Archived posting to the Leica Users Group, 2000/03/08

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Subject: [Leica] F stop
From: R.P.A.C.Savenije@AVV.RWS.minvenw.nl
Date: 08 Mar 2000 09:46:40 +0000

Rick Garrelfs wrote:


> What is the difference between a 1.4 summilux and a 1.5 Noktor?

>  Y = 1.5
>  X = 1.4

>  Ln(1.5) - Ln(1.4) = 0.06899;  Ln (sqrt 2) = 0,34657

>  difference is:   0.06899/0,34657= 0,19906, or about 0,2 f-stop.

Sorry Rick I do not agree...
I see a small mistake.
Just have a look at my previous calculations.

In fact it does not matter wether you use log or ln but
by calculating the difference between f 1.5 and f 1.4 you have to use the
real values.
So 1.4 is not 1.4 but sqrt 2

{ln(1.5) - ln(sqrt 2)} / ln (sqrt 2) = 0,16993
and this equals +/- 1/6 stop.

You kept me wondering why both our calculation methods gave a different
answer, because mathematicaly they are both the same.

So the Nokton is even a little bit more closer to the 1.4(1.41421...)!!

Richard.