Why Tangential and Sagittal Rises and Falls Reversely in Many Leitz Lenses

[Mustafa Umut Sarac]

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I found at many older leitz lenses , when sagittal mtf rises , tangential mtf drops and reverse is also true.

Why ?

Nowadays , all mtf is flat but older elmar 50 reintro have the same effect as above.

 

Thank you,

Mustafa Umut Sarac

Istanbul

 

 

 

 

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[fernando_b]

Hi Mustafa.

Tangential lines measures tangential pattern, while sagittal measures sagittal pattern. Old lenses where computed manually, recent lenses where computed by computer having small computing power, present lenses have been computed by nowadays computer and software... Computing approximations and errors reduce with computing power.

Regards,

Fernando.

[wizard]

To the best of my knowledge, when tangential and sagittal resolution figures are different from one another, this usually means that the lens shows uncorrected astigmatism. Modern lenses are inherently higher corrected than older lenses, as more resolving power is needed with todays high MP sensors.

Cheers, Andy

[fernando_b]

Andy,

right! However once they couldn't spend so more time in computing, since it required "years" of computing time. Now it requires much less time. For example one of the first supercomputer of '80 years (Cray 1) could do 100 MFlops (100 million of floating point operations per second); now my PC (not a supercomputer!) can do 100 times more.

The best lenses (Leica lenses ARE in this league) was example of human genius... without powerful computers (Mandler WAS a genius).

Fernando.

[eilert anders]

In olden days, one option to achieve a flat field was to compute a lens with some astigmatism - a trade-off.  You see this more often with wide angle lenses where astigmatism is not such a problem.  The modern Leica wide angles display minimal astigmatism, as do the lenses designed for digital sensors, which demand a flat field and no astigmatism.

[adan]

Specific to the 50 Elmars: they are Tessar designs with only 4 elements (8 surfaces - but two of them usually cemented and thus stuck with the same curvature), and that restricts how much correction a designer can achieve in any era, with any means of computation.

With the understanding that an ASPH surface can sometimes do the work of two regular surfaces - an Elmar-ASPH might be able to tighten the spacing between tangential and sagittal MTFs.

Constraints on lens size and weight also restrict the degrees of freedom a designer has to correct things. The Leica SL lenses often outperform their M equivalents - but who the heck wants to carry a 35mm Summicron the size of a beer can? The primary function of a 50mm Elmar is to be tiny.

https://leicarumors.com/2019/02/28/leica-summicron-sl-35mm-f-2-asph-lens-to-be-released-on-march-1st.aspx/

Since this is the R Forum, I'll note that the 35 R lenses are jumbo-sized for a different reason - the need to clear a moving SLR mirror. Thus the 35 f/1.4 Summilux-R is also the size of a (stubby) beer can.

Edited May 4, 2020 by adan