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The Barnet Book of Photography A Collection of Practical Articles

Others prefer to express the relative exposure necessary


[Illustration:

Fig. 6.]

The best way to mark stops is, for example, _f_/8 and _f_/16, as these expressions are universally understood, but some persons think that the relative rapidities or intensities are better, others prefer to express the relative exposure necessary, and every system of numbering on these plans has a unit which is merely empirical, not one of them adopting the only true or scientific unit of _f_/1.

Zeiss has recently changed his unit from _f_/100 to _f_/50. Dallmeyer marks some of his lenses now with the practical expression. The following table may be of service to those who happen to have lenses with their diaphragms marked on any of these empirical systems.

------------------------------------------------------------------- | Royal | | | | | | Photographic | | Paris | Zeiss | Zeiss | _f_/ | Society. | Dallmeyer. | Congress.| (old). | (new). | ------------------------------------------------------------------- 3.16 | | 1 | 1/10 | | | 3.2 | | | | 1024 | 256 | 4 | 1 | | | | | 5 | | 2.5 | 1/4 | | | 4.5 | | | | 512 | 128 | 5.66 | 2

| | | | | 6.3 | | 4 | 4/10 | 256 | 64 | 7.07 | | 5 | 1/2 | | | 8 | 4 | | | | | 8.66 | | 7.5 | 3/4 | | | 9 | | | | 128 | 32 | 10 | | 10 | 1 | | | 11.3 | 8 | | | | | 12.25 | | 15 | 1.5 | | | 12.5 | | | | 64 | 16 | 14.14 | | 20 | 2 | | | 15.81 | | 25 | 2.5 | | | 16 | 16 | | | | | 17.32 | | 30 | 3 | | | 18 | | | | 32 | 8 | 20 | | 40 | 4 | | | 22.36 | | 50 | 5 | | | 22.6 | 32 | | | | | 25 | | | | 16 | 4 | 27.36 | | 75 | 7.5 | | | 31.62 | | 100 | 10 | | | 32 | 64 | | | | | 36 | | | | 8 | 2 | 38.7 | | 150 | 15 | | | 44.72 | | 200 | 20 | | | 45.2 | 128 | | | | | 50 | | 250 | 25 | 4 | 1 | 54.77 | | 300 | 30 | | | 63.25 | | 400 | 40 | | | 64 | 256 | | | | | 70.71 | | 500 | 50 | | | 71 | | | | 2 | | 100 | | | | 1 | | -------------------------------------------------------------------


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