There's been some discussion of field correction choices lately. I don't mean to run it into the ground, but while folks are thinking about it seems like a good time to raise my question.
I use a Fujinon FMT-SX 7x50, mostly for astronomy. The F stands for "flat field", according to Fujinon. It shows almost no distortion of long straight lines, like telephone poles, near the edge of the field. It also exhibits the "rolling globe" effect when panning in the daytime. As Henry Link explained, this is like the perspective seen looking down on a flat checkerboard. So, it makes sense to call the field flat.
But, the binocular exhibits another effect, related to the sharpness at the edge of the field. My eyes have very little focus accomodation, and I find that if I focus an object in the center, and move it to the edge, it appears blurred. But, it can be made sharp again, by refocusing to a more distant setting, different by 2 diopters, read off the eyepiece scales. This makes sense in light of the above checkerboard, since the edge of the flat checkerboard is farther away than the center.
Now, I don't really have any problem accepting the reality of this situation, but I don't know what to call it. Over on the CN forum, this kind of edge blur that can be focused away, unlike astigmatism or coma, is called "field curvature".
So it appears that the binocular could be said to simultaneously possess a field that is flat, and curved. Of course that is nonsense, but it points out a semantic question. What are these things properly called, so we can communicate clearly when talking about this stuff?
Thanks, Ron
The F apparently stands for "sorta flat for older people"
quote:
But, the binocular exhibits another effect, related to the sharpness at the edge of the field. My eyes have very little focus accomodation, and I find that if I focus an object in the center, and move it to the edge, it appears blurred. But, it can be made sharp again, by refocusing to a more distant setting, different by 2 diopters, read off the eyepiece scales. This makes sense in light of the above checkerboard, since the edge of the flat checkerboard is farther away than the center.