I agree with Profiler, these diagonals are great value for money and will be a match for more expensive diagonals most of the time. I have seen the vignetting using a 24mm Panoptic in an f6 refractor. The Tele Vue Everbrite is more compact but actually has a larger opening and does not restrict the light. It is 2 or 3 times the price though.
This may not be a problem in a longer focal ratio scope. Certainly not a problem using eyepieces with smaller field stops.
