Summarit 35mm f2.4 vs f2.5 aspherical difference

[UliWer]

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I just saw a 2009 video interview with Peter Karbe, in which it was discussed how Leica produces their MTF charts - actual lab tests, or based on the computer modelling/predictions (theoretical). Karbe said Leica used the computer modelling. But in the process the interviewer asked "What about the 35 Summarit - those MTF charts suddenly changed?" And Karbe responded (with a sheepish grin) "Yes. I think we did a mistake - we mixed [up] the MTF graphs!"

 

Note the interview is 2009, long before the f/2.4 version came out. So be sure you are comparing late (post-2009) f/2.5 MTF charts to the f/2.4.

 

About time-mark 8:40 here: vimeo.com/6595625

 

I notice in Leica's current lens brochures, they have now added the disclaimer "specifications are subject to change without notice." - and publish an "effective date."

 

 

The admission of a "mistake" in publishing the MTF-graphs is interesting.

 

Though he doesn't say, if the originally published curves were a mistake, or the changement...

 

I downloaded the sheet for the "old" version of the 35mm Summarit in 2013 when I bought the lens - long after the interview, which I didn't know until today.

 

I am not quite sure, though I think I downloaded it after the website had a major update. Before this update there where many, many mistakes on the website. Some of them were rectified, also with a little bit of "exterior assistance"   - though of course nobody outside the lens department could rectify the published MTF-graphs and I think people who really know are not constantly looking on their own website. So if one took the pains to look very close with a lot of background knowledge one would find many other mistakes on their website still today. 

 

When you compare the curves from 2013 with those published now, there are quite a lot of differences and nobody - outside the lens department - could tell, if both are right, so there are real differences, or one of them is wrong, and if so, which one:

 

Objektiv_35_Summarit-M.pdf

[Michael Geschlecht]

That's right. But you are confusing well-defined points on a scale with actual data points.

 

The aperture number half an f-stop below f/2 is 2.3784 which for simplicity's sake gets called "2.4". However when a lens' maximum aperture is given as f/2.4 you cannot derive that "exactly half a stop less than f/2" is meant. It might just as well be rounded down from f/2.35 ... or up from f/2.44 ... or whatever. We don't know.

 

Also, we don't know if 1:2.5 written on an actual lens really is supposed to mean "exactly two thirds of an f-stop below f/2," or 2.5198. It might just as well be rounded up or down from some other number.

 

So in the case of the Summarit-M lenses, all we can do is to take the nominal numbers literally, and compute the difference, in f-stops, between 2.4000 and 2.5000. And that's 0.117787, or about 2/17, or slightly less than 1/8 f-stops.

 

Of course, that's just some pointless academic exercise, as we already know that "2.4" and "2.5" aren't really meant to be taken literally. Neither are they supposed to mean, "half or two-thirds of an f-stop below f/2, respectively". Instead, they both stand for basically the same actual value which probably is somewhere close to 2.44 or thereabouts.

 

 

Hello 01af,

 

Actually, the nominal numbers for the 1/2 stop & the 1/3 stop ARE 2.4 for the 1/2 stop & 2.5 for the 1/3 stop respectively

 

And the difference is 1/6 of a stop nominally.

 

Just as the nominal full stops are: 2 - 2.8 - 4 - 5.6 and so on.

 

These are universally accepted relationships just as: 3.5 (1/3 stop) 4.5 (other 1/3 stop) 6.3 (full stop) 9 (full stop) are.

 

Please remember: The general rule is to round "F" stops to 1 decimal place.

 

Also: Beyond the decimal place kept: numbers less than 5 are dropped.  5 or over means +1 to the number kept.

 

So: 2.3784 = 2.4

 

Because: 784 is more than 499

 

None of these are numbers "picked out of a hat".

 

Best Regards,

 

Michael

Edited October 12, 2017 by Michael Geschlecht

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[01af]

Sigh ... I guess I'll never understand just why so many people are having so many problems with elementary logic

[Michael Geschlecht]

Sigh ... I guess I'll never understand just why so many people are having so many problems with elementary logic

 

Hello 01af,

 

I agree.

 

Altho in this instance it is a system where people have simply agreed to accept certain "discoformities" such as: 8 - 15 - 30 - 60 - 125

 

Best Regards,

 

Michael

Edited October 12, 2017 by Michael Geschlecht