Working Out exposure with a Pro-Tessar 115 on a Zeiss Ikon Contaflex Super

I just got one of these little honeys: Dead Link Removed and I have a quick question about the maximum aperture.

The lens says f/4, but lenses on this camera are simply a swappable front element (or in this case, elements) with no diaphragm. I am assuming that the f/4 designation means that this lens adds a full stop (the diaphragm goes up to f/2.8).

My question is: Is my assumption correct, or is there something I am missing?

An f/stop is always a fraction whose value reveals a relationship between two factors - the focal length of the lens and the diameter of the opening (the aperture).

For a given aperture, a change in focal length will result in a change in the resulting f/stop.

If by changing the front elements you double the focal length, the effect on the resulting f/stop is that it will be twice as much - i.e. two stops different.

If by changing the front elements you increase the focal length by a factor of 1.4 (square root of 2) the effect on the resulting f/stop is that it will be 1.4 times as much - i.e. one stop different.

So if you are comparing your recent acquisition to lens elements that gave an effective focal length near 80mm, and if the diaphragm built into the camera yields a maximum f/stop of f/2.8 with those 80mm elements, then it is true that the resulting maximum f/stop for that diaphragm and your new acquisition will be f/4.

If your lens (or the resulting optical system of whatever assembly) is 115 mm of focal length, and is declared as being f/4, then the front lens should have a diameter of 115/4 = 28,75 mm. Conversely, if your front lens has a diameter of 28,75 mm, and the lens has a focal length of 115 mm, then its maximum f/value is 4.

If on the diaphragm ring the maximum aperture is marked 2.8, than with the tele extension that becomes 4, and 4 becomes 5.6 etc.

Your assumption sounds therefore correct.