Per Corell
Goerz D.f. 99
The D.F. 99 had a special kind of individual focus ocular, the so-called guide-bar [Geradfuehrung] ocular. Turning the ring moves the eye cup and lens up and down, but the ocular does not rotate. [helical focus] (Registered design no. 115394, 1899, 'Ocular focusing for prism binoculars, specified by the fact that the ocular travels in a straight line and is moved by a turnable slide-bar with cord and partitions.')
*The helical focus is still used today, for example see fig. 82 and 120, but for reasons other than those of 90 years ago. The author cannot confirm if the following is valid: 'The ‘guide bar’ of the oculars was conceptualized, to avoid rotation of an ocular with several lenses.
Early lenses which could not be ground so perfectly, could have changed the image [when rotated].
From the text of a brochure for the D.F. 99:
Our prism binoculars of 7 power, which were introduced according to the Royal Prussian ordinance of January 1903 are, as made known in the order of the royal general command department, offered by us to members of the German Army and Navy for their own use, also at the discounted price, agreed upon by the department, of 95 Marks, if the order is made by the Military. The instruments are officially called: ‘Doppel-Fernrohr Modell 1899’, and are engraved as follows: ‘C.P. Goerz, Berlin’ on the left telescope tube and ‘D.F. P. no.’ (i.e. Double Telescope Model 1899 Private No.)
Goerz D.f. 99
The D.F. 99 had a special kind of individual focus ocular, the so-called guide-bar [Geradfuehrung] ocular. Turning the ring moves the eye cup and lens up and down, but the ocular does not rotate. [helical focus] (Registered design no. 115394, 1899, 'Ocular focusing for prism binoculars, specified by the fact that the ocular travels in a straight line and is moved by a turnable slide-bar with cord and partitions.')
*The helical focus is still used today, for example see fig. 82 and 120, but for reasons other than those of 90 years ago. The author cannot confirm if the following is valid: 'The ‘guide bar’ of the oculars was conceptualized, to avoid rotation of an ocular with several lenses.
Early lenses which could not be ground so perfectly, could have changed the image [when rotated].
From the text of a brochure for the D.F. 99:
Our prism binoculars of 7 power, which were introduced according to the Royal Prussian ordinance of January 1903 are, as made known in the order of the royal general command department, offered by us to members of the German Army and Navy for their own use, also at the discounted price, agreed upon by the department, of 95 Marks, if the order is made by the Military. The instruments are officially called: ‘Doppel-Fernrohr Modell 1899’, and are engraved as follows: ‘C.P. Goerz, Berlin’ on the left telescope tube and ‘D.F. P. no.’ (i.e. Double Telescope Model 1899 Private No.)