Telescope Reviews: Comparison of 24mm-26mm Eyepieces

This review is exhaustive and much attempt is made to make comparisons valid.

There are a few quibbles I have with his test procedures however:

1) Comparing a 68 degree eyepiece's edge correction with a 50 degree eyepiece's edge correction would seem to be valid, given that the widefield user will be looking for definition at or near the edge of the field just like the narrower field user. However, if we assume that the edge correction of a 50 degree eyepiece is less than perfect at the edge, while a 68 degree eyepiece is perfect at the same 50 degree point in the field, but deteriorates more at the edge, then which eyepiece actually has the better correction? To give a valid comparison, I would want to see the correction obtained at 20, 25, 30, and 34 degrees off axis for each. It could be the widefield eyepiece yields a better-corrected 50 degree field than the 50 degree eyepiece. If so, the edge correction would have to be "field size compensated" to yield a relative ranking.

2) Bill seems to have some confusion about rectilinear distortion and angular magnification distortion and which aberration causes what in the image. Perhaps a good way to think about these two distortions is to picture map projections. If you reduce a globe to a flat representation, then there will be geometric distortions in the map, worst at the edges. We see this all the time in standard flat maps of the world. This is similar to what we would see as rectilinear (geometric) distortion.
If the globe itself is presented and rotated, a country will come around the edge of the globe, come toward you, and move away from you, changing size as it goes, but it will not be geometrically distorted. yet, it will appear to change in size.
Case one represents rectilinear distortion, while case two represents (it's an inexact analogy, I admit)) angular magnification distortion.

Would we want the planet to appear smaller at the edge of the field than the center? Probably not, though this wouldn't seem to be an issue in a driven scope with the planet's image held in the center. So, in the undriven scope, angular magnification distortion would not appear to be a desirable trait, especially in star fields or for viewing double stars.

But, the wider the field, the more the correction of one aberration leads to more of the other, so an eyepiece corrected for AMD will, necessarily, have RD. Since our eyes tend to have a little barrel distortion (positive RD)(it's a brain thing) when viewing the edge of a field, some amount of negative RD (pincushion) seems to result in an undistorted image to our eyes.
If there is too much RD, the object will appear to stretch and change shape as it nears the edge, similar to that Mercator map projection we talked about.

So what's ideal?
Well, in an undriven scope, a low level of AMD for sure. And for the least distortion visible to our eyes in images, a certain amount of RD. Not surprisingly, many good eyepieces are designed this way.

But what about the person who pans across the field? Well, if there is AMD in any significant amount it would appear as if the image is rolling across the surface of a globe. I have a kaleidoscope that does this--the edge appears farther away than the center because the object changes in size as it crosses the field and our brains tell us we are watching a globe rotating.

With a lot of RD and no AMD? Well, we'd see the object change its shape as it moves across the field, but the field could still appear flat. And if the RD amount is chosen to leave only a tiny bit of AMD in the field, we might see very little distortion in the field.

But, extend the field even more, to 82 degrees, or 90 or 100 or 110 degrees. The curves of solution for AMD and RD diverge even more. Having that *just right* about of RD in the field will result in noticeable AMD. So, a designer could solve for AMD and let RD be a larger amount. Given that most of these eyepieces are deep-sky eyepieces, that is probably the right choice. Used as terrestrial eyepieces, the edge of the field would appear quite distorted because we know how tree trunks and building edges are supposed to appear.
We might even tolerate that, though, if the eyepiece was free from astigmatism and the edge was sharp.

But it does indicate that the demands of designing the perfect astronomical eyepiece and perfect terrestrial eyepiece can be quite different.

I refer you to the discussions of RD and AMD which exist on the web, especially as they relate to binoculars and terrestrial usage.

3) Bill also missed the opportunity to discuss astigmatism and field curvature in the eyepieces. Field curvature's visibility might be related to the scope in which the eyepiece is used, but astigmatism is not. Since most of these eyepieces have narrow fields, where astigmatism should be tightly controlled, this could have been an illuminating comparison. I suppose some of that could be garnered from the comparison with Paracorr used, since, if coma and field curvature are more or less corrected, what's left is astigmatism [excluding chromatic aberrations].

All-in-all, a good step toward trying to be fair in comparisons. It was illuminating to see some highly-touted eyepieces come in down the list, and I applaud Bill's lack of that annoying drone about "tinted" eyepieces.