[Kevin Purcell]

I posted this paper deep on the CamerlandNY Mystery bin thread but I've posted this up here too so more people see it.

The paper is "Progress in Binocular Design" from Konrad Siel at Swaro in 1991. Very interesting and not too technical read on the state of binocular design in 1991 (just at the transition to phase correction in roof prism bins).

What he says is still relevant today!

http://www.optics.arizona.edu/optome...eil%201991.pdf

One of my favorite quotes:

Where compactness is a dominant criterion we found that the Schmidt-Pechan prism systems the best choice. In all the other cases we use Porro prisms.

He talks about how to mount prisms, how prisms (like thick plates) increase aberrations off axis, the effect of the quality of prism glass, how multilayer AR coatings reduce contrast in Schmidt-Pechan prisms systems(!) and how phase coatings work (and what impact they have when they aren't used).

It's interesting to get an insight into the number of compromises that you need to make to get a binocular. I'm not surprised we can't agree on the "best" bin.

Conclusion:

The basic condition to achieve high quality binoculars are an adequate lens design and production of optical and mechanical components with very narrow tolerances. For optimum performance the mounting of prisms and lenses, the quality of the glass and the different coatings of the prisms are essential.

That was written 17 years ago in Austria. But it's pretty much a guide to what the Chinese optical industry has been doing recently to make better binoculars:

• Better optical design.
• Better optical glass.
• Better optical manufacturing (to repeatable high tolerances).
• Better AR coatings.
• Better phase coatings.
• Better mounting systems and automated assembly (glue, check and set).

[ceasar]

Odd!
The link supplied won't download for me in this thread but will in the "Mystery binocular" thread.
Bob

[elkcub]

I can't download it either, but would like to read it.

[Kevin Purcell]

Gack. The forum truncates long URLs (in the visible text) so when you copy and paste the text you get a mangled URL. Sorry about that.

I fixed the link above and here it is in working form.

http://www.optics.arizona.edu/optome...eil%201991.pdf

[henry link]

Thanks for posting the link, Kevin.

The new information there for me is the bit about contrast loss at the roof faces that both reflect and transmit in a Schmidt-Pechan. I had always assumed that the TIR occurred at the surface of the glass even when it was coated. Just one more strike against the S-P.

Interesting to note how the future brought unforeseen changes in Swarovski designs. A few years after this paper the SL Porro line was discontinued while the older Traditional Porros still hang on today. The internal construction of the SL might have looked like the wave of the future, but the actual binoculars were not so attractive. Focus was at the front of the bridge and was very slow, probably because the large waterproof moving eyepiece tubes needed a high gear ratio to be easily moved. Only the 8x56 had adjustable eyecups. The field was very narrow in the 7x42, 8x56 and 7x50. The coatings had a strong yellow tint. They never went over with birders.

The basic optical design of the early SLC models (8x30/7x30) illustrated in the paper is still retained in the current 8x30 which uses a moving objective for focusing, but the later SLCs would use internal focusing lenses. I think in 1991 only the Leica Trinovids used a focusing lens and a little later the Zeiss Night Owls.

Phase coating is certainly essential for decent image quality, but I suspect that the quality of the phase coating varies in different binoculars and that in cheap phase-coated binoculars it's not very good. I noticed that the phase coating of an inexpensive roof prism bin I tested recently did not appear to be completely effective. I tested it by sighting my LCD computer screen through the objective end while rotating a polarizing sunglass lens in front of the objective. One half of the illuminated circle got very dark as I rotated the lens which I think indicates incomplete correction. This test can also be done without the LCD screen by using two polarizing lenses, one behind the eyepiece and one in front of the objective. There's an obliging retailer nearby so the next time I visit her store I hope to try this test on a number of binoculars.

Henry

[ronh]

Kevin,
Thanks so much, this looks delightful!
Ron

[ronh]

After reading, I have a couple of questions, maybe somebody could help me?

(1) The interferograms are not explained, but seem to boil down to something like a snapshot of wavefronts traveling though the glass, from left to right (or right to left?). If this is the case, then the ideal result would seem to be the top and bottom halves of the small insert(presumably corresponding to wavefronts that have bounced off of the two roof edges) showing straight vertical lines of similar good contrast, and lining up perfectly at the interface. If this hunch is correct, the phase coating works very well, in that the wavefronts in the two halves line up perfectly. But, the curvature still indicates a full wave of error across the entire wavefront. This seems pretty bad. Is this typical, and acceptable at, say, 8x?

(2) And who, on this forum, best fits Konrad's profile of the "experienced and pretentious user of a binocular"? I like to think I'm pretty darned pretentious, but experienced, no way.

Anyhow, when I shine a flashlight into my Trinovid now, I will no longer be so dismayed at the brightness of the reflection off the prism, compared to, say, a Fujinon FMT-SX. Hey, they've thought about it!
Ron

[henry link]

Ron,

I imagine this image indicates the overcorrection of the prism when it's inserted into a converging light cone. I believe this would be combined with an undercorrected objective in the binocular, but even after that there will probably still be considerable overcorrection. At least, that's what I see in most binocular star tests. Low magnification is very forgiving and, of course, in daylight the corrections will improve considerably just from the eye's pupil size stopping down the objective. Your 7x50 Fujinon's aberrations probably look pretty bad in a full aperture star test at 70x, but will clean up nicely when it's stopped down to 20-25mm in daylight.

Henry

[Kevin Purcell]

The interferograms show the phase consistency of the light bundle coming through the optic.

Neither of these two articles are that good but they give you a basic idea: take a coherent beam, split it in two, pass one through the item under test then combine the two beams to see the interference fringes.

If you don't have any "device under test" in the path you should see a series of straight (vertical) fringes.

http://en.wikipedia.org/wiki/Interference
http://en.wikipedia.org/wiki/Interferometer
http://en.wikipedia.org/wiki/Michelson_interferometer

My interpretation:

Fig 6: nice vertical regularly space fringes through the glass sample. The sample is homogeneous at the measurement wavelength.

Fig 7: interferogram is twisted and contorted through the glass so all the rays in the bundle do not have the same phase delay.

Not all optical glasses are created equally. Note that I suspect that if you looked through these glass blocks with the naked eye you wouldn't see a difference. I suspect you might not even see a problem in porro prism design either (as you on't need phase coherence for that to work properly) but it makes a difference in a roof prism where the input beam is split in two and recombined. That's why you need phase correction. But phase correction won't do any good if the beams coming though the prism alrady have their phase "mangled" by the glass.

Figure 13 and 14 are views with two different Schmidt-Pechan prisms in the light path. You can see the outline shape of the prism (take a look at the WP article for a diagram) with the roof to the right.

Figure 13 shows a phase-corrected prism where the phase-correction is working. The output beam shows consistent slight fringes across the exit phase of the prism. OK, the fringes you can see are very slightly bent. I think in this case they've tuned the delay in the reference path so they show minimum fringes (if this was perfect you'd cancel out the fringes on the image through the prism would be black). By doing this you can see the deviations from perfect phase. So you can see the phase is uniform across the prism.

Figure 14 shows an uncorrected roof prism. You can see bright fringes from one side of the roof and dim fringes from the other side of the roof so the light passing through the prism does not have the same phase delay for each side of the roof (for the path twisting to the right and the path twisting to the left). If you change the delay in the reference path (i.e. change the phase of the reference beam) you'd see the bright pattern dim and the dim pattern brighten until eventually the upper part would be bright fringes and the lower part are dim. The phases are different.

Note the MTF measuresments too: the contrast increases at higher spatial frequencies with phase correction. The phase correction is (most) needed to improve resolution when the "lines" in the "resolution chart" are perpendicular to the roof edge direction.

http://en.wikipedia.org/wiki/Optical_transfer_function

One interesting point is I've looked a quite a few roof prisms bins to see if I can see the roof edge. Sometimes you can; sometimes you can't. I would hope you couldn't see that edge. The sharper the knife-edge of the roof is the more difficult it should be to see (but at the lower end of bins I find the correlation is rather weak). And the sharper the edge the less it should scatter (rather than reflect) messign up overall contrast (another reason why cheap roof look worse). I noted the orientation of the roof isn't a standard direction. I've not seen one where the roof is either vertical or horizontal (a feature perhaps to avoid messing up vertical or horizontal lines which we have in the built environment). And of the ones I've seen the roofs are at 90 degrees to each other in each barrel. I suspect this is a deliberate effort to compensate for the reduction in resolution: when one eye is "good" then the other is "bad". Of course it doesn't quite work when you have a dominant eye (which I think almost everyone has). But every little helps.

BTW, I suspect the images were a lot better in the original publication ... I think this might be a scan of a photocopy.

The more I understand about roof prisms the more I appreciate porro prisms bins

[Kevin Purcell]

Originally Posted by ronh

But, the curvature still indicates a full wave of error across the entire wavefront. This seems pretty bad. Is this typical, and acceptable at, say, 8x?

I think at the low magnification you do get away with this without many problems. Think of how many figured or flat surfaces you'd have to get right and parallel to each other to get a tenth wave or better across the aperture!

That highlights the difference between astronomical optics and binocular optics.

[ronh]

Thanks, Henry and Kevin, that was helpful. I hope you guys will bear with me through one more issue I'm trying to grasp here.

In a 42mm f/4 binocular operating at 8x, 40 lines per mm works out to about 4 arcmin at the eye. That does not sound hard to see. So, the plotted (to 40 lpi for the glass types, but only to 20 lpi for the phase coating comparisons) differences might be quite noticeable. The disturbing impression I am left with is that the differences between uncoated junk glass and the finest roof prism binos made by Swaro in 1991 would be small, compared to the great dimunition in quality imposed by any binocular, compared to the naked eye. Shouldn't a binocular's MTF be flat at near 100%, to look essentially perfect to the eye? Down to 10% at easily-resolved wavelengths sounds completely horrible.

But, that's not how it seems. Binoculars of top quality give views that seem excellent, thrilling, spectacular, or we wouldn't be here! So, either I'm not very critical, legally blind more like it, or I have misinterpreted or been misled somehow by the results of Konrad's paper.

To help put it into perspective, the MTFs of the naked eye and Porro prisms should also be used, but I haven't easily wandered up on either, unfortunately.

End of rambling, bottom line question: At what contrast dropoff does the eye start to notice a loss?
Ron

[Kevin Purcell]

Originally Posted by ronh

In a 42mm f/4 binocular operating at 8x, 40 lines per mm works out to about 4 arcmin at the eye.

At what distance? Seil doesn't define his experimental setup: no distance to target, size of objective, magnification, etc. All which would change the conversion from spatial frequency to angular frequency.

So I don't see how you can extract an angular resolution measurement from the presented data.

I think he presents the data just for qualitative effect so you can see the effect PC coatings have and that they're dependent on the direction of the roof.

Of course, I could be wrong too

[Kevin Purcell]

Originally Posted by ronh

And who, on this forum, best fits Konrad's profile of the "experienced and pretentious user of a binocular"? I like to think I'm pretty darned pretentious, but experienced, no way.

I beleive I resemble that remark.

I noticed the odd phrasing when I read the paper. I presume it's either a spello or the original paper was written in German and they picked the wrong sense of the word in English.

Or perhaps there is no English word for it. Like schadenfreude e.g. the feeling a porro prism bin user gets looks at a roof prism bin costing twice as much and finds the view not as good as his bin.

[ronh]

Kevin,
I'm sorry to seem so obtuse. Here's how I did it. A 42mm f/4 lens has a focal length of 168mm. A magnification of 8x requires an eyepiece of 1/8 this, or 21mm. When you put your eye up to the eyepiece, you are using the eyepiece to view objects in its focal plane, effectively one focal length away. The eyepiece doesn't "magnify", so much as allowing your eye to effectively be placed only 21mm from the object, an impossibly close distance to focus on without optical aid. So, at 21mm, 40 lines/mm subtends (1/40)/21 radians. A multiplication by 57.3 deg/rad, and another by 60 arcmin/deg, gives 4.1 arcmin.

By the way, I found an MTF for the eye, in angle space, 3/4 way down this page:
visual MTF
This only reinforces my puzzlement. At a spatial frequency of 15 lines/degree, or 4 arcmin, the naked eye is only down to 80% of its maximum contrast senstivity, where the top bino, nay, merely the top prism, in Konrad's paper is down to a much smaller fraction of its maximum.

Ron

[Surveyor]

Ronh;
The optics will not focus at 0 (infinite diopter). If the optic will focus at 1.68 meters then the resolution will be 1/10 your figure, or about 24 arc seconds and proportionally smaller for further distance. I do have some optics that will focus down to about 15 mm from the objective though they change power drastically. As Kevin points out, without the test distance you have no reference. I hope I understand your question correctly.

Best
Ron

[henry link]

Ron H, notice that the spacial frequencies in the MTF charts are in line pairs/mm, not lines/mm. Now, I'll be happy for you guys to thrash out the math, but I don't think it will tell us much since we don't know anything about the objective used in the tests.

[elkcub]

Henry,

I could be mistaken, but I think lines per mm and line pairs per mm mean the same thing. A line pair is a black line and a white space, representing a full spatial frequency cycle.

Ed

[henry link]

Ed,

I'll follow Kevin's lead and resort to Wikipedia. It may be wrong, but it agrees with me, at least until someone edits it. ;-)

http://en.wikipedia.org/wiki/Image_resolution

Henry

[elkcub]

There seems to be some confusion in the ranks.

Also read:

http://madsci.org/posts/archives/200...2204.Mi.r.html

As a basic principle of human behavior, if there are two ways to do something, half the population will do it one way and half the other.

Ed

[elkcub]

Henry,

I'm quoting from the "ultimate" source: Warren J. Smith, Modern Optical Engineering, 2nd ed, pg., 345, the MTF.

"Note that in optical work the convention is to consider a "line" to consist of one light bar and one dark bar, i.e., one cycle. ... To avoid confusion [with TV parlance], "optical" lines are frequently referred to as line pairs."

So, if we're talking optics I still think they mean the same thing regardless of the wicked Wiki. :)

Ed

[Kevin Purcell]

See this for a rather good explanation of MTF (on the same page a link to the CSF image provided by Ronh).

http://www.bobatkins.com/photography.../mtf/mtf1.html

Particularly this graph

http://www.bobatkins.com/photography...l/mtf/mtf2.gif

on page

http://www.bobatkins.com/photography.../mtf/mtf2.html

Real lenses rarely, if ever, come close to the theoretical maximum MTF at apertures below about f8 as stated above [...]

Though the measured numbers for the prisms I think show how much worse life is when looked at through a roof prisms. The MTFs are worse than say a "real f/4" lens that you would expect in the objective.

And low magnifications (and a low resolution detector ... the human eye) let you get away with quite a lot. It seems, given the typical curves for a good f/4 lens shown above, that the roof prism is the worst component in the bin. But as others (like surveyor) have shown (and discussed) elsewhere on the forum (and on Cloudy Nights) the resolution of a good bin is well above that of the human eye at that magnification (that's why you need a booster scope to do a star test with a bin)

So what is the MTF for a typical porro prism set up? What is it for a thick plate?

Some of this is prior art in this patent

http://www.google.com/patents?q=5978144

but I'm not to sure of the scale (see figure 3c): is that really a maximum of 7.00 lp/mm? The prior art device specified is an 8x21 telescope with 7 degree FOV and 13mm ER. So the Swaro prism looks good compared to this!

One thing that is clear from this patent is the MTF falls off as the beam is widened too. That's another parameter not mentioned in the Seil paper (is he measuring on axis or off axis?).

The other thing to note is the graphs presented in the paper commit the cardinal sin of graphical data presentation: they don't use the same axes scales. Essentially all the coated lines would lie over the top of each other (they all hit about 0.45 at 20lp/mm).

For lp/mm it's slightly exactly as Henry says: a black line and a white line is a line pair. So 5 spaced lines (lines per mm) is 10 line pairs per mm.

http://en.wikipedia.org/wiki/Image_resolution

All this reminds me of this thread (one of the others to mention MTF).

http://www.birdforum.net/showthread.php?t=102792

Here endeth the rambling ...

[ronh]

Just a whimper as I go down for a final time...

Ron,
I took 40 line pairs per mm to mean the resolution in the focal plane, suggesting a real image that then requires an eyepiece to convert to visibility. I believe this is consistent with Henry's remark that the objective is a vital part of the system, and for me to speculate very deeply without knowing its MTF is futile.

Henry,
You have actually cleared it up for me, as you see from the above. The basic MTF we see is that of the objective, and it is perturbations in that that are the aim of the paper's study of the effects of prism.

Kevin,
Thanks again for the original link, and the boatload of others, which I intend to read at least part of.

Does one realization, amid mid many more unanswered questions, count as learning? You bet! Just, sort of nonlinear is all. Thanks guys, I am proud to have met such an experienced and pretentious bunch!
Glub...

[NWBirder]

what a bunch of optics maniacs!! Just kidding. After seeing all these discussion, I want to sign up to ASU's optical mechanical master degree to understand everything discussed here. :) it truly turned my hobby into some kind of curiosity to try to learn more about this topic. Thanks, everyone.

[elkcub]

Henry,

I agree with your possible implication about editing the Wikipedia article. I should have pointed out that the first reference at the end of the Swaro paper is Warren J. Smith's book. So I have no doubt that they meant what he meant by lines and line pairs.

Unfortunately, much of Wikipedia is oriented to digital technology, written by sincere people, no doubt, but in this case not vetted in optical theory and its conventions. In my opinion, when used as a primary source of knowledge, Wikipedia can be very misleading and can sometimes do more harm than good. The quality of the articles is quite variable, and as an aggregate doesn't substitute for a good text.

Ed

[Surveyor]

Ronh;
Sounds good to me. I had got stuck on the object/target side of the objective, neglecting the image side. Strange for me to overlook this since my collimator has a USAF 1951 target at the focal plane, duh.

Henry/Ed;
I hope you carry this topic on awhile. I have trouble sometimes figuring out just what format people are using. In most situations, engineers and surveyors use lines per mm (each black bar and white bar are counted) most of the time. I only converted to line pairs/mm when I started viewing the BF. There are a lot of examples that I can bring up but will keep to a minimum for now. Attached is a picture of a typical level/stadia rod, a device that is mimicked for a lot of different fixtures and procedures. Notice that a black bar counts as one unit and a white bar is another unit. Between 10.6’ and 11’ there are 40 divisions, 20 black and 20 white, each being .01’ increments (50 line pairs/foot or 100 lines/foot). These spaces, both black and white, are often further subdivided by 10 by use of a vernier attachment.

I refer to the two standards frequently. Most of my work related figures are what I call detectable resolution. For instance, a lot of our cheaper instruments are only capable of 2.4 arc seconds RMS so I have to be careful that the target should at least be that size, whether it is a plumb bob string (1.5mm @ 100m=3.1” and you can see it a lot further out), a 30 mm range rod @ 2km=3.1” (easily seen at 3km), triangulation lights used at night that are 400mm diameter that we try to stay within 25km of and larger targets from mountain top to mountain top. Over the last 20 or 30 years some lp/mm requirements have come into surveying practice. The point of the above is that we can detect antennas, wires and other objects considerable smaller than Dawes would indicate and measure them but if two are close together; we may not be able to separate them. NGS and USGS, when they require instrument calibration/verification, you report resolving power in lp/mm and the angle split accuracy (the 2.4” above) is the RMS value of a number of direct and reversed observations to a single bar to measure the pointing accuracy. Some agencies still want the USAF measurements in lines/mm.

There are a lot of other applications where single line resolution is needed. A typical IR thermometer or FLIR thermograph with specifications stated in mrad have to be computed to make sure the sensitive area is completely contained on the target face to keep from mixing the background temperatures with the target value. Distance measuring equipment has the same limitations.

So as you can see, as far as I am concerned, there is a difference between lp/mm and l/mm. Until I came to BF the only time I ran into line pairs were in photography or printing applications, with an occasional pixels/mm thrown in to really confuse the issue. Still, the only time I think in lp/mm is here on BF and I try to stay with that convention. I guess the biggest difference in my optics is that most are measuring optics instead of just viewing optics.

I look forward to more clarification and standardization on the subject.

Best to all.
Ron

Also attached a copy of my worksheet for resolution that I have inserted columns for line pairs.