Does anyone have more information regarding on axis sharpness in a Clave? This excerpt below was taken from Chris Lord’s “Evolution of the Astronomical Eyepiece” page 31. He states that the sharpest imagery occurs 30% towards the edge of the field of view. Has anyone noticed that? I did last night on M5 with a Clave 6mm. I was a little surprised to see the dense area of M5 tighten up when viewing that area slightly off axis. Then I recalled reading something about on axis sharpness and found this. I intend on checking again tonight. My Zeiss 6mm Mono and TMB 6mm Mono were very sharp on axis in comparison.
“However, unlike the Abbé Orthoscopic and its derivatives, where longitudinal spherical correction is zero on axis, the assymetric form of the Plössl leads to a zonal correction and the sharpest imagery does not occur on axis but some 30% towards the edge of the field of view. At low to medium powers this is of no consequence, but it is noticeable at high powers (exit pupils less than 1.5mm).”