Binocular Collimation Quick and Easy Method without Prism Adjustment

Preamble

I was initially put off from correcting an optical fault in an old pair of Porro prism binoculars because all I had seen or read online involved prism adjustment. Not only did this mean finding the screws, often by peeling back and possibly damaging the casing cover but it also allowed for the possibility of compounding, or even introducing new, errors.

Introduction

Some time ago I bought a second-hand pair of Frank Nipole binoculars from a camera shop. Although I tested them before purchasing it was only after I had started to use them for longer periods that I realised there was a discrepancy between the views within each eyepiece. The presence of the fault was made obvious when the instrument was taken away from the eyes and I experienced a slight double vision. I realised that my eyes were rapidly and subtly adjusting when at the eyepiece but were not so rapid in their adjustment to unaided vision.

By looking through the binoculars at a distant television aerial and closing first one eye and then the other it became apparent that the image from one side of the instrument was perceptibly higher than the other. 

The question was: could this collimation error be corrected?

Collimation Method

Fast forward a few years and I had gained more experience in using optical instruments, specifically for astronomy and had acquired a 200mm telescope and a fine new pair of 7x50 binoculars. The Nipoles were sitting in their case and I realised I should try and collimate them. As previously mentioned, I had come across several 'Youtube' videos showing how to achieve collimation by adjusting the prisms in the device but I still refrained from undertaking this specifically because the screws for adjustment were not readily visible. It was then that I became aware of a book entitled 'Choosing, Using and Repairing Binoculars' by J.W. Seyfried' and I purchase a copy from the UK.

Here is the US Link: 'Choosing, Using and Repairing Binoculars' by J.W. Seyfried'

In the chapter entitled 'Collimation' the author clearly describes the eyepiece views of incorrectly collimated binoculars and the several ways of correcting the problem. As far as I was concerned the singular most important fact about the binocular optics of which I was unaware was that, for the majority of binoculars, the objective lenses (the largest ones at the opposite end to where one looks through the instrument) are mounted in eccentric rings. Thus by loosening the screw mounting on the objective lens cell it is possible to rotate the lens in its eccentric mount and hence move the optical axis of the lens.

This sounded so promising and logical that I made a binocular mount to fit onto my telescope tripod so that once attached the binoculars could be pointed at a suitable distant 'target' and held there whilst adjustments could be made. 

I decided to opt for the 'free hand method' as described in the book. Attaching the binoculars to the tripod which was positioned at the open door of the workshop, I looked through them and selected a view of a distant (approximately 100m) neighbours' television aerial. The view in the eyepieces clearly showed that one image was higher than the other. 

I chose the left-hand side of the binocular to be the one I would adjust so I unscrewed and removed the dew cap from the front of the objective revealing the lens cell. I loosened the cell from the body by unscrewing it a fraction. The cell and mount could be rotated as I looked through the eyepieces. It was very readily apparent that the image quality markedly improved if the cell was rotated between 90° and 180°. So, what I wanted was the objective lens to be in the new position when the cell was tightened into the end of the binoculars. I thus had to slacken off the retaining ring securing the lens in the cell and rotate it through the appropriate angle counter-clockwise and re-tighten it in the new position.

To be able to undo the retaining ring I fabricated a 'spanner' from a piece of stainless steel sheet. The retaining ring had two diametrically opposite slots in its front face i.e. the face of the ring not touching the glass and I needed to cut the spanner so that the edge of the steel could engage into these slots and not come into contact with the surface of the objective lens.

If you can't find the correct gauge of steel then you could invest in an optical spanner set (UK) or for the USA optical spanner set.

I needed to have a mark on the lens to indicate the angular displacement. To do this I stuck a sliver of masking tape onto the lens face. I thought that a lightly adhered piece of this tape should do no damage to the coating on the lens surface. This mark needed to align with a reference point on the body of the binoculars and for this I cut a hole the size of the binocular body out of a piece of white card and pushed the card onto the body just beyond the lens cell. With the cell in the tightened (uncollimated) position I made a pen mark on the card coincident with the tape mark on the lens. A second pen mark was made on the card coincident with the tape mark when the lens was rotated to the collimated position.

I next unscrewed the cell from the body and placed it onto a clean surface. The card with the two reference marks on it was pushed onto the lens cell perimeter with the first mark coincident with the tape mark. The locking ring was slackened off using the home made spanner (photo below) and the objective was rotated to align the tape mark with the second mark on the card. I used an optical glass cleaning cloth to touch the surface of the lens in order to rotate it.

By checking the mark alignments as the locking ring was re-tightened it was easy to check the lens had not rotated any further. The card was then removed and the cell screwed back into the binocular body and tightened. A final optical check confirmed that nothing had shifted in the tightening procedure and that the images in the eyepieces were coincident.

Success!

After all those years of collecting dust the ten minutes or so of time spent in actually correcting the fault seem insignificant. I reckon it took a further half hour to an hour to make the binocular to tripod mount (photo below) and now I've a good second pair of binoculars without having to resort to fiddling about with the prisms.

Observations

I was fortunate that the spanner I made was well away from the front surface of the objective lens but it may need to be curved on its lower edge so as to have a clearance. This obviously depends on the thickness of the retaining ring and the curvature of the front face of the objective.

The arbitrary selection of the left hand side of the binoculars was not a precise one in that although now collimated with respect to each other the axes of the two optical tubes of the binoculars may not be parallel to the hinge axis of the device, but for my requirements I believe this not to be relevant.

...and now, if you'd like to, sit back and watch the film:

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Until next time!

Cheers, Andy
© Andy Colley 2016

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