Some preliminary results with add-on lenses held by hand directly in contact with the front of Pentax Papilio 6.5x21 binoculars.
These binoculars have a minimum focusing distance of 0.5m and are designed for close-focus observation. The objectives approach each other sideways at close focus (only 7 mm distance between the clear inner edges of the objectives), which reduces vignetting with an add-on lens. The outer clear edges of the objectives remain about 52 mm away from each other at closest focus.
Sigma Life-Size Attachment, 52 mm thread diameter (but a clear diameter of only 35 mm):
Binoculars focused at infinity: about 38 mm wide FOV. Difficult to superpose the two images into a single one. Apparently the outside periphery of each image is strongly vignetted by the add-on lens.
Binoculars focused at 0.5m: about 25 mm wide FOV. Much better image, much easier on the eyes.
Raynox M-250 (clear diameter 34 mm):
Binoculars focused at infinity: about 16 mm wide FOV. Same problems as with the preceding lens with binoculars focused at infinity.
Binoculars focused at 0.5m: about 10.5 mm wide FOV. Much better image, much easier on the eyes.
This suggests that, even with these binoculars that have objectives very close to each other, one needs a CMO lens with clear diameter of 52 mm or more with the binoculars focused at closest distance, or at least 60 mm with the binoculars focused at infinity. I believe I have a 2-3 element 58 mm Canon add-on lens which might be worth testing, but unfortunately cannot find it.
Eyepiece w. the smallest magn. for the self-made stereo mic
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