Kern Focalpin (1 Viewer)

[LPT]

I recently acquired a Kern Focalpin 10X60 in excellent condition.There was some haze on the focusing lens and a little on the prisms which cleaned off easily, and all optical surfaces including coatings are now clear and in almost unblemished condition (except a few old rub marks on the outside of one objective lens). Focusing is smooth and build quality is very high and also very interesting. However, optical performance is disappointing because of a grey/green sort of coloring to the view. Initially I though this was because the binocular had too many light reflecting air/glass surfaces (air spaced objectives and a third internal objective lens for internal focusing) and not adequate type AR coatings to properly transmit light. Although I still think this is in part correct, I can't believe that a company like Kern would have marketed a binocular with a view like this. The contemporary Hensoldt Dialyt 10X50 and CZJ Jenoptem 10X50 blow it out of the water! I now suspect the problem may have to do with an unusual covering on one of the sloping sides of each of the ocular prisms. The coating is white and at first looks like paint but upon closer inspection looks like an adhesive material. Here is a picture:
http://www.flickr.com/photos/binocwpg/6448260983/in/photostream
Does anybody know what the purpose of this covering is? It is certainly not a baffle. My wild guess is that for some reason the prism is silvered on the coated side (why would a Porro I prism be silvered??), and the silver has tarnished affecting the view. Any ideas?
Here are some more pictures of the binocular and internals:
http://www.flickr.com/photos/binocwpg/6448282533/in/photostream/
http://www.flickr.com/photos/binocwpg/6448277103/in/photostream/
http://www.flickr.com/photos/binocwpg/6448268527/in/photostream/

 

[ronh]

LPT,
Thanks for your report and photographs of this unusual binocular.

Henry Link has one, and he too has reported disappointment. Interesting apparently does not always equal good. He'll probably belly up to the forum in a day or two. You could do a search for his comments, in the meantime.
Ron

 

[mercedes_sl1970]

Hi - thanks for reporting on the Kerns. I had the 7x50 sibling of your binocular which I kept for several years (picked up locally secondhand). It was a pleasure to use, light, beautifully balanced and obviously very well made. However, the views just could not compare to modern binoculars with up to date coatings.

Andrew

 

[henry link]

LPT,

As Ron said, I ran into a NOS pair of 7x50 Focalpins a few years ago. They look like they've never been out of the box.

I think most if not all of the green tint you notice comes from the coatings. I photographed my Focalpin along with a Kern 8x30 from 1980 below. You can see some very different ideas about how to tune the single layer coatings. The bandwidths of the coatings are probably about the same but the Focalpins appear to be tuned for a transmission maximum at a higher wavelength than the 8x30. The image through the 8x30 looks very warm and the Focalpin looks yellowish green to me. I think you can pick up on the difference in the color of the white paper in the photo as viewed through the right sides of the binoculars. Could be the tuning of the Focalpin was intended to place the transmission maximum closer to the maximum sensitivity of the dark adapted eye. If I compare the Focalpin to a Leitz 7x50 Marseptit with a similar green/yellow color bias the Focalpin image looks about as bright, but, of course, neither compares favorably to a multi-coated binocular.

I'm not too keen to open up my pristine pair to see if they have the same odd prism coating as yours. The only possible reason I can imagine for silvering the prisms would be if the objective focal ratio was so low that its light cone edges fell below the critical angle for TIR, even in Bak4 glass. But then it would seem that all the prism reflection surfaces would need it. I've seen a photo of prism tarnish in an old Trinovid taken from the objective end. It looked like a mottled pattern of brown stains. If you see a clear view I doubt that you have tarnish.

Henry

 

[James Bean]

I've had a Kern 8x30 Pizar for some years now, and the optics are quite good, but nothing to get excited about, especially compared with Nikon SE or even B&L Customs, but I have to say my Kern is beautifully made and oozes quality. My Pizar is exquisitely finished, more like a Swiss watch than a binocular, particularly as to cosmetic appearance, its black paint looking more like stove enamel, and its leather case is a work of art and so finely stitched. As a 'user' binocular it's just okay, but as a 'collectable' it's one of my prized possessions...

 

[ronh]

LPT,
An internally focusing Porro is still pretty cool, a good idea that can't seem to quite make it off the ground, though a few have been attempted in the last decade.

It appears that one of the actively reflecting surfaces has the white coating. That is very perplexing. The smattering of optics I know describes how internal reflection works in glass with a refractive index about 1.5 or 1.6, when the material on the other side of the boundary is air, index 1.0. The index of the outside stuff is crucial to the performance. What the heck is the index of opaque white adhesive? Sheesh, it looks like a mistake.

Perhaps some talented experimenter in possession of a Porro component (ahem!) will kindly dab some liquid "white out" or something similar on a surface and see if the performance is affected.
Ron

 

[LPT]

Hello Ron,
I don't think the prism surface is white because it looks like the adhesive is on top of some some sort of coating on the prism. Look closely at the picture and you'll notice a very thin border at the edge of the adhesive at the prism top looking as though there's a coating underneath. I've never examined a silvered prism but I wouldn't be surprised if the silvering were covered by a protective layer. I think Henry's comments about the prism being silvered to accommodate focal length could be an explanation. The build quality of this binocular is exceptional and the makers must have had a good reason for doing what they did. I think Henry may also be correct when he says the AR coatings could have been intended to give maximum light transmission for the dark adapted eye. Yesterday I used the binocular under low light conditions and found its performance to be better than its daylight view led me to expect.

 

[ronh]

LPT,
White protection over silver is likely correct, if the binocular is wide fielded and/or the prisms are BK7. It would then become a way of reflecting even the rays that would be lost in normal internal reflection.

But it looks like if that was the case, then every reflecting surface would need the same treatment, since all the incident angles are maintained throughout the whole prism cluster.

Anyhow, it's a beauty.
Ron

 

[Renze de Vries]

Hello all,

This is a very welcome thread, as it addresses the same problems I had (have) with my Focalpin 7. Yes, it’s an extremely well built and technically interesting, even historically groundbreaking design, but the view is disappointing. Like you I don’t know exactly why, so all I can do is add the information I have.

Firstly, allow me to make a remark on the production years of the Focalpin range. Usually the Focalpins are dated to the 1970’s or ‘80’s, probably because Kern’s serial numbers seems to suggest so. However, their system of serial numbers is confusing as they seem to use different sets of serial numbers for different models. To this day I haven’t been able to successfully decipher the system, but what I do know is that the Focalpins were introduced to the market as early as 1950 (at the Foire Suisse de Bale, the international trade show at Basel, on April 15-25). As the design, at least to the best of my knowledge, was never updated or modified, I think it’s reasonable to date the Focalpin years of manufacturing back to the 1950’s.

When I bought my Focalpin 7 (used) I saw the view was very sharp at the center but compromised by a notable lack of clarity which I thought could be caused by dirty lenses and prisms, so I decided to have the binocular professionally serviced. It was done by Simone Hauptmann (OPTIXXX) in Arnstadt, Germany, who appeared quite interested in the binoculars’ construction. If I remember correctly she was - also - very much intrigued by the prism covering, however unable to come up with an explanation. Also, she sent me photographs of the work in progress, showing the binoculars’ internal parts.
Unfortunately her good work was largely lost on the Focalpin. Even with the glasswork cleaned, its overall view remained somewhat dull, greyish, lacking the transparency and sparkle one would expect from a 7x50.
Now, when LPT speaks about ‘a grey/green sort of coloring to the view’, and Andrew (in a 2008 thread on Cloudy Nights) mentions ‘a fair bit of flare and greyness’ I seem to understand what they are talking about. And to me this is not exactly what Henry is looking after: instead of a subtle difference in color tone caused by coating tuning it looks like a more serious flaw in light transmission and/or flare treatment to me.

Could the Focalpin possibly have been intended for use in low light, where ‘greying’ of the image is less critical? I don’t think so, as Kern in an old ad clearly points to daytime marine use (yachting). Looking at the design my impression is that Kern must have been especially concerned with weight and dimensions (and of course waterproofness). Although they certainly were successful in that, there’s a possibility the optical quality suffered. Note that the specified field of view of the Focalpin is disappointing already (6.6 deg. for the 7x50, 5.7 deg. for the 10x60) while in practice only part of it is usable because of a considerable amount of astigmatism toward the edge. So, a smallish view and a bit grey as well, that’s not too good for a binocular with a top tier price tag. I wonder what Kern did wrong, optically. Smallish prisms maybe?

Renze

 

[Renze de Vries]

Here's the ad I was referring to

renze

 

[Swissboy]

How is this thing focussed?

Interesting thread as the Kern Focalpin was very high on my desired binoculars list when I was a teenager. Of course, it was way beyond my financial possibilities. But it was the model both my brother (and fellow birder) and I kept dreaming about. Looks like this was money well saved, however.

But I'd still love to know how the internal focus works on this porro. The posted pictures don't seem to give any hints. Or else I can't recognize them.

 

[henry link]

Renze,

Thanks for the information and the fascinating photos.

I have another guess about the mysterious prism coating. Look at the internal view below. Notice that the ray trace of the reflection from the coated prism face is not horizontal as you would expect. It looks like this first prism is not a normal Porro with the reflection surfaces at 45 degree angles to the entrance and exit surfaces. The first reflection appears to be a little less than 90 degrees (below the critical angle for TIR) and the second one a little more than 90 degrees. I'm not sure why such a thing would be done unless it is related to space saving with an asymmetrical prism. That would explain the need for silvering the first reflection only.

Robert, between the internal view and the parts photos you can probably figure out how the long pins (the focalpins) act as levers to move the internal focusing lens.

As a footnote I included a photo of my 7x50 pair with all the stuff that came with it, including the superb leather case. I notice the same sailboat on my box and the ad posted by Renze. Do you know the date of the ad, Renze?

Henry

 

[henry link]

Another thing just occurred to me. Maybe the disappointment in the light transmission is because the prism mirror coating is aluminum rather than silver.

 

[LPT]

Another thing just occurred to me. Maybe the disappointment in the light transmission is because the prism mirror coating is aluminum rather than silver.

Very possibly. And this binocular also has a high number of air/glass surfaces (air-spaced objectives, internal focusing lens, prism faces, four(?)element oculars) which adds up to a lot of reduction in light transmission.

 

[Renze de Vries]

But I'd still love to know how the internal focus works on this porro. The posted pictures don't seem to give any hints. Or else I can't recognize them.

Robert,

Have a look at the pics of yet another Focalpin, the 6x40. Taken from page 86 of Hans T. Seeger's Feldstecher, Ferngläser im Wandel der Zeit. The internal view is a bit sharper than Henry's 7x50 picture and while the focus wheel of the 6 is located between the oculars (contrary to the 7 and 10) the focussing works the same: axial movement of a spindle in the central shaft, levers in ball bearing moving the focussing lens to and fro.

Renze

 

[Renze de Vries]

I have another guess about the mysterious prism coating. Look at the internal view below. Notice that the ray trace of the reflection from the coated prism face is not horizontal as you would expect. It looks like this first prism is not a normal Porro with the reflection surfaces at 45 degree angles to the entrance and exit surfaces. The first reflection appears to be a little less than 90 degrees (below the critical angle for TIR) and the second one a little more than 90 degrees. I'm not sure why such a thing would be done unless it is related to space saving with an asymmetrical prism. That would explain the need for silvering the first reflection only.

Ah, very intriguing suggestion! I'm almost prepared to take my Focalpin apart and check (but I won't, at least not yet). Suppose you're right, would this be a novelty, an asymmetrical prism like that? Also, take a look at my (Seeger's) picture of the Focalpin 6. Here the light ray in the prism is horizontal again. While the body design of the 6 and 7 is the same. Reason for doubt, it seems to me.

Your suggestion of aluminium coating: yeah, why not.

No, unfortunately I don't have a date for the sailboat-ad. I took it off eBay, from a seller offering a Focalpin 7 probably. As there's a page number visible, I guess it's from a brochure.

Renze

 

[henry link]

Thanks once again, Renze. This thread is turning into a treasure trove of Focalpin information.

I can see in the 6x40 cutaway that the focuser is a doublet, something that I could never make out for certain in my tiny illustration. I also notice that the 6x40 objective is a cemented doublet instead of the air-spaced tele objective used in the 7x50.

Of course it's not a novelty for Porros to be asymmetrical so that the second reflection face is smaller than the first, but I've never heard of one with asymmetrical reflection angles. It makes no sense to me since it takes away the advantage of TIR at each face, but it does neatly explain why there appears to be a mirror coating on one face of this prism.

Henry

 

[ronh]

The funny angles look like drawings of 90 degree angles in 3-D perspective to me. The reader is not looking perpendicularly at the vertex of the angle, but down on the binocular, so the viewed angle is different from the actual angle. The path drawn through the second prism also is not horizontal to the page, consistent with this.

If it was mine I'd scrape that stuff off a there!
Ron

 

[henry link]

Ron,

Yes, I had always thought that too, until this coating question came up. Everything else in the drawing is not in 3-D perspective, but I suppose the ray trace might have been added later and is just inconsistent with the rest.

Henry

 

[ronh]

Henry,
I am referring to the right hand picture that Renze gave us. The whole picture is in a consistent perspective, but, curiously, shows the first angle, in the page, the same as in your first picture. Nothing in that picture looks like non-90deg prism angles. I think there may be just a screwed up drawing, yours namely, not much case for anything so unusual.

Anyhow, you were supposed to get mad. I did recommend a massive invasion of a rare collector's item, just to satisfy curiosity, you know! The next owner might get suspicious if the view wasn't famously dull. I'll keep trying...
Ron