It occured to me that a pair of zoom eyepieces could compensate for a focal length difference between objectives that results in a magnifcation difference exceeding the 2% Mil standard. The only drawback is that most zooms have a disappointingly narrow field at low powers. But it would work. A better idea would be a pair of very weak, sliding Barlows ahead of the eyepieces that could be adjusted to compensate for the focal length difference. The focal lengths could then be matched, albeit at a slightly higher primary f-ratio, but you could still use low-power, wide-field eyepieces. And it would only have to be set once. It might take some hunting to find such weak negative achromats of the appropriate diameter. Hmmm... On Mon, Feb 18, 2013 at 2:57 PM, Chuck Hards <chuck.hards@gmail.com> wrote:
Thanks Mat. I've had them for a few years, with the idea of building a giant bino. I have the monster prisms needed for the erecting systems/interpupilary adjustment as well. I'm hoping that the focal lengths are close enough to permit the project.
These are the Surplus Shed 127mm dia x 700mm focal length objectives in thermoplastic cells. The US Mil standard for magnification differences between the two halves of a binocular is 2%, above which the stereo effect is lost. The eye can accommodate for differences below 2%.
That would mean that with 25mm eyepieces, for example, yielding 28X with a nominal 700mm focal length, the other objective could be mismatched about +/- 12mm before we exceed the Mil standard. So it's not as bleak as I first thought. But something like 5mm mismatch or less would be almost ideal. The closer the match, the higher I can push the magnification and stay within the Mil spec.