Hi Bill,
Sorry, but I have to disagree with you on two counts.
Firstly, I think the method you describe is not very precise, and secondly the distance of the object you use to set the diopter is absolutely immaterial.
When we view an object with a telescope or binocular we set the focus so that its image appears at or near infinity, i.e. the image formed by the objective is in or near the focal plane of the ocular. The myopic would, without glasses, place this image slightly inside the focal plane so that rays emanating from the eyepiece would diverge. Conversely, the far sighted would place the image outside the focal plane so that rays from the eyepiece would converge. The actual distance of the object from the binocular doesn't matter because its focussed image is always in the same place.
To verify this concept I have just performed an experiment under fairly critical conditions. I think I am even your senior (see my nick :-() and have very little accommodation in my right eye and none in my left after cataract surgery. I used a 10x binocular (a 10x42 Swarovski SV), which has a shallow depth of field on a tripod.
Swarovski roof prism binoculars have a very stable diopter setting due to the spring-loading of the focussing elements.
With the right objective covered, I focussed on the branches of a tree 115 m distant and then retracted the focussing knob to disengage the left barrel and displaced it to an arbitrary setting before adjusting the diopter with the left objective covered. The diopter was on zero and I repeated the the procedure indoors at a distance of 2 m to end up at zero again. This was with my glasses on and a second test without them resulted in +2 diopters at 115 m and 2 m.
Old eyes are almost always a handicap but in this case the lack of accommodation aided the repeatability. I wouldn't necessarily suggest that younger observers go to these lengths, merely that they choose an object with fine detail at any distance to set the diopter.
John
PS:- I hope you soon recover your original dexterity.
Hi, John,
I sure wish others wanting to dispute something I have said, or take me to task, could do so as courteously.
Sorry, but I have to disagree with you on two counts.
— Firstly, I think the method you describe is not very precise ...
The brain does not work with the mathematical precision that can only be replicated on a computer screen. Thus, the “precision” is only as accurate and relevant as the individual OBSERVER’S ABILITY to control his or her focus by staring. This is an individual matter. And, what could be more precise for an observer than his or her own focus?
The screenshot attached is the spot diagram of a Houghton telescope I designed 18 years ago. On axis the visual wavelengths look like a pea laying in the center of a plate, that plate representing the Airy Disk. Even at the edge of a 1.5-degree field, the visible wavelengths are well within the disk. I performed this operation just to see what I could do.
Science and medicine have shown that a star image (or other pinpoint) that falls within the Airy Disk can’t be recognized from “perfect” by the eye/brain companionship. Pretty good designing for a little kid ... huh? My Image didn’t just fall within it; it fell within a FRACTION of it. * That was calculated BB stacking on my part. So, how much computer-generated precision can the eye/brain companionship really resolve?
— and secondly the distance of the object you use to set the diopter is absolutely immaterial.
As you say, “sorry.” Your comment is as intuitive as it is wrong.
With both eyes open, an observer will be trying to focus one telescope with background stimuli from the other telescope getting involved, and this could be from efforts at spatial accommodation, no matter how slight. The greater this distance the less of a factor this will be, and it is the misleading effects of background stimuli that neurologist and Visual Science professor, Dr. Edward Adelson, of MIT has worked so hard at showing that what we THINK we see ... we often don’t. This, of course, would not be an issue if using the "cover-the-objective" techniques. But, remember, I'm trying to teach people to stare, which is more useful in the long run.
I hope you understand my rationale. If you don’t, please let us agree to disagree, agreeably.
PS Thank you for your well-wishes. I offer you the same! When I was in my 20s, I could have sworn getting older took a lot longer. ‘Guess not. The artifacts from my stroke are getting worse. I fell in the garden twice last week while trying to get as many tomatoes in as I could before the frost.
About 10 years ago, I decided I was going to live forever and ... SO FAR, SO GOOD! :cat:
Cheers,
Bill