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Turning Prism Tilt Screws is Not ??? Collimating

Started by EdZ, 11/22/2002 06:47AM
Posted 11/22/2002 06:47AM Opening Post
I’m amazed the general user population hasn’t figured this out yet. Binoculars are a more complex optical instrument than a telescope simply because there are two optical paths with more optical surfaces that must by critically aligned.

edited 11-26-02
If images are not aligned, but you are starting with nice round exit pupils, you can tilt the prisms to merge the out-of-collimation images, but you might then be moving the prisms out of alignment.

True collimation is the aligning of all the optical elements along the binocular optical axis. No optical element should be tilted from the optical axis.

I must admit that I own binoculars that needed serious alignment and I achieved making these binoculars usable by turning the prism tilt screws to merge the images.

Tilting binocular prisms can have the dramatic affect of considerably reducing the amount of light through the binocular optics. The shape of the binocular exit pupil image can be observed to directly assess the amount of light being lost. Perfectly round exit pupil images should be observed for a binocular to pass 100% of the light. Binoculars that have tilted prisms will exhibit a pointed oval shape to the exit pupil image indicating a light loss thru the binocular.

Ignore, for sake of this discussion, the sometimes seen squared off edges in the exit pupil image caused by undersized prisms. Unless grossly undersized, this vignetting will not impart as significant a loss of light as might be induced by tilting the prisms.

Precise measurement of exit pupil images with a micrometer is required to calculate the amount of light loss induced by tilted prisms. The results of one example calculation are given here. A pair of 20x80 binoculars that should have a fully round exit pupil of 4mm was adjusted to acquire a merged image. The exit pupil measured still a full 4mm in one dimension, but only 3.2mm in the narrowed dimension. An accurate calculation of the area obscured by the tilted prism proves that nearly 26% of the area is lost.

Simplified A=pi r2, 80mm lenses, if passing 100% of the light would pass 80x80= 6400 sq. mm. of light. Reduced by 26%, the same lenses pass only 6400 x 74% = 4736 sq. mm of light. The square root of 4736 gives the resultant of the equivalent diameter objective lens as 68.8mm. Therefore, 80mm binoculars with tilted prisms resulting in an exit pupil image narrowed by 20% in one dimension are effectively reduced to the equivalent of 69mm binoculars.

If you have tilted the prisms in only one barrel of your binoculars and they exhibit these symptoms, try using the same eye to compare the images thru both barrels of your binoculars. The light loss should be readily apparent.

I have written more on this subject and I would encourage discussion. You may refer to this article for more information. edz
Posted 11/22/2002 11:33AM #1
Ed, I just wanted to tell you how much I have learned from your posts on this subject, particularly your Cloudynights article. Thank you for sharing this important information.

You may have noticed from recent high-end bino threads that I have been researching the very expensive (for me) Kowa 82mm and Vixen 125mm interchangeable EP binoculars, as well as others. My guess is that 45deg viewing adds still more complexity to an already complex light path.

Now you have added a new element to consider. Do you (or anyone else) know if either the Kowa's or Vixen's have adjustable prisms or objective cells, or would even need it for that matter? I have also considered the Astromeccanicas, which have easily adjustable diagonal mirrors. But, as you say, that's not really collimation, is it?

I tend to favor grab-and-go viewing over lengthy setups and adjustments, and would not want binoculars that were any more difficult to use than my telescope.

Thanks and clear skies, Milt
Posted 03/24/2003 11:10PM #2
Welll... I've got an old pair of 7x50s that seem very very clear but... it "strains" my eyes to use them for any extended period of time. At first it feel strange. If I concentrate, my eyes will focus better, but when I look away it feels strange again. Seemed to me like they are "out of alignment."

I do not see any cat's eye shaped light at the exit...

Alternately closing eyes shows me a slight difference left to right, but it's the exact same behavior with my new Orions which give me no eye strain even after hours.

I do not see any prism adjustment screws (but at this point I dont assume they would help).

The binocs are heavy weight, and move very smoothly in all other respects.

What's written on them is "Le Gran Model No. 129 Fully Coated Optics No 0480 7x50 Field of View 400ft at 1000 yds"

It's kind of a bummer, because they are more clear and have better contrast than many other binocs I've played with, but the strain is no fun at all after a while.

Any advice out there..?