Hi Lars..thanks..I was pretty sure of this ,but not completely ..First I thought the oversized prism maybe was only for the ED82 and the 60 could have used a slimer version,but i got that pretty much sorted looking at some nikon brochures...Now I have the confirmation...
The straight and angled scopes naturally use different prism designs. I would not read to much into the angled Nikon prism "advantage" if comparing to the latest angled designs that use a dielectric coating.
Jerry,
There's more than one way to make an angled scope. The 60, 78 and 82mm Nikon fieldscopes used a monoblock Schmidt prism that was oversized and offset so that the roof edge did not split the light cone. That kind of prism has only 2 glass to air surfaces and 4 reflections like a Porro, all with total internal reflection. It needs no phase correction or mirror coating.
The 50mm angled Fieldscope uses a Porro followed by a semi-pentaprism. That has 6 glass to air surfaces and 6 reflections, one of which requires mirror coating.
IMO the new angled Fieldscopes have taken a step backwards. They use centered Schmidt prisms, so the roof edge splits the light cone. They require phase correction, but not mirror coating.
Henry
I assume that the angled share the same prisms as well.
The straight and angled scopes naturally use different prism designs.
Jerry,
I haven't seen the new EDG scopes, but changing from an offset to a centered Schmidt prism creates new opportunities for optical defects that the old scopes never had. The new arrangement requires high precision roof angles, a well made roof edge and phase correction. All of that comes at a higher cost and a higher risk of defective specimens. The other current alpha scopes also use centered Schmidt prisms and I've already seen a few badly made roof edges in star tests. It was impossible by design for the old Fieldscopes to have that defect. You could say the same thing for the change from Porro to Schmidt-Pechan roof in the straight versions of the EDG scopes.
Henry
mayoayo,
Yes, the centered prisms can be smaller and lighter, but I doubt that the difference amounts to more than a few ounces.
I have a question, when Nikon changed from Porro to Schmidt-Pechan roof in straight EDG scope, that's the reason the scope body is straight like telephoto lens than the ED scope which is offset to the top ?
Can Nikon still use Porro on straight EDG scope ?