Today I had a rare eureka moment. The subject was off-axis aberrations, which I've been mulling over recently as I read the review of the Zeiss 10x42 FL by James Holdsworth here and followed a thread about the Nikon 18x70 at Cloudy Nights (a place where I find myself in some kind of limbo. For some reason I can't log-in, change my password or start a new account).
I know from my own experience that evaluating off-axis aberrations is harder than it seems at first. That's because "fuzziness" away from the field center is a brew of different interacting characteristics that can change dramatically at different light levels and pupil positions. Essentially three things act together: field curvature, astigmatism and vignetting and the combination results in some highly variable subjective observations from different viewers about the same binocular.
Today I realized there is a simple backyard test that can reveal exactly what is happening at at any point in the FOV for a particular binocular, observer, pupil position and light level. All that's needed is an out-of-focus star point. For a daylight test focus the binocular at infinity and place the a glitter point of the sun (tiny reflection from a shiny curved object) close enough to see an out of focus disk in the center of the field using one eye only. Now move the disk toward the edge of the field and notice how it changes.
In a perfect optic you would see no change in the disc anywhere in the FOV.
Field curvature alone will cause the disk to shrink as it approaches the edge.
Astigmatism alone will cause the disk to stretch into an oval shape with the long axis pointing from the center to the edge (you may see an oval in the center which indicates astigmatism in the binocular or your eye. Rotate the binocular. If the astigmatism doesn't rotate it's in your eye).
Vignetting alone will cause the disk to form a cat's eye shape toward the field edge with the long axis parallel to field edge and at the very edge the cat's eye will collapse into a arced line that follows the field edge.
In many binoculars you'll see all of this happen at once, but in some field curvature or astigmatism will dominate. You'll also notice that vignetting is highly dependent on pupil positioning. It's maximized if you move you eye and pupil in the direction of the disk at the edge and minimized if you move your eye in the opposite direction and then sight across the center toward the disk on the opposite side of the field. Maximizing vignetting actually improves the appearance of edge sharpness. In my case I find my natural eye movements cause vignetting to be most pronounced at the 9 and 3 o'clock positions at the edge and least at the 6 o'clock position.
I've used this technique before mainly to evaluate vignetting, but only today did it occur to me that it works to make sense of all the off-axis aberrations. I should add that this method won't tell you how "sharp" the edge of a binocular will look to you, but it will explain why you see whatever you do see.
Henry,
the point is well taken. But I think you're mistaken what really happens.
Curvature of field, as I see it, is not equal to shape distortion at the edge.
To me, curvature of field is rather related to, or very similar to spherical aberration. This means that you need to refocus the binoculars to achieve edge sharpness.
Astigmatism, either on axis or close to the edge, will be bad for the edge sharpness since vertical and horizontal image lines (or 45 and 135 degrees as another example) will not get into focus simultaneously. When you turn the focuser one way, the vertical parts of the image will become sharp, and then you need to refocus to get the horizontal parts sharp.
(Astigmatism; a= non, stigma= cross, i.e the two lines that form a cross cannot get into focus at the same time)
The two other first-order aberrations are coma and chromatic aberration. The latter needs no explanation as the subject is well-known at the forum.
Coma means that needle-point objects, like stars, show up as lines, pretty much like a comet.
Looksharp,
Thanks for your response. I'll try to address your points.
First, field curvature causes the size (not the shape) of the out-of focus disk to change as it is moved from the center to the edge. Essentially the star point comes closer to focus at the edge than at the center, so the disk shrinks. This is quite different from spherical aberration which makes good focus impossible to achieve at any point in the field, including the center.
Astigmatism causes a star to be cross shaped only at the point of best focus. Astigmatic out-of-focus star points are seen as oval shaped disks with the long axis of the oval switching by 90 degrees on either side of focus.
I've never seen coma in a binocular except for some defective specimens with a poorly collimated telescope. Lateral color is the one important off-axis aberration that isn't addressed by this method. A focused image works better for that.
Henry