This is an early Tessar from 1911, with max aperture f/6.3. Introduced in 1902, in 1904 there was already a 4.5 version that became available commercially in 1907. Later, in 1930 it went up to f/2.8 and equipped millions of 35mm and medium format cameras. To see lens diagram and some more info go to my page: Tessar 210 f/4,5.

In this page I would like to take the opportunity to talk about lens aperture because this Tessar has an interesting feature: instead of one among regularly used aperture scales it brings the iris diameter in millimeters.

The role of aperture in lenses is, in first place, to control the amount of light passing by. Very much like a faucet.

The photographic emulsion (or any other light sensitive recording system including digital sensors) needs a minimum amount of light exposure to start recording anything at all. There is also a maximum quantity of light beyond which there is no perceived difference when more light is shed onto the sensitive surface. Within those limits, there is photography.

The way photographic equipment controls the amount of light reaching to the film is based on two things: time and lens aperture. Time is controlled by a shutter system (there are many) and aperture means how open or close the lens is, how much of the usable lens surface is actually used. Again, very much like using a faucet to fill up a container: the more you open the faucet the less time you need and the other way around.



In order to have some evenness of results when photographers use different lenses it was created a certain convention on the way we indicate lens aperture. f/5,6 will yield the same exposure under the same shutter time and light conditions in any given lens. No matter whether it is wide angle or tele objective, large or small format, one brand or another.

The convention is that the f number is the focal length divided by the diameter of the entrance pupil. Before talking about the entrance pupil it is worth to stop and understand this ratio. It makes sense. Imagine a normal lens. Foci is about the frame's diagonal that it covers. Roughly 300 mm for a 18x24 cm frame and 80mm for a 6x6 cm frame. If the lens spreads light onto a 18 x 24 cm negative it will need a 'bigger hole' compared to a normal lens that spreads light onto a 6x6 cm negative. The larger the negative surface to be covered, the more light needs to be in-taken by the lens. Hence, the longer is the lens and bigger the frame the larger must be the hole and that is what makes the ratio foci/hole a nice standard to talk about aperture.

Now understanding the 'hole'. It can't be the iris diameter itself. Simply because there is glass in front of most of the irises. The front lens element augments the effective size of the hole just like a reading glass augments the letters we read. The above mentioned Entrance Pupil is like the virtual hole size that takes into consideration this effect produced by the front lens element.

Coming back to this Tessar: the millimeter scale shows the following series: 48, 34, 24, 17, 12, 8, 6, 4, 3. I assumed the 48 mm is equivalent to f/6.3.



Considering that f = foci / Entrance Pupil (EP)

then 6.3 = 360 / EP and EP is 360/6.3 = 57.1428 mm for this aperture f/6.3. EP divided by 48 mm tells us what is the 'iris enlargement' procured by this Tessar front element.

57.1428 / 48 = 1.19047 that means all the marked mm correspond to an EP that is given by:

EP = 1.19047 x iris in mm

Using the series of f numbers proposed by Franz Stolze (1830-1910), that has f/6.3 in it, and was used at that time, I calculated the iris aperture for the following figures:

6.3 9 12.5 18 25 36 50 71 100 (this is the Stolze's series)

That was simply dividing each one by 1.19047. The result i got, certainly not by coincidence, was exactly the sequence marked in mm in the lens scale as shown in the chart below.

mm marked on lens	aperture old scale	mm	aperture modern scale
48	6.3	48	6.3
		45	8
34	9
		33	11
24	12.5
		22	16
17	18
		16	22
12	25
		11	32
8	36
		8	45
		7	45
6	50
		6	64
4	71
		4	90
3	100

Using the same reasoning, I calculated as well the iris diameter when starting at 6.3 but taking the modern and widely accepted series 8 11 16 22 32 45 64 90

The results are in the 3rd and 4th columns in the same chart. That is useful when using this lens with a modern light meter.

What is the difference between Stolze's and current scale?(I could not identify an author for that). Well, basically they just use the same principle with different figures. The actual amount of light taken by a lens is proportional to the area of a circle having the entrance pupil as diameter. Double the area and you double the light. In both cases the ratio of two subsequent numbers correspond to doubling or halving the amount of light. This is very convenient because shutter speed is the second lever we have to control exposure. You certainly remember that it also doubles/halves in each step: 1 1/2 1/4 1/8 1/15 1/30 1/60.... By having the two scales, aperture and shutter speed following that pattern, one can easily understand that one step up combined with one step down in both scales corresponds to the same exposure! Several 35mm cameras, before the automatic exposure, offered the possibility of, once a certain exposure was picked as a target, having both adjustments, up and down, combined in only one movement. That is the case of Voigtlander Vitessa.