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Converting a 10x50 binocular to an 8x50

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#1 brocknroller

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Posted 24 June 2006 - 10:46 AM

The paucity of high quality, wide field 8x50 PORRO prism binoculars makes me wonder if it's possible to make my own (or have an optics expert make one for me)?

If I had two binoculars from the same series (8x40 and 10x50) and could switch the EPs (i.e., they had the same occular size and threads or were adaptable), could I convert the 10x50 model to an 8x50 by switching the EPs?

I know longer FL increases the magnification of a given power EP, so if the 10x50s were longer, the magnification would be slightly larger than 8x, perhaps 9X, but if the FLs were the same, and at least according to the specs, some 8x and 10x models in the same series have the same height, the power should stay the same.

Would the FOV stay the same with the transplanted EP? That is, would a 10x50 with 5* FOV with an 8x (6.5*) EP show a 6.5* TFOV?

Any possible "side effects" in doing this "power change operation"? Vignetting at the field edge? Distortions?

If this turns out to be more complicated than I can do myself, are there any optics repair shops that could make this modification for me?

Thanks!

#2 EdZ

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Posted 24 June 2006 - 10:49 AM

If you had two binoculars from the same series (8x40 and 10x50) and could switch the EPs (i.e., they had the same occular size and threads), could you convert the 10x50 model to an 8x50 by switching the EPs or would the different focal lengths change the power and produce a 9x50 instead of an 8x50? If the FLs WERE the same, would the modification make an 8x50?

Would the FOV stay the same with the transplanted EP? That is, would a 10x50 with 5* FOV with an 8x (6.5*) EP show a 6.5* TFOV?



The focal length of the objectives would generally be different. Therefore, switching eyepieces, as you suggest, will NOT give the desired result.

If the 8x40 and the 10x50 were nearly identical in physical size, you'd start out with a greater chance that the objectives are very close to the same focal length. But that is usually not the case. Generally, binoculars are much closer in f# than in focal length.

The Nikon AE 10x50 and 12x50 are the same physical size, have the same objective focal length, and use diifferent focal length eyepieces to get 10x and 12x. Switching eyepieces would simply switch which one is a 10x50 and which is a 12x50.

However, the Nikon AE 10x50 and the 8x40 are not the same physical size (the 8x40 is shorter), they have significantly different objective focal lengths, potentially the same f#, and may use exactly the same eyepieces to get 8x and 10x. Switching the eyepieces between the 8x and the 10x would not change anything. All three use eyepieces with almost identical Afov.

The Fujinon 16x70 and 10x70 have the same objective focal length and because both are 70mm also the same f#, and use different eyepieces to get 16x and 10x. Switching eyepiecces would simply swap powers and really change nothing.

I would predict, for example using the Nikon AE series, that switching the eyepieces between the 8x40 and 10x50 would result in nearly identical magnification. I suspect both use approximately the same focal lenth eyepiece, approx 20mm. However, if you switch the 8x40 and the 12x50, which probably have eyepieces of 20mm and 16.67mm respectively, you could get approx 9.5x40 and a 10x50, different but not what you are looking for.

The eyepiece Afov will not change, so the Tfov you get out of it is simply Afov/mag.

If the 8x40 and the 10x50 were identical in physical size (how tall they are), you'd start out with a greater chance that the objectives are the same focal length and the eyepieces are different. If that were the case, the eyepieces from the 8x40 binocular would have a longer focal length and would produce a lower power in the 50mm binocular, giving you your desire result. But, I can't think of any instance where this would hold true.

edz

#3 patter1

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Posted 24 June 2006 - 08:15 PM

Brock, I sort of did that, but with 1.25" telescope eyepieces; I took an old 20x50, removed the bridge and eyepieces and inserted a 25mm plossl on one side. I was lucky; it fit perectly. If I did it on both sides, I'd have a sharp 8x50 binoc of around 6.5 degree FOV, and long eye relief, but fixed focus. (To make it work with the focuser bridge wouln't have been worth the effort or even possible). I didn't look specifically for it, but I didn't notice any vignetting.

So yes it's optically possible, but mechanically? If it's too much work or expense, it raises the question of whehter or not it's worth it.

#4 Bob W6PU

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Posted 25 June 2006 - 02:44 PM

Brock, your post made me remember the USN Bausch and Lomb 7x50 to *9x50 conversion.

Apparently, this conversion was officially authorized, and the finshed work was stamped 9x50( this may have been 9x60?)

It seems that these were presented to high ranking naval officers. I'm thinking that there may also have been a USN 7x50 to a * 9x60 Mod, but my memory fails me on that one...perhaps this was the actual conversion that I remember seeing?

I would enjoy reading some feedback on these conversions, and just how common they were?

Bob in NM

#5 Henry Link

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Posted 26 June 2006 - 07:26 AM

The Peter Abrahams web page lists a 9x63 Mk 37 from 1944 which it describes as a Mk13 7x50 with "modified objective".

I have an Edmund Scientific Co. catalogue from about 1960. In those days Edmund still sold WWII US military binoculars and parts. They sold a 9x50 ($60.50) which was made by removing the objective from a 6x30 and screwing on a sawn off objective barrel from a 7x50. I'm not sure this was ever official military issue. I think it may have been cobbled together by Edmund from its parts stock.

#6 Bob W6PU

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Posted 26 June 2006 - 12:57 PM

Thank's for the Info. on the Mk 37 binocular, that is what I must have been thinking of. I remember seeing it for sale, at an auction, years ago, but had forgotten the details!

Now that you provided us with the Mk 37 I.D, I will do a Google search to try and learn more!

I understand that Bausch and Lomb had a display in their Rochester NY factory, of their limited production binoculars.

There were some very unusual and rare specimens on exhibit! I wonder what has become of these, now that the Bausch and Lomb factory no longer exists?

Upon first recieving my Hensoldt Zeiss 7x50, I did a very carefull visual comparison between those, and my USN 7x50.

The B&L 7x50 had the sharpest image! It is also taller than the Zeiss made 7x50, so it probably uses a longer focal length objective!

My feeling are that the WW2 B&L binocular was better built and sealed than was my Mil Spec. Zeiss built 7x50!

What a shame it is that Bausch and Lomb no longer manufactures their binoculars and telescopes, the company was an American treasure!

Bob in NM


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