Telescope Reviews: Convenience, 45 ° or 90 °?

What about 60 deg, such as the WW II Zeiss 12 x 60, which was to be 12x70 for 1945-46 production, or its big brother/sister, the WW II 25 x 100 (f/4.5)?

60 deg is very comfortable for zero to 90 deg. target altitudes. The 25 x 100 has one roof prism and two other non-roof. They are optically contacted, rather than cemented. I believe that the roof prism is working entirely, or for most of the beam , in mode II, in which the beam is not split by the roof. I have never noticed any roof spikes in WW II CZ 25 x 100. Their prisms are not metallized. One of the prisms in the 12 x 60 is metallized.

See Hopkins' chapter on mirror and prism systems, in Mil Handbok 141, visible via a link in the reference list of this forum, for drawings of roof prisms in the split and the unsplit beam modes.

As followers of this forum know, several recent Chinese 90 deg. inclined designs with pentaprisms plus Porro II have had several problems . That is a long path in glass ( see Hopkins, op. cit.) , so the Pentaprism should be large aperture, with a fat body to house it, to keep vignetting of a big final pupil within the usually accepted design rule of thumb of 50% at the field edge. There have been non-circular exit pupils , symptomatic of design/integration problems.

The 90 deg. inclined view Fuji from the mid- 1980's had noticeable lower field illumination than the straight view version, which used the same body. I believe that the pentaprism used, ahead of the same Porro II cluster as in the straight version, was the largest which could be fitted without a new body casting. Coincidentally(?), a suitable pentaprism was at hand from a sighting instrument for a Japanese armored vehicle, I was told by a person in a position to know. The IPD bearings are very short, much shorter than the straight version, which seems to be a symptom of the the constraint faced by the designer without a new body casting. But with the exception of the weight, about 65 pounds for both straight(f/5) and 90 deg. ( f/5 or close to f/5) versions, the 90 deg is a desirable instrument. Some of the heavy brass, more of which was found in the early versions of the first type Fuji straight view 25 x 150mm, was "aluminized" for the mechanisms of the 90 deg, inclined version. Some "aluminization" of the mechanisms of the Fuji I occurred in the later 1970's, mostly for reduction of galvanic brass-aluminum-salt water spray corrosion , I believe as the result of my suggestions/complaints. This had the side effect of weight reduction.