The rationale behind the focal lengths

[ho_co]

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doesn't seem to work...i.e., what sequence of focal lengths do you get based on 50mm?

 

It's a suggestion made by John Brooks of the Leica School, I've since remembered, but it's only an approximation and rule of thumb.

 

16 mm would give you a 24 mm next step.

18 mm would give you a 27 mm next step.

21 mm would give you a 31.5 mm next step, splits the difference between 28 mm and 35 mm.

24 mm would give you a 36 mm next step.

28 mm would give you a 46.4 mm next step, close to 40 mm and to 50 mm.

35 mm would give you a 52.5 mm next step.

52 mm (long-time approximate design focal length for so-called "50 mm") would give you 78.8 mm.

75 mm would give you a 112.5 mm next step.

90 mm would give you a 135 mm next step.

135 mm would give you a 202.5 mm next step.

 

In regard to the 52 next step (your question): Wasn't there a 73 mm Hektor in the lineup at some point? And didn't Zeiss offer an 80 mm?

 

I agree with Andy that trying to ascribe rhyme or reason to the sequence is overthinking, though fun speculation. (And a lot of good points have been made above.)

 

The question in Leica's mind would be "what can we sell that's different enough from what went before to be photographically useful?" (Emphasis is on the word "sell." )

 

I am doubtful that anyone sat down with a formula to derive the "next focal length" in the line. And the gaps have been filled in gradually after the original steps.

 

Just my 2¢ worth.

[Guest #12]

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thanks; I think 1930's lineup was 28, 35, 50, 73, 90, 105, 135