You were 100 % right. I was just going to send it back and ask for a refund thinking perhaps it's was just in the cheap design of a $59 eyepiece, but some of the reviews seemed to suggest otherwise which kinda confused me. Agena contacted ES in my behalf who in turn called me and offered a replacement with free return of the old eyepiece. I knew there was a chance that the replacement eyepiece would be no better than the first one, that it could possibly just be in the design of an inexpensive eyepiece but turns out the reviews saying that it had a flat field were correct. I'm really happy with mine and would suggest anyone having a 3mm ES 52 degree eyepiece that displays anything looking flat only in the central 60% area have it checked out.
I contacted ES by e-mail about the distortion in my 3mm 52-deg and requested an exchange. The distortion begins at around 40% from the edge in all of my refractors.
My take from their response is that they think the 3mm 52-deg’s edge distortion in my sample is not as big a problem as I think it is. They didn’t accept my exchange request or deny one in their response. I then told them I would rather return the eyepiece and they offered to repair it. I’m glad for that, as long as they can see for themselves and confirm what I am seeing or give me an explanation that would convince me to keep the eyepiece.
I’ve sent it back and am waiting for their evaluation. It seems to me that ES is taking this issue seriously, but being cautious and standing by the quality of this eyepiece. The other three eyepieces I have of the 52-deg series showed no distortion so far away from the edge, and all four are a huge step up in workmanship and eye relief compared to the Orion Sirius Plossls I used to own. My Celestron 17mm Plossl on the other hand had distortion from about 30% of the edge. At 3mm FL, I’d be ok with distortion from about 20% of the edge.
I think the 3mm is an eyepiece worth getting, but if the edge distortion is too great, whether or not to keep it depends on how tolerant you are about it, or if the on-axis view is good enough for you. It just isn’t for me.
Edited by Rock22, 27 April 2019 - 01:42 PM.