7 resulting in brighter images.

What I think actually happens is that bak4 has a higher refractive index and so the light path needed in a bak4 prism is shorter than that of bak7, so less light gets lost.

I think I read that the bak7 also has a higher dispersion value, so you might get more colourful images, than with bak4. I suspect the focal ratio also comes into play in these circumstances.

I hope that is of some use. Apologies though, I dont have any numbers to back any of this up.

Sheers,
~Albert

About half-way down this thread:

www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/44730

are some links to other threads about BaK-4 vs. BaK-7. Complicated subject.

-- Jeff.

Thanks guy's.
After much reading I am still unsure of what best suits my particular needs.

The cloudynights thread had some great links.
At the outset I thought the BAK7 was a better all round spec for porro bino's and was surprised that Al seemed to differ.

However Zeiss confirm that BAK4 is a much better and more expensive glass. It has a much better refractive index and is definitely the way to go when choosing bino's.

However when using binoviewers on a fast scope, the better dispersive properties of BAK7 is much preferred.

Zeiss and OPT seem to contradict each other.


clear skies

Johnno

(Astigmatism is a pain in the ass)

Hi John,

I can’t find any listing for a BaK7 (BaK series only goes up to 6 in my catalogues!) so I’ll substitute in BK7 which is the bog-standard optical glass used in most optics since God was a boy! The difference between the glass types is in the amount they refract the light at different wavelengths (nr, nC, nd… nh) which allows the optitian to select a pair or three types when designing a lens to correct chromatic aberation to different levels (simple achromatic, apochromatic, apo-ED (Extra Dispersion) and super-apochromatic.) In binocs, the type of glass used in the prism is not usually important – they all have the same transmission value (more or less.) It is not likely that the prism plays any part in the correction of chromatic aberration in either binocs or bino-viewers. What bino-viewers are you looking at and I’ll see if I can help with a comparison for you. I’ll be away until Sunday evening before I can get back to my PC BTW!

All the best,

Phil.

BK7
d-digit No. 517642
Cost v BK7 1.0
Density 2.51
Nr 1.51289
NC 1.51432
Nd 1.51680
Ne 1.51872
NF 1.52238
Ng 1.52669
Nh 1.53024


BaK4
6-digit No. 569561
Cost v BK7 1.02xBK7
Density 3.10
nr 1.56402
nC 1.56576
nd 1.65883
ne 1.57125
nF 1.57590
ng 1.58145
nh 1.58609