Jono Slack Review: APO-Summicron-SL 28 f2 Asph – Dreams of Summertime

[jaapv]

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The question is: Is the designation "APO" snake oil? The answer is ambiguous Technically, no, Quality-wise, mostly no but partly yes,  sometime a full yes.

Although it appears to be a mistake that I was mentioned, I am quite happy to chime in .

The designation "APO". is not some marketing label, but a technical term in lens design.
Chromatic aberration is the failure of a lens to focus all colours in the same point, which gives rise to coloured fringes. Normally purple  or blue for Longitudinal Chromatic Aberration (failure to focus R,G, and B in the optical axis, or red/green for Lateral Chromatic Aberration (outside the optical axis)
When a lens is able to focus two colours in the plane of focus, it is called an Achromat, when it is able to do so for three it is called an Apochromat.
The assumption is that a lens with Apochromatic correction is better corrected, and for Leica lenses -and undoubtedly some other brands-  that is the case.

However, a lens can be an APO lens and not be particularly good.
Firstly, although a lens may focus in the same place for three frequencies in the spectrum, -thus being an APO lens-,  the performance for the rest of the frequencies is not defined, so the focal plane for the rest of the light spectrum can be quite wavy.
Secondly, the definition does not address the correction outside the plane of focus, so a lens can be free of Chromatic Aberration in the plane of focus, but horribly off in the OOF areas.

With Leica you need not worry,  they make sure that the focal plane is congruous throughout the spectrum and extends well into light frequencies adjacent to the visible part of the light and take great care for the OOF rendering, so the label really designates a high level of correction throughout.

Then we have the question: can MTF curves be of help in judging the performance of an APO lens?
Once again, the answer is ambivalent :To a certain extent. We can assume that a lens wilth flat and high curves will be a good performer, but it is no more than an assumption and of limited  use for comparing lenses, especially between different focal  lengths and different brands. 
MTF curves are resolution measured for different contrast ratios and not specifically meant to represent acuity. They are only valid in the plane of focus and not in the OOF areas. Sometimes they are measured (e.g Zeiss), sometimes they are calculated (e.g. Leica), and the spectral content of the light is not defined, in general assumed to be neutral white - which means very little.
A better measurement would be O(ptical)T(ransfer)F(unction) graphs: pretty rectangles which are easy to read and - importantly - take OOF performance into consideration, but OTF graphs are complicated and expensive to produce, especially for magazine reviews,  thus not in general use.

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[scott kirkpatrick]

One subtle addition that I noticed in Jono's review is that he posted MTFs for the new lens calculated for focus at infinity and also at 0.5 meters.  The close up performance is not in the announcement literature, perhaps because Leica is sensitive that this lens, unlike the 35 APO-SL-SC, does seem to lose a little resolution at the edges wide open and close up.  (The article acknowledges the fact.)  But if you look at the results stopped down and close up, they are almost identical to the lens performance at infinity and better in some places.  So when shooting close, where I normally stop down quite a bit, this is not a problem.  The new lens focuses down to 24 cm.  But 24 cm from what point and how far from the end of the lens?  I'll be curious to find out.

Edited February 21, 2021 by scott kirkpatrick

[proenca]

On 2/21/2021 at 1:34 AM, jaapv said:

The question is: Is the designation "APO" snake oil? The answer is ambiguous Technically, no, Quality-wise, mostly no but partly yes,  sometime a full yes.

Although it appears to be a mistake that I was mentioned, I am quite happy to chime in .

The designation "APO". is not some marketing label, but a technical term in lens design.
Chromatic aberration is the failure of a lens to focus all colours in the same point, which gives rise to coloured fringes. Normally purple  or blue for Longitudinal
(…)
A better measurement would be O(ptical)T(ransfer)F(unction) graphs: pretty rectangles which are easy to read and - importantly - take OOF performance into consideration, but OTF graphs are complicated and expensive to produce, especially for magazine reviews,  thus not in general use.

I’m an user of this forum for years and just found this little post from @jaapv.

I have to say, its the most informative post that I’ve read in years. 

Learned a lot, simple and direct, thank you Jaapv ! 

[hdmesa]

On 2/21/2021 at 3:24 AM, scott kirkpatrick said:

...The new lens focuses down to 24 cm.  But 24 cm from what point and how far from the end of the lens?  I'll be curious to find out.

Old post, I know, but: MFD is always measured from the sensor to the subject.

[Alberti]

On 2/19/2021 at 10:28 PM, beewee said:

I think it was meant as a joke because this thread got posted in the ‘Other L-Mount products’ forum

It is nice to read something outside my ‘bubble’. It looks like a remarkable lens.