Bolex H16 Rex prism

[Mmechanic]

The double prism of the Paillard-Bolex H RX cameras introduces a longitudinal aberration. Specially calculated and manufactured RX designated lenses have been introduced. Berthiot, Schneider, and Angénieux made RX lenses, too.

The longitudinal focus error becomes negligible with lenses of 50 mm focal length and more and with lenses stopped down to f/3.3 and more. Likewise RX lenses can be used on other cameras under the same conditions.

The double prism does not diminish resolution. Light loss by the partially mirrored inner surface is 25 percent. Light loss due to reflection on the air-glass surfaces is of the order of two percent. It is correct that the double prism must be very accurately aligned in order to secure overall focus. That is done by adjusting two set screws within its mounting and the aid of an autocollimator and an optically flat mirror in the film plane. The set screws are lacquered.

You have three manufacturers who offered the turret-and-rackover focusing system, Bell & Howell, Paillard-Bolex, and ETM. The latter French company is less well known. The corresponding cameras are named P 16 and P 9,5. I haven’t found evidence of an ETM slide or displacement accessory until today but I can furnish the geometric data, if someone has a P. Paillard-Bolex was slow with delivering a rackover device after Toledo Cine Engineering made a first one in 1953. Bell & Howell had a Focusing Alignment Gauge to all of their serious cameras, beginning with the Standard in the summer of 1912 (part of tripod head), the Filmo 70 in May 1931, the Eyemo 71-N, -O, -P, and -Q in June 1937, and the Filmo Eights in July 1939. These systems are very accurate and don’t add any optical complications.

[Phil_F_NM]

I really want to find a Filmo 70 focusing alignment gauge at a price that isn't outrageous.

Phil Forrest