[spitfiretriple]

Prisms are heavy. Why not replace the four prisms in a porro binocular with eight dielectric mirrors?

[oleaf]

I would think alignment would be next to impossible with 8 mirrors.

maybe I'm wrong?

Cheers

[John Dracon]

My one and only experience with mirrors versus prisms in a binocular came about in the early days of ebay. I bid on a 8x30 porro appearing binocular for 20 bucks including postage. The seller was a real BS artist, extolling the pristine condition of the binocular. It was one of those plastic glued together replicas. Looking through it was like looking through the bottom of a Coke bottle. So I took a hacksaw to it to look at the innards .Where prisms normally would have been were four mirrors. It produced a lot of laughs with my optics buddies.

John

[Pinewood]

Hello Spitfiretriple,

As glass has a higher relective index than air, prisms shorten the optical path.

Happy bird watching,
Arthur Pinewood

[spitfiretriple]

Originally Posted by Pinewood

As glass has a higher relective index than air, prisms shorten the optical path.

Hadn't thought of that. I can see that a shorter optical path would make for a more compact binocular. But on the other hand, there is quite a weight difference. Has any manufacturer (excluding John's $20 coke bottle supplier) ever tried to do things differently?

NB I am aware of a couple of binoculars that used concave mirrors rather than lenses, but I am talking about replacing not the lenses but the prisms.

[henry link]

I thought it worked that way too, Arthur. I only recently learned that the refractive index of the prism glass increases the physical distance between the objective lens and its focal point, compared to air, rather than decreasing it (the optical focal length doesn't change). The subject is covered on pages 18-19 of Rutten and van Venrooij, "Telescope Optics". The refractive index of the prism glass is helpful in the overall design because it causes spherical overcorrection that naturally compensates for the undercorrection of spherical aberration in fast binocular objectives. The lenses and prisms are made to work together.

I happen to have a few loose Porro prisms around which I weighed. The Nikon 8x30 E/EII prisms for one barrel weigh 3 oz. (they're two different sizes and the big one is asymmetrical to save weight and decrease size). The prisms from one barrel a Fujinon 16x70 weigh 4 oz., but they're both the same size and symmetrical. They could be lighter if they were done like the Nikon.

Spitfire,

The Leitz 6x24 Amplivid used a combination of mirrors and prisms.

Henry

[Kevin Purcell]

Some more reasons ...

Because porro prisms are cheap to make (really they're quire low tolerance), very rugged (it's the mounts that are fragile), keep themselves in alignment (glass is pretty good at that) and have higher reflectivity (total internal reflection beats even dielectric mirrors).

The production engineering in jigging and getting the eight mirros aligned would be no trivial (and would add weight).

And then the are the other reasons porros have gone out of fashion which would mean that a much more expensive one that is not any brighter would sell how much more?

Asymmetric-sized porros as Henry points out are a good way to save weight. Nice to the that Nikon did that. I know the modular US Army bin did that too (the second porro was tiny).

[Pinewood]

Hello Henry,

If you thought the same as I did, I am not too embarassed.
Would not dielectric mirrors be more expensive than Porro prisms?

Happy bird watching,
Arthur

[spitfiretriple]

Henry: thanks for the Amplivid info; I had never heard of that model. To my eyes it looks closer to a roof bin than a porro bin. I have to say, when I first peeked onto this forum, I never imagined there would be so much to learn about binoculars. For those interested, here's a useful page of info I subsequently found.

Quote from that page: The metallized reflecting surfaces and small aperture also contribute to low image brightness Fair enough, but reflective coatings have come a long way in 50-odd years, and who says we have to limit a mirror bin to a 24mm objective?

@Kevin/Arthur
I recognise an 8-dielectric-mirror bin would be more expensive to make than the average porro bin. But I am not expressing surprise that mirror bins have failed to sweep the world clear of porro bins; Edit: Okay, in my original title this is exactly what I was saying! I am expressing surprise merely that not one manufacturer seems to have tried it. Yes, a pure-mirror bin would definitely be more expensive than a prism bin, but it would surely also be lighter. The world being what it is, there will be someone somewhere prepared to pay a premium for lightness that he couldn't get elsewhere. Actually, there would be quite a few such people - enough to support one small production line in a small factory somewhere?

Henry's Amplivid is an interesting old relic, but even the Amplivid wasn't a pure mirror design. No doubt there's a good technical reason why there isn't (and would appear never to have been) a pure mirror bin- beyond mere manufacturing cost. But I don't think we've yet established that reason(s) "beyond reasonable doubt".

Is it the difference in reflectivity that Kevin mentions? I guessed this might be a factor, but modern dielectrics are pretty "shiny". And if the difference is just a couple of % IN TOTAL, the lightness would still win the day for certain users in certain applications.

Is it simply(?!) the spherical aberration correction that Henry mentions? And Henry says The lenses and prisms are made to work together. Okay, fair enough. Would it not be possible then to select a lens, or series of lenses, that would complement a mirror system?

I'm curious about such things. I'd like to find out for sure.

[elkcub]

Hello Spitfire,

I think you're looking for a convincing reason (or reasons) why dielectric mirrors have not been used to replace standard prisms. Being a skeptic myself I'd have to ask first, is it really provable that a mirror-based system would weigh less? Second, Henry came up with a rationale for the use of prisms that's quite persuasive:

...The refractive index of the prism glass is helpful in the overall design because it causes spherical overcorrection that naturally compensates for the undercorrection of spherical aberration in fast binocular objectives. The lenses and prisms are made to work together.

Could that advantage also be obtained with a mirror system?

Ed

[spitfiretriple]

I admit, I assumed, without evidence, that mirrors would weigh significantly less than prisms. It seems to me to be a reasonable assumption, but I accept it is an assumption. Can anyone shed any light on this?

As regards Henry's erudite (as always!) contribution, whilst I understand what he is saying, I admit I do not understand the physics of how prisms would correct (at least partially) spherical aberration. I have put some thought into a variety of light paths, and can't envisage anything to support the prism-corrects-SA concept. I am probably being dim here, again if someone could explain the concept I'd be grateful.

I've done some researching on reflectivity. Apparently the latest dielectric coatings can achieve 99.9%+(!) reflectivity across the visible spectrum. So even after four reflections, there should be plenty of light. I am therefore inclined to rule out reflectivity/light-transmission as a reason for the non-existence of mirror bins. I don't think cost can be a factor either. As I suggested previously, even if mirror bins were expensive, somebody somewhere would be prepared to pay, in return for lightness (assuming 4 mirrors weigh less than 2 prisms). Which brings me back to Henry's SA-correction. Which I don't understand!

[Nessus]

Has anyone done a side by side with an old Nikon Monarch and the new Dielectyric ones to see how big a difference there really is? Those two bins present a a perfect testing scenario.

[etudiant]

The real question is why have prisms or mirrors at all.
The prisms primary purpose is to lengthen the optical path enough to allow fairly simple lenses to be used. Presumably with sufficient lens making wizardry they could be eliminated entirely.
Given the superb lenses now on offer for cameras, it may be possible to envision a new design binocular with a straight through light path, just like the old Galilean telescopes, but with more acceptable optical performance.
The question then would be whether the weight of the new lenses exceeds the savings from the elimination of the prisms.
Maybe some of the experts on this forum could put out a couple of informed guesstimates on that.

[PatrikIver]

Originally Posted by etudiant

The prisms primary purpose is to lengthen the optical path enough to allow fairly simple lenses to be used.

The prisms primary purpose is to turn the image right side up.
Without the prisms the image would be turned 180 degrees.

Gallilean optics do not need prisms because their eyepieces do the job of the prisms, but at the cost of severe restrictions on field and ER.

I'm sure some real expert will correct me if I'm wrong... :)

[PatrikIver]

About four years ago I started a thread on Cloudynights under the heading "Why prisms and never(?) mirrors?".

Link:
http://www.cloudynights.com/ubbarchi.../o/o/fpart/all

Some ideas and answers can be found in that thread.

[Kevin Purcell]

Patrik is right ... binoculars require an image erector (unless you are an astronomer ) hence the prisms. That prisms extend the optical path is an added bonus but you still need an erection system.

Camera lens design is a very different thing from binocular or telescope design with very different requirements (e.g. a flat field is mandatory). They have more elements so more glass so they're more complex and heavier (especially as the sensor size for a full size SLR is rather bigger than the biggest human eye). And they don't have an eyepiece to deal with just a flat plane.

[etudiant]

Kevin:
Could the needed image erection be achieved as well by another lensing stage instead of a prism?

Are the camera constraints on the camera lenses actually greater than those on binoculars?
If so, should we not see real benefit soon from the gains in camera optics?

[Kevin Purcell]

You can erect an image with a system of lenses but you make the optical system physically much longer (and introduce a few other undesirable features too) e.g. telescopic sights and some folded path spotting scopes use this technique. Henry Link has written about this on BF before.

e.g.

http://en.wikipedia.org/wiki/File:Te..._internals.png

Their different and the solutions are different. I doubt camera lens design is having much impact on bins. If there were any impact you'd expect to see it in spotting scopes but even there simple optics win out.

If we move to "digital binoculars" you might see some impact.

[elkcub]

Originally Posted by PatrikIver

About four years ago I started a thread on Cloudynights under the heading "Why prisms and never(?) mirrors?".

Link:
http://www.cloudynights.com/ubbarchi.../o/o/fpart/all

Some ideas and answers can be found in that thread.

Patrik,

That was a very good thread, yes, indeed. Of course, being a psychologist I got as much insight into the personalities as the optics, maybe more.

Ed

[etudiant]

Kevin,

Thank you very much. That was a very enlightening link.
Plus you are of course right, the technology read across from digital cameras to binoculars to date has been too small to measure.
The emergence of new players such as Zen Ray may have been fostered by the market for low cost precision optics to serve the digital camera industry, but that is a separate issue.

[spitfiretriple]

Originally Posted by PatrikIver

About four years ago I started a thread on Cloudynights under the heading:
"Why prisms and never(?) mirrors?"

Some ideas and answers can be found in that thread.

Excellent! I didn't think I'd be the first person to ask the question!

It was a shame though that the thread went off topic so quickly - even though it continued to be interesting and enlightening. Okay, that's what threads often do, but it was a shame when you had raised what was (to me at least) such an interesting question.

And I'm not sure I'm convinced by many of the answers that were offered to you. To paraphrase:

"Mirrors get tarnished over time"
Well, sure, silver mirrors will, especially if they're in an oxygen-rich environment. So put the mirrors in nitrogen or argon - cheap and easy to do these days. Alternatively, just don't use silver...

"Mirrors aren't reflective enough; too much light would be lost."
Well, sure, aluminium mirrors - fours in a row - would lose too much light. But this is the 21st century - we have dielectric mirrors now, and they are shinier than aluminium. Much shinier.

"Mirrors would be more expensive than prisms".
Yeah. So what? Dielectric is more expensive than aluminium. Doesn't stop the alphas using dielectrics. Hell, doesn't stop Zen-Ray using dielectric.

"Fashions have moved on; people want roofs now".
People want the Next Big Thing. That's fashion for you. But I'm not asking why porros aren't replaced by mirrors overnight. I'm merely asking why there aren't at least some mirror bins on the market. The mirror bins mentioned in the CloudyNights thread, the ones I could track down at least, may have had mirrors somewhere in their construction, but as far as I could see were not the porros-but-with-mirrors that I have been floating. Note: I can't be sure about this, I failed to dig up any "exploded" diagrams, I had to rely on written descriptions which were not as clear as they could have been.

Overall though, I'd argue the anti~ arguments don't stand up to scrutiny.

The only argument I've seen so far that I can't refute is the one raised and discussed by Henry, on this thread and on the CloudyNights thread: Spherical Aberration.

And even Henry (on CloudyNights at least) doesn't rule out mirrors entirely. He merely says that an well-designed optical system will have its components designed and selected to work together. (Correct me please Henry if I am misrepresenting) Replace any one of those components with a different spec component (or remove it altogether), and the system will fall down. Henry illustrates his thinking with his account of his dismantling of a bin, and his removal of the prisms, and the image now being terrible.

Okay, fair enough, replacing a pair of prisms with a quartet of mirrors will unsettle a system. Replace any single component of a bin without changing the other components will almost certainly give a worse view. But who says we can't redesign/reselect the other components of a system the better to work with mirrors instead of prisms?

[spyglass]

I think prisms work pretty well. Witness the Nikon SE's, Swift 820's, Zeiss Oberkochens, to name a few. I guess for the sake of tech talk/opining/discussions/arguments anything can be pondered or proposed......Me? I think if it ain't broke, don't fix it.

[spitfiretriple]

And you would no doubt be content driving a car with a steam engine?
And flying in a plane with reciprocating engines?
And surfing the net at 56k dial-up, while peering into a cathode ray tube?

Etc etc

In technology, there's always room for innovation and improvement. And such innovation and improvement should be explored and exploited.

I agree though, that we aren't going to change the world here, we're just chewing the techno-fat. And it seems there's quite a few of us here who enjoy doing just that!

[Hans Weigum]

Apart from the Amplivid, other once produced binoculars using one or more surface mirrors instead of prisms are:

1. Hensoldt Diarex (mirrors only)

2. Bushnell X-trawide/Bresser SWA /(Kamakura) 7x32 and 10x50 (mirrors and Porro-prisms)

3. MOM 8x40 (roof prisms (?) and mirrors)

[NWBirder]

Leupold's GR compact spotting scopes use two mirrors according to their catalogue.