• Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.

    Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

Are BK-7 prisms that inadequate or inferior? (1 Viewer)

J

John Dracon

John Dracon
Without plunging our readers into a virtual wilderness of optical terminology, I have begun to question the de-emphasis given to BK-7 glass as a binocular prism material. In terms of refractive index, it can't match the density of BK-4, of course and hence must be considered as inferior with less light gathering ability. But does this mean that binoculars crafted with BK-7 glass aren't useful and in fact are inadequate in transmitting a quality image to the human eye? I have found to the contrary that some binoculars with the BK-7 glass to perform quite well. I might add that all of these binoculars are built to a high mechanical standard.

I have several of these lower status binoculars that provide a sharp and colorful image, which is virtually indistinguishable from its high status companions. One, a Bushnell 8x30 CF Zeiss clone, at least 60 years old, that functions flawlessly, will out perform any of my alpha glasses of similar power on the resolution charts.

Another, a Bushnell Banner ( recognized as a second tier Bushnell product) CF 6x30 with a 393 foot field came out of a pawn shop for $20. It is at least 60 years old and shows wear. But its coatings are still unblemished, and the sweet spot is quite wide. And the edges are very good. The deep eye cups are easily removed revealing a flat surface which allows the eye glass wearer full view without contacting the ocular surface. Several of my friends have remarked what a pleasing image it presents. It is light, handy, and focuses easily.

Of course both of these binoculars subjected to 30 X magnification will show their optical shortcomings, but for the ordinary persons, these binoculars are adequate. The thought of forking over several thousand dollars for a pair of the alphas, even if cost wasn't an object, is to depart from reality.

It is similar to tire shopping I did the other day. One set was rated to handle sustained speeds of 130 mph with wear rating of 50,000 miles. The other set, which I bought, is rating to handle sustained speeds of !00 mph with a 60,000 wear rating. Since I never drive at either of these speeds, why spring for the higher rated and more costly set of tires?

John
 
I wouldnt argue with your thoughts, I have an old pair of Tokyo Optical 7X50 that are excellent. I think you have to look at the difference implied in inferior and inadequate. They may be inferior in the technical sense to the current crop of alpha or near alpha glass, but inadequate they certainly arent.
 
Bear in mind...the difference for a Porro is less important than for a roof.
Also, sticking to 7.5 degrees asks a lot less of the prisms than going truly extra-wide.
You can see that clearly in the size of the 'blue zone' for each BK7 looking through
backwards. A Porro with an excellent eyepiece and limited to 7.5 degrees like that is
usually a gem, if clean. The Banner narrows in general had excellent precision and comfort,
and 6x doesn't hurt. Banner narrows were design as a more economical version of Featherweights
with the same eyepiece....that's the 'pleasing view'. The 7x35s are also very easy to look through
and accurate.

You will see a little 'veiling glare' from their chamber-n-prism-hat scheme,
but if you avoid provoking it, it's fine. For the same thing, a little lighter and
deeper contrast, no veil, that's the Featherweights (most brands) or
Featherlights (Bushnell, IF), or the Kowa Prominars.
On a nice overcast day, those Banners will look the same.
 
Last edited:
John Dracon said:
Without plunging our readers into a virtual wilderness of optical terminology, I have begun to question the de-emphasis given to BK-7 glass as a binocular prism material. In terms of refractive index, it can't match the density of BK-4, of course and hence must be considered as inferior with less light gathering ability. But does this mean that binoculars crafted with BK-7 glass aren't useful and in fact are inadequate in transmitting a quality image to the human eye? I have found to the contrary that some binoculars with the BK-7 glass to perform quite well. I might add that all of these binoculars are built to a high mechanical standard.

I have several of these lower status binoculars that provide a sharp and colorful image, which is virtually indistinguishable from its high status companions. One, a Bushnell 8x30 CF Zeiss clone, at least 60 years old, that functions flawlessly, will out perform any of my alpha glasses of similar power on the resolution charts.

Another, a Bushnell Banner ( recognized as a second tier Bushnell product) CF 6x30 with a 393 foot field came out of a pawn shop for $20. It is at least 60 years old and shows wear. But its coatings are still unblemished, and the sweet spot is quite wide. And the edges are very good. The deep eye cups are easily removed revealing a flat surface which allows the eye glass wearer full view without contacting the ocular surface. Several of my friends have remarked what a pleasing image it presents. It is light, handy, and focuses easily.

Of course both of these binoculars subjected to 30 X magnification will show their optical shortcomings, but for the ordinary persons, these binoculars are adequate. The thought of forking over several thousand dollars for a pair of the alphas, even if cost wasn't an object, is to depart from reality.

It is similar to tire shopping I did the other day. One set was rated to handle sustained speeds of 130 mph with wear rating of 50,000 miles. The other set, which I bought, is rating to handle sustained speeds of !00 mph with a 60,000 wear rating. Since I never drive at either of these speeds, why spring for the higher rated and more costly set of tires?

John
Click to expand...

To armchair speculators who read too swiftly--yes.

To optical engineers and technicians--no.

Bk7 is a more homogeneous glass that has been at the top of the visual optics food chain for years.

It is "inferior" to BaK4 only in that it's MATHEMATICALLY "inferior" in faster f/r instruments. When dissecting that with the realities of human pupils in daylight and dark, however, even that argument falls apart.

The argument will never go away. That's why touching on those boring realities, every so often, is, perhaps, a good thing. :cat:

"However beautiful the strategy, you should frequently look at the results."--Winston Churchill

Bill
 
Last edited:
John,

It is ancient knowledge (from the 60s at least!) that the shadowy incursions into the exit pupil, seen in Porros with with BK-7 prisms, is a view dimming evil. But that's the only problem with the stuff, and doesn't seem to be a huge problem to me. I've never seen the brightness of the shadowy zones quantified, but they look to be maybe only 10-20% dimmer than the fully illuminated part of the exit pupil. If everything else about the binocular was first rate, I bet you'd hardly notice, in the view quality. (Judging from your post, I'd be right.)

If you don't need the higher index of BAK-4 to get the whole field of view though, the cheaper and clearer BK-7 would be the obvious high quality choice. Zeiss uses BK-7 in its notoriously bright Abbe-Koenig prism binoculars.

Ron
 
As I understand it the higher refractive index of of something like BaK4 is important for the total internal reflection of a prorro, but otherwise, the lower dispersion and higher transmission of BK7 and other glasses may be the better choice for mirrored prisms.

I suspect there is another layer of confusion caused by many cheaper models claiming the use of "BaK4" glass. However Chinese glass manufactures use the BAK4 designation for a PSK3 specification glass and BaK7 for a BaK4 so we probably have little idea what's actually inside most roofs in particular. Schott specification BaK4 is most likely used a lot less often than we imagine.

David
 
Last edited:
typo said:
As I understand it the higher refractive index of of something like BaK4 is important for the total internal reflection of a prorro, but otherwise, the lower dispersion and higher transmission of BK7 and other glasses

David
Click to expand...

Almost....on a few counts.
The higher index of BAK4 (all caps) is more important for roofs,
and not so important for Porros. As is the 'dielectric' coatings,
not needed for Porros at all. And...the BaK4 (with a little a) is a third glass type. The slight 'bluing' of the BK7 image actually counteracts
the 'yellowing' of the single coatings, so many
are pleasantly surprised by the result.

I really like the "Bak4" I sometimes see, but I am not sure why.

BK7 actually has less dispersion than BAK4, that is, not as much
chromatic diffference in the focal length of the gadget.
This may cause confusion when BK7 (esp. narrow fov)
pairs seem to be very sharp.
 
Last edited:
O_N

I should have explained that a lot better. You are correct that both porro and roof prisms use total internal reflection but the as far as I've been able to find examples, the diamond shaped exit pupils were were typical of wide angle, low f-number, BK7 porro designs which are rarely encountered in roofs. I have only one roof that I know to be BK7, a 6* 10x42 and the exit pupil is round, but my BK7 9.4* 8x30 porro has the diamond. I suspect many roof designs would be fine or even better with BK7 glass but you may have better information.

BaK is the German abbreviation for Barium Crown and BaK4 is one type. However Schott also use BAK in their catalogue listing so I guess both are correct. The Chinese BAK4 is quite different and not a Barium Crown glass at all so BaK should not be used but often is in practice. Rightly or wrongly BaK4 and BAK4 are interchangeable in both parts of the world but are totally different glasses. Which, if either, is used may not always be correctly disclosed I suspect.
http://www.schott.com/advanced_opti...s/optical-glass/index.html?so=uk&lang=english
http://www.chinaopticsnet.com/sop/19110000.htm

David
 
Last edited:
BAK4, BaK4, BAK-4 and BaK-4 are usually all the same glass. If in doubt just compare the descriptions in the glass catalogues.

The odd Chinese BAK4 has a critical angle for TIR (40.1º) which falls between Schott BAK4 (39.6º) and Schott BK7 (41.2º), so as a prism glass it would be capable of TIR with lower focal ratio objective lenses than Schott BK7 (around f/5) but not go quite as low as Schott BAK4 (around f3.5).

Roof prism binoculars do not have a greater need for BAK4 glass than Porro prism binoculars. As Ron pointed out the reflection angle in the first prism of an Abbe-Konig is well above the critical angle for BK7. The problem with Schmidt-Pechan is that one reflection falls below the critical angle for any glass and so needs to be mirror coated. The other angles of reflection in S-P are no more acute than Porro.

One last point. The critical angle for TIR applies to axial rays as well as off-axis rays, so a narrow field binocular is no different from a wide field. That's why you see the diamond shape when you move your eye straight back from the eyepiece to examine the axial exit-pupil. It's the focal ratio of the objective, not the field width, that determines which glass type is needed for TIR.
 
Last edited:
. Thanks for that explanation, Henry.
Most interesting.

Does that mean that spotting scope prisms are made from BK7 glass?
 
Last edited:
"
The critical angle for TIR applies to axial rays as well as off-axis rays,
so a narrow field binocular is no different from a wide field.
"

I am having trouble with this logic.
Rays coming from different angles outside strike the prism at different angles.
The only way they could all reflect at the same angle is at dead center for all.


If your inference is correct, there should never be any variation in the
coloring across the exit pupil....at all. It would all-white or all-blue-grey
if the above were true. And...there the variation is. I see it.
Are my binoculars under some spell?

The critical angle is a fixed thing, yes. But: some of the rays are higher than that,
and some are lower. That is how we see the variation. Axial rays and off-axis rays
must necessarily hit the prism faces at different angles. How else would the field be carried?
 
Last edited:
The point is that the cords cannot be eliminated by reducing the field width. In the center you will always see the same 4 cords forming a diamond shape impinging on the exit pupil by an amount determined by the objective focal ratio no matter how wide or narrow the binocular field. The size of the diamond remains the same for off axis exit pupils but the center of the diamond will shift within the exit pupil so that as 2 cords disappear from one side of the exit pupil the two opposite cords widen.
 
I'm not sure who intended to get rid of the blue patches entirely.
You could, actually, but the field width would be way too narrow.
I was referring to the amount, not the total removal.
If the point deals with total removal, we are talking on different planets.
 
OPTIC_NUT said:
I'm not sure who intended to get rid of the blue patches entirely.
You could, actually, but the field width would be way too narrow.
Click to expand...

No amount of narrowing the field will reduce the cords even slightly. The diamond will stay the same size relative to the exit pupil unless the objective focal ratio is raised or a higher index prism glass is used.
 
henry link said:
No amount of narrowing the field will reduce the cords even slightly. The diamond will stay the same size relative to the exit pupil unless the objective focal ratio is raised or a higher index prism glass is used.
Click to expand...

I ran this through Google translate - yet it still came out like this!;)
 
james holdsworth said:
I ran this through Google translate - yet it still came out like this!;)
Click to expand...

I think unless you have enough geeky interest to sort out the technical stuff it's probably good if ALL of it sounds equally nonsensical. At least that way you won't be mislead by the stuff you read here that really is nonsense. ;)

Henry
 
whatever the hullaballoo,
I don't think I can ever believe that all the rays from all the different angles all
hit at exactly the same angle on the prism and experience exactly the same relation
with the critical angle. The mere existence of the blue patches tosses that overboard.

Calling off-axis and on-axis equally affected by the critical angle in the face of
of what is known and seen is a "hasty generalization" in Critical Thinking and Science.

Not coincidentally, noting an especially bad veiling issue in one particular pair and
declaring all pairs with that power and aperture must have that problem is also
'hasty generalization', as dozens of pairs in my stable show. I have a special 10x50
can-scope for camera use...no prisms, a wide body, just an eyepiece and objective.
And....zero veiling. Why? The eyepiece iris is a proper design. Every opening must be,
whatever the power or the front.

I'll take time to look at the diamonds again tomorrow, across a few pairs. It is what it is.
 
Last edited:
Thank you Henry for your reflective and illuminating comments. I had not considered what you say about f/ rather than FOV.

Ron
 
Last edited:
Warning! This thread is more than 9 years ago old.
It's likely that no further discussion is required, in which case we recommend starting a new thread. If however you feel your response is required you can still do so.

Similar threads

J
What binoculars are as good as Nikon Superior E?
2 3
Replies
46
Views
4K
Jaywalk
J
chill6x6
Birding after surgery...
2
Replies
32
Views
2K
hopster
hopster
M
Six 5X, 6X, and 7X reverse porros compared
Replies
4
Views
1K
Canip
C
hopster
Optical comparison of Habicht 10x40 and HT (or FL) 10x42
2
Replies
34
Views
3K
hopster
hopster
kimmik
A comparison of prism types, and why the Schmidt Pechan seems to have won the premium binoculars
3 4 5
Replies
83
Views
10K
kimmik
kimmik

Users who are viewing this thread

Total: 2 (members: 0, guests: 2)
Back
Top