The reader is right in that the Edmund catalog states 68 degrees, but this figure was based on a set of optics that was not actually the final product. Anyone with any observing expierience can hold this eyepiece up to their eye and see that the apparent field is far greater than 68 degrees. The field stop on the Edmund is 42.7 mm, which gives it a slightly larger true field than the 31 Nagler. The 31 Nagler has a field stop that is 42 mm in diameter. The field stop diameter x 57.3, divided by the focal length of the telescope in mm will give you the true field at the eyepiece.

I've owned a 31 Nagler for some time.  It is used in a 12.5" f/5 with a Paracorr.  The Paracorr was designed before the 31 Nagler, so the optimum setting for the 31 (6) is not quite reachable with the tunable top on the Paracorr.

Nonetheless, this eyepiece produces a bright image, with no obvious vignetting, all the way to the edge.

Evaluating sharpness of focus, I could not have had a more different experience than possible compared to the previous poster.

I am a fussbudget par excellence where sharpness of focus is concerned.  My sample of the 31 produces tiny pinpoint star images to nearly the 90% point in the field.  From that point on out coma starts appearing because the Paracorr is not at an optimum setting.  However, in perspective, even at the edge the images are still tinier than mediocre seeing makes all star images.  I just don't blame eyepieces for seeing problems.

The low power this eyepiece produces in my scope (59X) means the star images should be sharp or there's something wrong.

The 31 doesn't disappoint.

Blackouts--none noticed.  The eyepiece lends itself to easily holding ones head at the right distance.  The 19mm eye relief can cause blackouts if you tend to hold your eye too close.  The 32 Erfle would have a tighter eye relief so holding your eye too close would be less likely.  Also, sitting down when viewing makes it easier to hold your head in the right place. [Standing up is too tiring, anyway]

Astigmatism--the dominant aberration in the Erfle design, the 31 Nagler has none detectable.  Rolling the focuser in and out defocuses the star images but there is no astigmatism in the images, even at the edge.

Light transmission--The 31 behaves like a simple Kellner.  The images are very bright and there appears to be no noticeable dimming of the image compared to simple eyepieces like Plossls or Kellners.

Rectilinear distortion--The 31 has pincushion distortion so is not recommended for daytime use.

Angular magnification distortion--there is none noticeable, which means that owners of non-driven dobs will not see a distorted image in a planet as it drifts across the field of view.

Chromatic aberration--there is none on-axis, but a trace of lateral chromatic aberration exists.  This isn't a problem on stars, but may make the Moon image a little odd near the edge of the field.

Spherical aberration--well corrected.  None visible.

Field curvature--Because I use a Paracorr, which flattens the field slightly, I cannot evaluate the contribution from the eyepiece, but I can say it is a lot less than the 35 Panoptic I used before this.  I see no field curvature in this eyepiece, but a user in a shorter focal length when used without a Paracorr might have a different report.

Field of view--there is a psychological factor with this eyepiece that is a little different than other eyepieces of similar focal lengths--the field of view almost seems to envelope you and it seems you have to look left and right to see the outer parts of the field.  This isn't specifically true, since the entire field can be seen at one time merely by paying attention to your peripheral vision.  But it is more (and I don't like this term) immersive than other eyepieces--even others in the Nagler line.

Pros:

A larger sharp field of view than any other eyepiece with a similar field stop.

Comfortable to use

Good light transmission

Cons:

Weight (in some scopes)

Price

Lack of appropriateness in daytime use.

Rating: 9 out of 10