Carl Zeiss PL eyepeices:
They suppose to have flat field, but just in microscopes.
For telescopes, fast ones, are not excellent on edges, but still not bad like Kellner eyepieces are.
First of all, let me explain what mean numbers on eyepieces: if we talk about model PL 10x/18, this means that this eyepeice have 10x magnification on microscopes, and FOV 18mm (this is actually field stop diameter)!
But how they get 10x? All results about magnifications are devided always with number 250, which is actually some focal lenght, so it means 250 devided with 10, equals 25mm, and this is focal lenght of eyepiece.
This means, that eyepeice with marks 16x/16 have focal lenght 15.625mm (250/16), and field stop 16mm.
Most popular eyepiece model here is PL 10x/18 (non focussable model) with aspheic eyelens, which is most known model which lenses Markus Ludes takes for his legendary TMB 25mm aspherical ortho eyepeices. There also exists PL 10x/18 focussable eyepeice.
But there actually exists 4 models of PL 10x/18 eyepeices:
***one model have small aspherical eyelens, and thats we called it Aspheric. Markus named it ORTHO, but is is not really ortho design, becasue it consist only 3 lenses in 2 groups. Best model of all PL 10x/18 eyepieces. Provide also most neutral toned image.
***second model have bigger aspherical eyelens, with same performance as above mentioned.
***third model have concave eyelens, and this is newest model, with not so good edge performance as Aspheric design have. Have just bit more yellowish tone of image comparing to Aspheric design.
***fourth model have somewhat flat to little concave eyelens, oldest model, almost good as Aspheric design.
All four models are very very good, and very close in performance on axes, and there is little difference on edges as you could read, and colour rendering, but differences are very little.
Now, there are other PL versions like PL 10x/20, 10x/22, 10x/23, 10x/25.
All they differ in FOV, and date of producing, with aspherical, flat to concave, and concave eyelenses, but they are all very very good, especially PL 10x/25 versions, becasue really big FOV, and here newer model (444034-9000-000) produce slightly more neutral tone of image, while older version (444034) provide somewhat yellowish tone of image, but it have bigger eyelens and more imerssive look.
Here you can find some photos....