APO-SUMMICRON SL 35

[cbayon]

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Generally we see the new Summicron SL 35 as a  Summicron, but on the Leica Web site it has the title APO-SUMMICRON-SL 35 f/2 ASPH , so will it be the first Apochromatic 35 mm available?

[Peter Branch]

Generally we see the new Summicron SL 35 as a  Summicron, but on the Leica Web site it has the title APO-SUMMICRON-SL 35 f/2 ASPH , so will it be the first Apochromatic 35 mm available?

 

It is generally, but not universally, accepted that the modern use of the term APO by Leica on a lens does not necessarily mean is satisfies the classical criteria of Apochromatic correction, i.e. focusing three colours in the same plane.

 

In common with others, e.g. Zeiss, the term APO has come to mean a lens which focuses two widely spaced colours in the same plane but keeps the longitudinal and lateral deviation from that plane of all the colours in between to an absolute minimum.

 

Both in theory and practice this results in a lens with a significantly higher performance than one with classical apochromatic correction.

 

As far as I am aware it is indeed the first time Leica has used the term APO on a full frame lens of less then 50mm focal length.

Edited March 15, 2018 by Peter Branch

[Bill W]

Thanks for the explanation Peter. I am not technical enough to fully comprehend it but it helps. I think the first APO lens from Leica was the 180 R. Is that correct. 

[Peter Branch]

Thanks for the explanation Peter. I am not technical enough to fully comprehend it but it helps. I think the first APO lens from Leica was the 180 R. Is that correct. 

 

The first Leica of which I'm aware to use the term APO was the legendary 180mm f/3.4 Apo-Telyt - R.

 

Just to confuse the issue that lens, which I owned for some years, was actually Apochromatic - two visible light colours plus one in the near infra-red. 

It was a joint project by Leica, Eastman Kodak and the US Navy specifically designed as a surveillance lens.  I used it with Infra-Red films where it performed very well.

However despite its legendary status it was soon matched and then superseded by other Leica lenses for normal photography.  

[Bill W]

The first Leica of which I'm aware to use the term APO was the legendary 180mm f/3.4 Apo-Telyt - R.

 

Just to confuse the issue that lens, which I owned for some years, was actually Apochromatic - two visible light colours plus one in the near infra-red.

It was a joint project by Leica, Eastman Kodak and the US Navy specifically designed as a surveillance lens.  I used it with Infra-Red films where it performed very well.

However despite its legendary status it was soon matched and then superseded by other Leica lenses for normal photography.  

I owned one for several years in the late 80's. It was considered state of the art at the time.

[Jeff S]

More thorough explanation by Andy (adan) in this related thread...

 

https://www.l-camera-forum.com/topic/281798-does-anyone-expect-a-35mm-summicron-apo-in-the-future/page-1?do=findComment&comment=3461163

 

Jeff

[Peter Branch]

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More thorough explanation by Andy (adan) in this related thread...

 

https://www.l-camera-forum.com/topic/281798-does-anyone-expect-a-35mm-summicron-apo-in-the-future/page-1?do=findComment&comment=3461163

 

Jeff

 

Just like I said the two term,  APO and Apochromatic, are not universally accepted as being different.  APO can mean Apochromatic but demonstrably does not always mean three colours coincident.

 

This all stems from the fundamental mathematics governing the variation in focal length as the wavelength changes.

Focal length is a complex continuous functions of wavelength.

If such a function, (focal length), has the same value for three different values of the variable, (wavelength), then the function, of necessity, has at least two inflection points.

That's the problem.  The value of the function at the inflection points tends to be significantly different from its value at the coincident points. (The Δ is larger than is desirable.)

 

By using glass with anomalous partial dispersion it is possible to ensure that many wavelengths have a low Δ value.

That is why the top manufacturers have adopted such an approach.

 

In practice it is a much better form of correction than Apochromatic.