ZEISS DIAPHRAGM MARKINGS.

U

By HAROLD M. BENNETT.

INTIL 1895, when the VIIA Double Protar was introduced the relative fractional foci (F system) was used by the firm of Carl Zeiss for marking the diaphragm openings of photographic lenses. The VIIA Double Protar is a convertible lens composed of two single lens combinations of equal or varying focal lengths known individually as Series VII Protar. For example the Series VIIA, No. 10, is a combination of two No. 4 Series VII Protars each having a focal length of 1334 inches and combined, a focal length of 7% inches. It would therefore be necessary if the F system were used to have two rows of F markings, for 7% and 1334 inch foci, the largest F values being 6.3 and 12.5 respectively. A Series VII, No. 5 or No. 6, of 16% and 18% inches focus may be substituted for the front lens, the addition of the No. 5 changing the largest aperture to F 7 and the No. 6 to F. 7.7 the combined focal lengths changing to 8% and 91⁄2 inches respectively. By interchanging the four elements seven different focal lengths can be had, the combinations, resulting foci, and largest openings being as follows:

If the diaphragm openings of these combinations were engraved on the lens mount it would necessitate seven rows of figures which would be somewhat confusing. It was therefore necessary to use a system of diaphragm markings which would be suitable for the various foci. A simple solution. was to engrave upon the tube, a scale, the divisions of which correspond to a change of one millimeter in the diameter of

the iris aperture, in other words by adapting a regular millimetric scale to indicate the diameter of the iris aperture. On the scale half centimeters and centimeters are indicated by larger strokes, and only the 3, 4, 6, 8, 12, 17, 24m/m divisions are denoted by corresponding figures.

Prominence is given to these numbers since the squares of two consecutive numbers are related at 1:2 the exposures corresponding to these stops must be as 2:1 no matter what the focal length of the objective may be. Fig 1 shows the scale as engraved on the No. IV. Zeiss tube mount with the irisaperture adjusted to 15.6 m/m. The lines numbered on the IV tube mount, and relative exposures with any combinations of lenses which fit it, would be as follows: Numbers on No. IV mount

Relative exposures

24

3 4 6

8 12 17

64

350 285 224

24

2016

22 17

285 2244.

22 18 14

350 289

11:2

REL APERTER?

Ser. VIA
Foci m

200 163128

1:22.6

098

Double Protar lenses are provided with caps having diaphragm markings as shown in Fig. 2 (for No. IV tube mount) by means of which the relative F value of the millimetric markings can be easily learned. On these caps the focal lengths of the single elements are expressed in millimeters as shown

in the outside row of figures. Thus, for instance the table supplied with cap of the No. IV tube mount contains the necessary data for the Protar-lenses of Series VII having focal length of 350, 285 and 244 m/m respectively and also for their combinations in the form of Double-Protar Series VIIA. The outer row of figures indicates the focal lengths of the single elements.

The F and U. S. markings are indicated by the fifth and sixth rows, and the relative opening in millimeters for the various combinations, by the second, third and fourth rows. The second row shows the opening in millimeters for the focal length indicated in space above it in the outer row. Half of this opening would give the same F aperture for a combination of two of these lenses. For example, the opening in millimeters for a Series VII Protar of 350 m/m focus to correspond with F 32 would be II while with a VII No. 10 Double Protar composed of two 350 m/m lenses the opening for F 32 would be one half of 11 or 5.5 indicated by the 2 after 11. The third row shows the millimeter opening for a combination of two Protars of different focal lengths-thus with a combination of 350 m/m and 285 m/m F 32 would be 5 m/m and with a combination of 350 m/m and 224 m/m the opening would be 4 m/m.

II.

The center of the diagram shows the resulting combined focus of any two of the lenses indicated on the outer row.