I once bought a telescope (bargain clearance), and it was practically useless because you could only see an image if you held your eye in exactly the right position. I mean breathing was enough to put your eye out of alignment with the view. And when you could find the view, the field of view was very tiny, regardless of how magnified the image within that field was. What features (focal length /eyepiece ratio?) cause this, and how do I avoid it in the future?
Binocular separation has very little to do with that image flattening. Actually, we’d better be careful here, because “image flattening” has a whole nother meaning–that the edges of the image are not distorted or out of focus.
Lumpy, you are talking about the apparent field of view (i.e. how big the view appears to be, or the size of the magnified view), right? That is mostly determined by the eyepiece. It’s easy to make an eyepiece that produces a good image at the center of the field. But an eyepice that produces a sharp image to the edge of a wide field is difficult and expensive - TeleVue makes very wide field eyepieces, but their top models cost around $400, I believe. And there are surely much higher priced ones for special purposes.
As for binoculars and spotting telescopes (with non-interchangable eyepieces), the field of view should be stated in the catalogue, if not on the binoculars themselves. Sometimes they give the true field of view; you can multiply this with the magnification to find the apparent field of view.
About the flattening effect - I think what we (well, some of us) are talking about is perspective. Say you look down a short length of pipe. If you’re close to the pipe, the far end of the pipe looks much smaller than the closer end, so it looks like a fat ring. If you look at it from farther away, the closer end isn’t very much larger than the far end, so it looks like a thinner ring. And a smaller ring, fo course, but the diameter-to-thickness ratio changes as well, so even if you look through a telescope, it’ll be a different shape ring. If you look at it from infinitely far away with an infinitely high magnification, it will just be a circle.
>> Binocular separation has very little to do with that image flattening
No sir. I respectfully disagree. Build some binoculars and make the objective lenses be (say) 30" apart and you will get the stereo depth perception as if the object you are looking at is ten times closer (Which may still be not that much if it is very far away or may be a lot if it is closer). Make them 300" apart and it’s 100 times closer.
This reminds me of the optical telemeters or rangefinders used for airplanes during WWII. The operator looked into what looked like two periscopes on their sides so that his stereo vision was augmented to about 3’ wide. He looked at the incoming bombers and turned a knob until both images aligned and he would then read the distance. Have several stations do this repeatedly and by triangulation you get the bombers’altiture, course, etc. I have seen these used in documentaries and had an old veteran explain to me how he used them. It was fascinating.
Then the antiaircraft guns would make a curtain of explosions in their path. They were not aiming at anything in particular, just set off a lot of fireworks in the airspace the bombers would cross.
With this primitive technology I’ve always wondered why they didn’t wait to have a war later on with more effective technology. The bombers’ navigation equipment was so slow and primitive they pretty much had to fly in stable straight lines if they wanted to hit anything. And all the AA guns could do is set up explosions in their path with the vague hope of scaring them with the slight chance they may get hit.
But getting back to building binoculars with 3’ separation. You say you can’t do it. No problemo. I have seen this experiment done: Take two photographs moving the camera the required distance sideways and then view them through a stereoviewer. This can be done quite easily if you are photographing immovable objects but it will not work with your toddler.
Make the photos far apart enough and you can get good stereo effect at great distances. In the Air and Space museum, here in DC, you can see aerial photos shot succesively from an airplane for mapping purposes and you can look at them through a stereo viewer and the effect is amazing.
Say you get very good depth perception when the distance to the object is 5 times the separation of your eyes. Then, all you have to do to get the same effect is to have your binocular lenses or your photographs separated by five times the distance to the object. If you are photographing a mountain from a mile up your photographs would be separated by a fifth of a mile.
I have seen some interesting stuff done along these lines. You can take two cheap cameras and mount them on a common base and take simultaneous photos and they make good stereovisions.
getting back to the original question of whether making an object ten times bigger is the same as bringing it ten times closer the answer is no. It would be equivalent for a monocular telescope with zero aperture but not otherwise. To make an object appear ten times closer you would have to widen your stereo angle by a factor of ten. In that case yes, the object appears ten times closer.