Eyepiece Distortion
Rectilinear and angular magnification distortion

TeleVue.com: Advice Article List > Eyepiece Distortion 
"Distortion becomes especially important in widefield eyepieces. In discussing distortion, however, one must clearly distinguish between rectilinear distortion and angular magnification distortion. For terrestrial telescopes, it is often required that straight lines in the focal surface look straight in the eyepiece. For zero rectilinear distortion, the following relationship should apply:
y = f · tan ß
where y is the offaxis distance in the focal plane, ß the image angle from the optical axis, and f the focal length of the eyepiece.
For astronomical observation, however, it is important that the angular magnification remains
constant over the field. For instance, the angular distance between double stars should be the same and a round object (a planet) should retain its shape whether viewed in the center of the field or at the edge of the field. In this case, the following relationship should apply:
y = f · ß
where ß is expressed in radians.
With zero angular magnification distortion, straight lines on a focal plane appear curved in a pincushion fashion, with the curvature becoming greater the farther they lie from the center. It is impossible to correct an eyepiece simultaneously for rectilinear and angular magnification distortion."
— Rutten, H & van Venrooij, M. Telescope Optics. WillmannBell,(1998). p169.
For reference, the maximum angular magnification distortion of the 13mm Ethos is no more than 1% at any point in the 100° field.