Introduction

It’s been a long wait, principally due to difficulties sourcing materials as a result of the pandemic, but finally, here is the Leica 21mm Super-APO-Summicron-SL. I think we were all expecting the 24mm to be the sixth lens in the range (after the 90, 75, 50, 35, 28mm lenses), but perhaps 24mm has fallen out of favour. At any rate, here is the 21mm.

I have had a copy of this lens on and off for a while, so I’ve had a lot of time to learn about it, and how to make the most of it. 

APO lenses

This is a subject that has caused much confusion (and conviction) among people all over the internet, and I thought it might be worth reverting to the subject here. If you’ve read it all before, then you can skip this bit!

So, what exactly is an APO lens? The truth is that it’s rather a vague term and not to be confused with Apochromatic, which isn’t vague at all!

Longitudinal Chromatic Aberrations occur when different colours focus at different distances from a lens and cause colour fringing around high contrast edges. An Achromatic lens is one which is corrected to ensure that two wavelengths of light focus in the same plane — an Apochromatic lens is where 3 wavelengths of light focus on the same plane. This is well and good, but it doesn’t cover the bokeh, so although it’s relevant where an image is completely in focus, it isn’t necessarily when it’s not.

With Leica’s APO lenses, chromatic aberrations are reduced using low dispersion glass elements in the lens design. The intention is to reduce chromatic aberration in all areas, both in focus and out of focus. As more APO lenses have appeared, they have refined the design so that they have become more and more successful, culminating in the SL Summicron lenses (and the 35 & 50 APO Summicron-M lenses).

So, for practical purposes, we might define an APO lens as one which contains one or more low dispersion glass elements. 

The official Leica position

Indeed, Leica has made this statement:

Apochromatic and more 

The prefix ‘APO’ in the name denotes Leica lenses with the best imaging performance. From telephotos to wide-angles, all Summicron-SL lenses in the SL-System portfolio bear these coveted three letters before their names. For a long time, this distinction was reserved exclusively for telephoto lenses, as only they could achieve the exceptional quality required. It is now over 40 years since the first APO lens built by Leica made its appearance — the APO-TELYT-R 180 mm f/3.4, in 1975. In 2012, the APO-Summicron-M 50mm f/2 ASPH. became the first standard lens to bear this prestigious prefix.

Thanks to technological progress, more complex construction and new manufacturing methods employed in the SL-System; the design engineers were able to perfectly maximise the optical performance. To us at Leica, APO means more than just the correction of longitudinal chromatic aberration, for which the purely scientific definition of the term ‘apochromatic’ stands.

Here, Leica is emphasising that the Leica APO lenses (at least the modern ones) aim to remove chromatic aberrations completely. 

But this is not just about Chromatic aberration. As Peter Karbe explained to me, the lack of aberrations results in much more contrast at the point of focus than in an ordinary lens, and that this contrast falls off rapidly in front and behind the point of focus. This means that an f/2 lens can appear to have the same depth of field as an f/1.4 lens (or even less in the case of the 75 f1.4 compared to the 75 f2 APO M lens).

I think that this rapid fall off in contrast confers a special look on the Leica APO lenses, which are very detailed when in focus, but with a quick roll-off into gentle bokeh: Indeed, I think the whole look is ‘gentle’. 

It’s important to make the distinction between ‘sharpness’ and ‘detail’. Very often, older lenses, when stopped down, can appear extremely ‘sharp’ and even ‘crunchy’, this is because of tiny amounts of longitudinal CA enhancing the edges of high contrast areas. Leica APO lenses might superficially look less ‘sharp’ but actually show more detail than their less sophisticated cousins.

Leica 21mm Super-APO-Summicron-SL f/2

The 21 APO SL shares many features with the other 5 lenses of the system:

A maximum magnification of 1:53  

(smallest object field, 127 x 191mm)

This is as a result of the even closer focus distance of 0.21 metres. In fact, the other lenses have a barely perceptibly larger magnification (with a smallest object field of 120 x 180mm). This difference isn’t relevant in practical use.

The same body size: 102mm x 73mm

the lenses vary a little in weight, but are all between 700 and 800 gm without their lens-hood. The Leica 21mm Super-APO-Summicron-SL is 789 gm. All the lenses have an E67 filter thread.

AF System with Dual Synchro Drive

Autofocus speed is limited by the mass of the lens elements moved whilst focusing and the travel of the elements between the closest focus and infinity.

The APO SL lenses employ two extremely light focusing elements, which combine in their lens concept based on double internal focusing. 

Lens design

The Leica 21mm Super-APO-Summicron-SL has 14 lenses arranged in 3 groups, it has 3 Aspherical elements and 2 floating elements. In addition, most of the lens elements are made of a special custom glass which exhibits anomalous partial dispersion — where light is higher or lower in a limited range of wavelengths than in the rest of the spectrum.

The MTF curves are excellent, and show very little penalty of shooting at f2  (or close up) and bear out Peter Karbe’s “Why Stop Down” mantra. They are perhaps not quite as earth-shattering as those for the 35 APO Summicron SL, but quite as good as those for the 28mm.

Image quality

So much for numbers and graphs, does it take good pictures? Hopefully, the samples here answer the question!

As you would expect, for landscape and architectural photography it performs brilliantly, with very little distortion, wonderful colour and exceptional contrast. 

More challenging are the images with some sharp areas and lots of out of focus area, and this is something which often challenges wide-angle lenses (think of nervous bokeh and nasty roll-off between in and out of focus areas). The Leica 21mm Super-APO-Summicron-SL is properly part of the APO team, with incredible detail right up to the 0.21 minimum focus distance, a lovely roll-off to out of focus and then an excellent creamy and delicate bokeh. 

Conclusion

There are now six lenses in the Leica APO Summicron range for the SL cameras:

• 21
• 28
• 35
• 50
• 75
• 90

Hopefully, there will be more to come — it would be lovely to see a 24 and a 135 at least (if it would fit in the form factor). 

But the real point is that Leica has produced a complete range of really high-quality lenses for the SL system. 

More than that, all lenses are all the same size and form factor, and they all have the same magnification ratio. 

Leica have eschewed the bragging rights of very large apertures while still managing to produce the effect of the limited depth of field those lenses provide.

The image quality is exceptional, and consistent across the range of lenses. I think it’s a fantastic achievement — the ‘best’ is difficult to define, but there can be few better lenses than these.

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Acknowledgements

First, thank you to Emma, who not only puts up with me testing the gear, but also writing the articles.

Thanks also to Steffen Rau, Product Manage at Leica for being patient and helpful, and providing extra information required for finishing this article.

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Above, clockwise: Sunburst Leica SL2 1/2000th f/7.1 ISO 100; Rose Bay Willow Herb Leica SL2 1/320th f/16 ISO 100; Selfie at the Mothership
‍Leica SL2 1/100th f/3.5 ISO 100; D‍avid’s Ferrari Leica SL2 1/80th f/2 ISO 100; Leica, Leica SL2 1/100th f/2 ISO 100

Read more reviews from Jonathan Slack

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