elkcub
Silicon Valley, California
Sorry Ed, I still don't understand why the addition of field curvature by the binocular would be beneficial. The afocal light that falls on the eye directly from the natural world is inherently free of field curvature. Isn't it desirable for the afocal light that falls on the eye from the binocular eyepiece to be just like the direct afocal light except for magnification?
Henry
Hi Henry,
I don't think it works quite that way. Light that enters the eye directly from the natural world is formed into a retinal image by the eye's optics (cornea and lens), which constitute a focal system. The eye produces all the usual aberrations including non-zero field curvature.
When an afocal telescope if placed in front to the eye, the optics of the telescope couple with those of the eye, and the combined telescope+eye system is focal. Image field curvature varies with the optical design, but it isn't eliminated. Field flatteners added to the telescope reduce the curvature of the entire system, which I believe is why some people perceive "unnatural" images. All of this is not to say that flatteners don't serve a useful purpose for people willing to overlook these unnatural qualities.
The same reasoning was used to analyze depth of field, incidentally, where it was shown that the coupled telescope + eye system simply reduces the DOF of the eye by M^2.
Hope that explains my point of view.
Ed