Spotters that are “easier” to look through

[Spiral Horn]

Since you mentioned both Kowa and eye fatigue (something material but often overlooked), this is one area that IME Kowa Spotters shine. I can use any of my Kowa spotters for extended periods with very little eye fatigue. After directly comparing the Kowa 88mm to Swarovski and other major brands = Meopta S2 (also underrated), Vortex, etc. the Kowa was the one that ended up going home with me. The dual focus mechanism of the Kowa is also superior.

If weight/size is an issue the new Kowa 66A is only a slight step from the performance of the 88mm.