"The Absam ring occurs when you push it too far with the edge definition. It seems to be a kind of wave in the position of the image plane.
Without image field flattening, the intermediate image, which is produced by the lens, is not on a surface, but on a curved image shell - this is just the image field curvature, a well-known aberration. If you focus on the center of the image, the vertex of this image shell is in focus and the object appears in focus. Beyond the center of the image, however, this shell increasingly bends away from the focal plane and the image appears out of focus. Usually it is possible to keep about 50-60% of the image reasonably sharp (depending on the accommodation capacity of the observer), further out the star points will soon be washed out.
The Bildebnungslinse (also Smyth lens) is placed in front of the intermediate image, and it gives the image shell with an additional curvature in the opposite direction, so that it ideally becomes almost flat. Of course, this does not work perfectly, but then you can achieve a sharp picture up to 80-90% towards the edge. The Fujinon FMT-SX, or the old Swarovski EL were examples of this performance class.
Here, the Smyth lens has actually reached its limits. Recently, however, one tries to drive the game a little further: edge sharpness at any price! A two-piece Smyth group is bent in such a way that the image shell intersects the same focal plane not only in the center, but also at the edge, so that both the image center and the image edge appear sharp. However, this does not succeed in keeping the image completely flat on this plane over the entire visual angle range: the intermediate image lies with the vertex on the focal plane, then slowly moves away from this plane with increasing visual angle, at around 80% a turning point to reach and then turn back to the focal plane.
The absam ring is then the area in which the intermediate image is at a maximum distance from the focal plane before it is bent back. At the same time, the distortion of the image also changes in such a way that it switches from a slightly pincushion distortion (0-80% viewing angle) to a barrel-shaped distortion (near the edge of the image), which can give the impression of the globe effect when panned."
Holger Merlitz
Yes, it is normal!
Andreas