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12.5mm Zeiss 90* and 12mm Nagler Type 4:
I love both the Zeiss 12.5mm 90 degree and the 12 Nagler Type 4. The Nagler 4 is better suited in my faster scopes. The Zeiss is good in slower scopes. I never got a Paracorr because I wanted the eyepieces to do all the corrections. Perhaps if I had very fast scopes that would change. Also, I never really barlow, and I try to keep the number of elements at a minimum.
I really don't have any plans to sell the 12.5mm Zeiss eyepiece. It's very special to me. I like it very much, and it's a rare and wonderful eyepiece. For your information, I have been using the 12mm Nagler 4 almost exclusively. The main reason I bought the Nagler was because I like to observe double stars. Using the 155EDF refractor, stars look like seagulls at the edge through the 12.5mm Zeiss. The 12mm Nagler 4 eliminates this problem. Since I mainly use the refractor, I rarely use the 12.5mm Zeiss; but, I'm not planning on selling it.
In some ways the 12.5mm Zeiss is superior to the 12mm Nagler 4. In other ways the 12mm Nagler 4 is superior to the 12.5mm Zeiss. The 12.5mm Zeiss is easier to use since you don't have to worry about eye position. The 12mm Nagler 4's performance is greatly dependent on eye position, and takes some getting used to.
The 12.5mm Zeiss turns stars into little seagulls near the edge of the field in fast scopes and the 12mm Nagler doesn't.
The 12.5mm Zeiss has a larger field of view (90 degree) than the 12mm Nagler 4 (82 degree).
The 12mm Nagler 4 has greater eye relief.
I have detected very faint chromatic aberration through the 12mm Nagler 4. The 12.5mm Zeiss never shows even a hint of false color.
Both eyepieces are equal with regards to sharpness and contrast when used in a f/10 scope. In a f/7 scope the edge suffers with the 12.5mm Zeiss.
I remounted the Zeiss optical assembly making it almost parfocal with the Nagler. The original Zeiss mounting requires a lot of in-focus rendering it unusable on most refractors.
I added a set screw to the Nagler sliding eyecup to fix the cup to my optimal eye position. I didn't like the Instadjust sliding eyecup at all. Fixing the eyecup really improved the eyepiece, in my humble opinion.
Both are great eyepieces and it's a pleasure to own both. I use the 12.5mm Zeiss in my f/10 Schmidts, and the 12mm Nagler 4 in my f/7 refractors.
So, that's it for now.