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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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Analyzing BK-7 and BaK-4 prisms using refractive indices of 1.518 and 1.569, respectively, I've determined the fastest f/ratio objectives which can be used before the zone(s) of incomplete internal reflection intrudes into the on-axis light cone. This applies to prisms which have 45 degree reflective faces, such as a Porro...
BK-7: f/4.95 (objective subtends 11.6 degrees as seen from the focal surface.)
BaK-4: f/3.35 (objective subtends 17 degrees as seen from the focal surface.)
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1768
Loc: Connecticut
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Which makes me wonder why binoculars in the f/6 range wouldn't choose to use BK-7. My understanding is that BK-7 is a superior optical glass and the only reason to use BaK-4 is its higher index of refraction.
There are binoculars with focal ratios of f/3.0. If they used BaK-4 you'd have the same diamond effect in the exit pupil we commonly associate with BK-7. They use a glass with an even higher index of refraction than BaK-4.
Some very expensive long focal length binoculars choose BaK-4. I'm not sure why. Probably something mundane like cost or availability, not performance.
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 971
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?? Accordingc to some 1970's or 1980's Fuji Photo Optical specifications for the 25 x 150,which is f/5, the prisms are BK-7. There is no square shading in the exit pupil. But I have not tried the EdZ. laser test, nor the new LED flashlight/torch test or its solar variant to those yet.
Did you refract the converging cone at the prism entrance? The converging light cone inside the prism has a smaller included angle than the its angle in air. As seen in cross-section, one side of the cone inside the prism has a higher incidence angle on the reflective faces than the axial ray's incidence angle on those faces, and the opposite side of the cone has a lower incidence angle there. According to your analysis, as I understand it, that lower incidence angle ray would probably not be totally internally reflected in a Bk-7 prism, because f/5 is very close to your limit of f/4.95 .
I do not have an optical design program installed.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Glenn's finding pretty much agrees with what we've been saying here for many years.
This quote form our Best Of thread link regarding when to use BaK4 or BK7
posted 9-5-2003
<quote>
In fact, show me an application faster than F/4 in amateur
astronomy where a binoviewer is used. So my point was that even with an F3.5
newt (we do have a few users!!), one must employ the OCS to allow focus.
F3.5 becomes F/4.4. There is no application faster than about F/4.3 or so.
Therefore, BK7 with superior Abbe number and transmission (yes, transmission)
is the preferred glass for OUR applications. Binoculars are another story and
since BaK4 has a higher refractive index and is therefore more efficient in bending
steep cones that BK7, it is preferred for many binocular designs.The objectives
of the Binoculars will determine which prisms must be used. Has nothing to do with
poor quality. It's a matching up thing entirely.
<and...from within the original posting> from Dick Buchroeder. I hope he doesn't mind if I share this since he is highly qualified to make a comment on this matter:
"When the objectives are say f/5, as they were on many of the WW2
binoculars, BK7 was the proper glass to use. When more compact designs,
with objectives at f/4 or faster (eg, the 10x80 German Flak glasses),
BAK4 was used." R.D.Buchroeder, PhD (Optical Sciences)
<end quote>
Confirmation is always welcome.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 971
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Yes, I know that. I sent to R.A.B. the 10 x 80 optical radii, spacings, and glass types, from Busch plans in a Leitz file from postwar Allied investigators. Leitz: "Why should we show you everything, after what you have done to us?"( the Leitz plant was not bombed). Allied investigator: " Well, perhaps if we bring in some Russians, you will be more cooperative" . The Allied team was soon shown many things. I did my research some 40 years ago, but just learned of that conversation about three years ago, in a memoir of a retired archivist, who was involved in sorting the mountains of Allied microfilms of WW II German industry work.
The question remains: If f/4.95 is the cutoff for an axial or paraxial ray, and if the FPO 25 x 150 is BK-7,( and I believe the Nikon 20 x 120 I and II, f/5, are also BK-7) how does the lower ray of the converging cone, or a bottom ray from a cone of rays from a target away from the center of the field , become totally internally reflected, as it seems to be from the absence of square shading , or any obvious shading line(of course there is vignetting,( which does not have easily seen boundaries),because the prisms are not the enormous ones which would be required to fully illuminate the entire field ), in the round exit pupil?
I suppose that satisfaction this point requires application of ZEMAX, OSLO, or one of the other programs, with graphics. It would be possible to confirm/refute the identity of the prism glasses on an optical bench by measuring an apparent thickness, or by immersion in Cargille liquids, or by...
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Simon S
sage
Reged: 01/07/07
Posts: 391
Loc: Crawley West Sussex UK
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So with all the aperture variations is it still correct to assume an exit pupil with a vignette is a BK7 prism and without a Bak4?
I have also read somewhere that BK7 offers less spherical aberrations Would I notice the difference if I did swap the two prism types in the same binocular? I have viewed binoculars with low index prisms that offer a very bright view even in crepuscular conditions.
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My binocular collection recent first
http://www.flickr.com/photos/binoculars/
My binoculars in Alphabetical order http://www.flickr.com/photos/binoculars/sets/72157613812824211/detail/
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 971
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The answer depends on what you are calling " a vignette ". I suspect that you mean well-defined square shading inside a round pupil. Is it not better to name that "square shading", and to reserve the term "vignetting" for the gradual, not abrupt, loss of illumination away from the center of the field, which occurs in a prismatic instrument with prisms of practical dimensions and weights, even when total internal reflection has occurred to all of the rays captured by the prisms?
The ray path diagram to clarify this may/may not be found in Mil Handbook 141, available on this site, or in one of the books by Warren Smith(McGraw-Hill or SPIE). It is certainly available through experienced(which I am not) operation of ZEMAX, OSLO, etc.
It seems safe to assume that for specimens one is likely to see, square shading in the pupil does indicate BK-7, where BaK-4's higher index would have allowed total internal reflection of more field rays. But BK-7 does not, in general, imply square shading.
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KennyJ
Reged: 04/27/03
Posts: 12909
Loc: Lancashire UK
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< Is it not better to name that "square shading", and to reserve the term "vignetting" for the gradual, not abrupt, loss of illumination away from the center of the field,>
I would agree with that differentiation , Gordon .
The term vignetting seems to have become one that means too many different things to too many different people .
Kenny
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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The analysis I did was concerned with the *on-axis* light cone only. I did account for the differing refraction within the glass, differing critical angles for total internal reflection and the extreme marginal ray which srikes the prism's internal reflecting face at the steepest angle.
Obviously, for image points lying *off-axis* there will be some rays striking the reflective surface at even steeper angles. This will result in the blue-gray dropoffs becoming visible when one peers into the exit pupil from off-axis angles. I'll have to analyze this in order to determine how much of an effect this has.
What I can say right now is that the 'squared off' pattern shifts laterally as one's sight line changes with respect to the optical axis. It will be visible on one or two of the four sides only, never three or four. (Again, this is the case when the objective's f/ratio is at or very near to the minimum allowed. A rather slower objective will have enough of a margin so that the zones of light loss can never intrude.))
So the f/ratio limits I arrived at pertain to on-axis imagery only. For image points off-axis there can be some small amount of brightness loss, becoming a bit worse closer to the field edge. When the complete situation for all field angles is considered, the f/ratio limits will increase somewhat, but probably only slightly.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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JCB
super member
Reged: 10/04/04
Posts: 197
Loc: France
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Quote:
Analyzing BK-7 and BaK-4 prisms using refractive indices of 1.518 and 1.569, respectively, I've determined the fastest f/ratio objectives which can be used before the zone(s) of incomplete internal reflection intrudes into the on-axis light cone. This applies to prisms which have 45 degree reflective faces, such as a Porro...
BK-7: f/4.95 (objective subtends 11.6 degrees as seen from the focal surface.)
BaK-4: f/3.35 (objective subtends 17 degrees as seen from the focal surface.)
Glenn,
I have verified with Zemax. I have found the following limits :
BK7 : f/4.928
BAK4 : f/3.304
Jean-Charles
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JCB
super member
Reged: 10/04/04
Posts: 197
Loc: France
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Quote:
The question remains: If f/4.95 is the cutoff for an axial or paraxial ray, and if the FPO 25 x 150 is BK-7,( and I believe the Nikon 20 x 120 I and II, f/5, are also BK-7) how does the lower ray of the converging cone, or a bottom ray from a cone of rays from a target away from the center of the field , become totally internally reflected, as it seems to be from the absence of square shading , or any obvious shading line(of course there is vignetting,( which does not have easily seen boundaries),because the prisms are not the enormous ones which would be required to fully illuminate the entire field ), in the round exit pupil?
Gordon,
Are you sure that the effective diameter is really 150 mm ? Have you examined the exit pupil totally off axis, just before it disappears at the edge of the eyepiece lens ?
For a 25x150 binocular with BK7 prisms, f/5 objectives, 60° AFOV, and no vignetting at all, you should see an exit pupil at the edge of the field of view similar to the attachment.
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JCB
super member
Reged: 10/04/04
Posts: 197
Loc: France
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Quote:
It would be possible to confirm/refute the identity of the prism glasses on an optical bench by measuring an apparent thickness, or by immersion in Cargille liquids, or by...
You can easily measure the index of a prism by measuring the angle of minimum deviation :
http://www.mtholyoke.edu/~mpeterso/classes/phys103/geomopti/MinDev.html
Draw a line on a sheet of paper (this will be the horizontal red line in the simulation above), and put the prism on that line.
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JCB
super member
Reged: 10/04/04
Posts: 197
Loc: France
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Then you rotate the prism like in the simulation, and you observe the behaviour of the image of the line through the prism. When the angle reaches a minimum, draw a line parallel to this image. Then you measure the angle D between the two lines, and you apply the formula with A=45°.
I've determined that the Meade 10x50, that cost me 20 euros, has BAK4 prisms, as advertised. Since, when I read that BAK4 prisms are expensive, I am laughing.
Jean-Charles
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Robert A.
sage
Reged: 01/21/05
Posts: 294
Loc: Milwaukee, WI Northern USA
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That decent method reminds me of the method to discriminate between a fake diamond and real diamond. The fake diamond in question is Moissanite. They both have very bright sparkle. They are both very hard, on the extreme edge of the Mohs scale.
The similar to Jean Charles test was that moissanite is demonstratively doubly refractive than diamond.
Just babling....
Rob.
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 971
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I shall mount up a 150 and look. A dog bit me, and other activities, have taken precedence over hobbies, and are likely to do so for several more days.
That is an interesting prism test you show. But the 150mm. Fuji prisms are in a cemented Porro II cluster of three. There is one large prism at the bottom, and two smaller prisms are cemented to it . The exit prism, slightly smaller, has rounded corners. I have attempted to separate the prisms several times, in various solvents,including DMSO and methylene chloride( somebody told me to make a mixture of methylene chloride with something else, to "activate" the methylene chloride, but I would need to look for my notes of that conversation) with immersions up to several months, or more, without success. There was some edge attack on the glue, but not enough for physical separation of the prisms. I may have one of the components, separated by oven heating in oil, but will spend at least 1/2 hour just to find it . I could do another oil heating, on a savaged damaged cluster, but the kitchen oven is not working. We have become accustomed to the microwave oven, prefer it, and use the oven top as a flat table for general food preparation. I have a small laboratory vacuum oven, perhaps an alternate separation method? Doubtful, but?
Perhaps the test you show could be applied to an intact Porro II cluster. The entry and exit rays will be separated , with one of them well above the plane of the paper, which could be a source of error in the sighting.
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JCB
super member
Reged: 10/04/04
Posts: 197
Loc: France
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My understanding of the Porro II system is that there is a 45° angle between the external face and the internal reflection face, for both the entrance and exit prisms. If you consider only the upper half of the prism in my first picture, you should have one of those prisms. So, I think the test is still possible for those prisms, even if other prisms are glued to them on one side.
The classic prism is the middle cannot be tested this way, but there is no reason to choose a different glass for it.
Jean-Charles
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