Leica X Vario: Lens

Archive

On this page, I would like to discuss one the specific characteristics that lead me to buy the Leica X Vario, namely the lens. It is often the most prominent argument in favor of the Leica X Vario camera. Elsewhere, I mention that some users maintain that the X Vario's lens equals in quality - or is even better - than some genuine Leica M-mount lenses. In this vein, some users state that the X Vario is like a Leica M with four prime lenses: 28 mm, 35 mm, 50 mm, 70 mm - and in between. This makes up for a much more compact and lighter package than having to carry with you a Leica M with four primes... Leica also maintains that the X Vario's lens satisfies 90% of the "normal" shooting needs. On this page, I will investigate what reviewers and the available test results say, and I will also cover some lens specifics.

The Lens

The Leica X Vario features a fixed lens, a Leica Vario-Elmar 28 –70 mm f/3.5 – 6.4 ASPH. (35 mm equivalent; 18 – 46 mm real; 9 lenses in 8 groups, 2 aspherical lenses), with a zoom range of 2.5x, or the equivalence of four "classic" prime lenses: 28 mm, 35 mm, 50 mm, 70 mm. It features a distance ring and a zoom ring. The first seems to be "by wire" but feels like a mechanical ring, the second seems to be purely mechanical. While there are distance marks on the lens, it regrettably lacks depth of field indicators (as there are on old mechanical zoom lenses). See my pages about hyperfocal distances and depth of field tables for more information on this.

Figure 1: The "pure" Leica X Vario - front view

Figure 2: Leica X Vario seen from above showing the lens with two rings for setting distance and focal length - and the lens hood

The lens has a 43 mm filter thread for attaching filtera and close-up lenses. ´There you can also attach a lens hood, which you can either from Leica (see Figure 2), which is fairly expensive (90 EUR in Germany), or you buy a look-alike, which costs about one third of it (35 EUR). Leica praises the lens hood, but it does not really help, whenever you shoot against the sun. But it seems to protect the lens against rain and keeps you from touching the front lens...

I often confuse the distance ring with the zoom ring (and vice versa). A design with different surfaces for both rings would help avoid confusing them, but probably designers would find this ugly... There is a small difference in diameter between the distance and the zoom ring. But to feel it you have to move your fingers over both rings...

Find more technical information about the lens below.

A Few Technical Data

Data	Leica X Vario (Type 107)	Comment
Lens	Leica Vario-Elmar 18-46 mm f/3.5-6.4 ASPH. (corresponds to 28-70 mm in 35 mm format) 9 lenses in 8 groups, 2 aspherical lenses	For focusing, only one movable element is used. This is meant to speed up focusing.
Zoom	Optical zoom: 2.5x	The zoom range includes four "classic" primes: 28mm, 35mm, 50mm, and 70mm (equivalent).
Filter diameter	43mm	Some filters seem to cause vignetting.
Aperture range	From f/3.5 to f/16 (at 28 mm) / f/6.4 to f/16 (at 70 mm) in 1/3 EV increments	The X Vario lens is particularly slow at the tele end.
Distance setting range	30 cm/1 ft (at 70 mm focal length) to infinity; otherwise starts at 40 cm	Lens designer Peter Karbe states 20 cm as shortest distance in an interview. You can use close-up lenses for achieving higher magnification (+5 = 0.4, +10 = 0.5).
Smallest object field	Manual: 27 cm x 18 cm at a distance of 30cm and a focal length of 46 mm (70 mm equivalent)); approx. 12 cm x 8 cm according to own tests (same data)	Magnification: 0.2 for no lens, 0.4 for +5 lens, ca. 0.5 (1:2) for +10 lens

Maximum Aperture Versus Focal Length

Focal Length
Equivalent	28	35	50	70
Actual	18	23	33	46
Maximum f-Number	3.5	4.5	5.1	6.4

Actually, some data suggests that the maximum aperture for 23/35 mm should be f/4, but the camera insists on f/4.5...

Sharpness Data

Jim Fisher (PC Magazine) used Imatest to check the sharpness of the X Vario's zoom lens. Below is a tabular overview of his results:

Focal Length
Equivalent		28	35	50	70
Actual		18	23	32	46
Mimimum f-Number		3.5	4.5	5.1	6.4
Lines - Center		1,774	---	1,978	2,043
Lines - Edges		1,441	---	just below 1,800	1,900
Stopped down to f/5.6	Lines - Center	1,869	---	---	---
Stopped down to f/5.6	Lines - Edges	1,500	---	---	---
Stopped down to f/8	Lines	---	---	about the same	about the same

The lower resolution in the corners may, at least in part, be caused by electronic (software-based) distortion correction. Because pixels in the corners of the frame are "stretched" to correct for the distortion, there is, according to Imaging Resource, some loss of resolution in the corners.

A Collection of Opinions on the Lens

Leica

Let me start with what Leica itself has to say about the lens, particularly in combination with an APS-C sensor:

The integration of an APS-C format sensor in a compact camera is a highlight in itself. But the combination of such an unusually large sensor with a zoom lens in a compact camera is unrivaled in the art of camera engineering. The Leica X Vario is the world's only compact camera to offer this unique combination. A sensor that is extremely large for the camera's compact dimensions combined with the high performance Leica Elmar 28 –70 mm f/3.5 – 6.4 ASPH. (35 mm equivalent) lens ensures images of unsurpassed brilliance.
Even with a maximum aperture of f/3.5, this combination allows the creative use of planes of sharpness to lend images incomparable plasticity and depth. The wide-angle to telephoto zoom range of the Leica X Vario offers enormous scope for creative composition. Whether capturing indoor scenes that appear more spacious, impressive landscapes, or intimate portraits, Leica X Vario users can simply and quickly change the focal length to spontaneously capture the perfect moment of any situation in all its authenticity.

Maike Harberts, Product Manager for the Leica X System (from David Farkas, Red dot forum):

The greatest challenge within the project was definitely the lens – zoom range, auto focus, best optical image quality and aperture. All those things had to add up perfectly and do so for a reasonably large APS size sensor. And in the end it had to be producible. This was not as easy as it seems when you have the final product in your hands.
The AF lens of the X Vario is actually a unique synthesis between tradition, most modern optical design and precision mechanics. From an engineering point of view, a lens is always the interplay between performance, focal length, aperture and mechanical size. The focal length 28-70mm covers the most popular and most used ones by our customers. So that range was fixed quite easily. The aperture has a huge influence on the size of a lens.
The X Vario lens is designed to deliver the best image quality at any aperture and any focal length – so to achieve that, and on the other hand, not get too bulky a lens we decided to compromise in the best possible way and restricted the aperture range to f/3.5-6.4. In comparison to comparable lenses of other systems the Vario lens is actually a relatively small lens.

Actually, "paper is patient," as we say in German. The Ricoh GXR A16 camera unit was the first combination of an APS-C sensor with a (much more) compact camera, "compact" may not be quite the right characterization of the X Vario, and a maximum aperture of f/3.5 is not well known for creating photos with stunning plasticity... Nevertheless, some of my photos taken with the X Vario indeed show what Leica claims...

Peter Karbe designed the X Vario's lens. In The Leica Camera Blog, he talks about the design goals for the lens and also about the constraints. Here are some excerpts from the interview with Karbe (from The Leica Camera Blog: A Look through the Elmar-Vario Lens):

... here are the five things we really strived to achieve with this lens:
- Imaging quality equal to that of the best Leica M-Lenses
- Constant imaging quality from the minimum focusing distance to infinity
- The best conditions for fast autofocus
- A compact construction
- The familiar rugged and resilient construction of a Leica M-Lens.
Only one single element is used for focusing – namely the rear lens – while shifting the remaining lens group takes care of changing the focal length. The use of a single and relatively lightweight lens is particularly advantageous for the autofocus system, as the focusing motor now only needs to move a low mass. This in turn increases the focusing speed, which is a particularly critical factor for a contrast-detection autofocus system. Turning the focusing ring for manual distance setting doesn’t actually connect mechanically to the focusing element, but it says a lot about Leica design that it feels as if it does. In fact, the degree of rotation is sampled and electronically passed to the focusing motor.
The spec sheet for the Vario-Elmar was a sporting challenge – contrast transmission that has no need to hide behind that of the best M-Lenses. ... We could only satisfy these demands by lowering the bar somewhere else. This explains why the maximum aperture of the Vario-Elmar is relatively slow, between f/3.5 and f/6.4.
The abilities of the X Vario to isolate a subject against an unsharp background may be limited, despite its harmonious bokeh. However, in view of its exceptional imaging qualities, this is an acceptable limitation that may well be overlooked.
Imaging quality always comes in first place, be it the contrast in the zone of sharpness or a pleasingly harmonious bokeh in the out-of-focus areas. ... Thanks to its lens, the X Vario is now able to achieve this imaging quality in an even smaller format and, as a result, with an even smaller overall camera size, which is an outright winner in the “compactness” discipline of the pentathlon. This high imaging performance is maintained throughout the entire zoom range. And, as the autofocus rapidly and precisely locks onto subjects at any distance between 20 cm and infinity, the X Vario takes another two disciplines in stride.

By the way, in this interview, Karbe speaks of a minimum distance of 20 cm at the long end and 30 cm at the wide end. The manual, however, speaks of 30 cm and 40 cm - and reality is closer to this. This would have made a noticeable difference for close-up shots - so reality is a little disappointing...

Others

Here are a number of statements about the lens from some other sources:

Jono Slack: The truth of it is that the IQ is excellent, and the lens is quite useable wide open at all focal lengths, and is remarkably sharp from the center to the corners. Added to this, it has a really pleasing (to my eyes) bokeh and great color rendition. I'm not certain of a real definition of the "Leica look" but to my eyes photographs with the X Vario have a zing about them which I've come to associate with much grander Leica lenses.
Ming Thein: Let's talk about what is perhaps the most contentious part of the camera: the lens. Though the maximum telephoto aperture of f/6.3 is just a third of a stop less than most f/5.6 kit zooms, (and probably better in reality when transmission is considered, not just physical aperture) for most people seeing a "6" anywhere in the maximum aperture spec is a bit of a turnoff. In practical use, however, it isn't that much of an issue unless a) you intend to shoot handheld telephoto at night, or b) need subject isolation. In situation a), the camera defaults to being a 28mm f/3.5; not great, but not unusable either, given its decent high ISO performance and low-vibration leaf shutter. You could even attach the optional EVF (the same one as the X2 and M 240) and brace it against your face for a bit more stability.
Imaging Resource (from Review Summary): Though the 28-70mm equivalent Vario-Elmar zoom lens may not be the brightest in the world - providing maximum apertures of just f/3.5-6.4 - it's nonetheless incredibly sharp corner-to-corner. Combine the lens with an excellent 16.2 Megapixel, APS-C sensor and nimble processor, and the X Vario delivers tremendous image quality with incredibly accurate colors, with its high ISO results featuring a pleasing film-like grain.
Mike Tomkins (Imaging Resource, Review): The Leica X Vario's 2.5x optical zoom lens provides a 35mm-equivalent range from 28 to 70mm. (Actual focal lengths range from 18 to 46mm.) Maximum aperture of the Vario-Elmar branded optic varies from f/3.5 at wide angle to a decidedly dim f/6.4 at telephoto, while the minimum aperture is f/16 across the board. Taking into account the focal length crop, background blur would be similar to that from an f/5.3 lens on a full-frame body at wide angle, and by the telephoto position would equate to that of an f/9.8 lens.
The lens design features nine elements in eight groups, including two aspheric elements. The X Vario's autofocus system operates to as close as one foot (30cm) at the telephoto position.
The X Vario does feature image stabilization of a kind for both still and video recording, and it can be toggled on or off separately for either capture mode. It's not mechanical, however. For still imaging, the stabilization system simply captures two images in quick succession, and then merges them to create a single shot with reduced blur. It works only at ISO 1600 or below, between 1/4 and 1/30th second, and with static subjects. For videos, electronic stabilization is used.
Imaging Resource (Leica X Vario Review - Lens Quality - Summaries):
Geometric Distortion: Low geometric distortion from the X-Vario's 18-46mm fixed lens in JPEGs, though strong distortion at wide angle in uncorrected raw files.
Chromatic Aberration and Corner Sharpness: Low levels of chromatic aberration in JPEGs, though uncorrected raw files show higher amounts. Very good to excellent sharpness in the corners.
Vignetting: Some minor corner shading ("vignetting") is also noticeable from the difference in brightness of the center versus corner crops above, and it appears the camera is applying some shading correction.
Chromatic Aberration Suppression: The Leica X Vario applies lateral chromatic aberration correction to its JPEGs, as uncorrected raw files show moderate levels of green and magenta coloration at wide angle, and cyan/red fringing at telephoto.
Nick Rains (Luminous Landscape): Slow lens? Yes, it certainly looks that way. F/3.5 – f/6.4, what were they thinking? This seems to be the biggest gripe on the forums right now. ...
It seems Leica have looked at the cost, quality, and size equation and decided to go for high quality and small size as the priorities. This necessarily limits the max aperture but by concentrating on quality they have produced a stunner of a lens. It's tack sharp at all focal lengths, wide open and right into the corners.
Optically, it's at least as good as any mid range zoom I have used, no CA that I can see, sharp in the corners and with a lovely color rendition. In short it's a real Leica lens.
Going back to the maximum aperture issue, I would never use a mid range DSLR zoom lens at maximum aperture if I could avoid it. ... On the X Vario, f/3.5 is as good as f/8 so it's actually a faster lens in the real world because f/3.5 is still usable without compromising quality.
Jim Fisher (PC Magazine): I used Imatest to check the sharpness of the X Vario's zoom lens. It's a 28-70mm (35mm equivalent) design, with an aperture that starts at f/3.5 and dwindles to f/6.4 when zoomed in. The modest aperture means that you won't be able to create a shallow depth of field as you can with a faster lens, although it's still possible if you are working towards its minimum focus distance. That's 1.3 feet for all but the 70mm setting; when zoomed all the way in you can focus as close as 1 foot. This isn't a wide-aperture lens, so you have to work a bit in order to obtain subject isolation; it's not going to magically blur the background of almost any shot like a 50mm f/1.4 lens will do. There's no image stabilization built into the lens, but under certain conditions the camera will apply digital stabilization. ...
The more you zoom in, the better the X Vario's lens is. At 18mm f/3.5 it is just a little bit below the 1,800 lines per picture height that we require of a lens to call it sharp. Using a center-weighted test, the X Vario scored 1,774 lines at 18mm f/3.5. Edge performance was weak, just 1,441 lines, which brings the score down a bit. Stopping the lens down to f/5.6 brought the score up to a more respectable 1,869 lines, but edges still hovered around 1,500 lines. In visual terms, the edges of the squares of our test chart appeared just a little bit fuzzy at 18mm; it's something that you're not going to notice when viewing images at screen resolution, but if you're making a large print it could detract from its impact.
Things got much better around 32mm (the 50mm equivalent setting). Here the aperture narrows to f/5.1, but the average sharpness across the frame is 1,978 lines, with edges that just fall shy of 1,800 lines. Performance is about the same at f/8. When zoomed all the way to 46mm (70mm equivalent) the aperture narrows to f/6.4. The X Vario scores 2,043 lines here, with edges that approach 1,900 lines. Narrowing the aperture to f/8 delivers almost identical results. Distortion was modest, but present; at its best it's about 1.2 percent, and only 2 percent at its worst. You'll see some slight curving of straight lines, but it's easy enough to fix in Adobe Photoshop Lightroom, which is the included Raw converter.
David Farkas (Red Dot Forum): With all the design and under-the-hood improvements aside, the most visible and ostensibly major change in the X Vario is the newly designed zoom lens. With an equivalent focal length range of 28-70mm, this one lens can cover most shooting needs and features 9 elements in 8 groups with two aspheric surfaces.
The lens feels like a real Leica lens, with all-metal construction and standard distance engravings in meters and feet. I had always wished Leica would have added a manual focus ring on the X2 so this addition is most welcome on the X Vario. The ring turns smoothly and has a relatively short focus throw, allowing for reasonably quick and responsive focusing. Switching between AF and manual focus is extremely intuitive. When you hit the infinity mark on the focus ring, there is a resistance point. Turn just a little farther and the ring will click into the AF position. There is no release button or switch, just a stronger than usual detent which keeps the lens in autofocus mode when you want it and in the manual focus range when that’s your preference. This method works far better than the AF/MF button on the X2 as your left hand naturally falls on the lens while shooting and there are no selections to be made in a menu.
In front of the focus ring is a manual zoom ring with nice damped action. Both controls feel truly mechanical, rather than electronically linked to a separate motor, the norm for other cameras in this class. Perhaps equally significant is that both zoom and focus have start and stop positions instead of the more common freely turning setup. This allows you to preset the lens focal length and/or focus position by feel before aiming the camera. The one thing missing is the DOF scale, which is understandable as the DOF varies immensely between the wide and telephoto ends of the zoom. Perhaps this could be displayed on the LCD as is done with the X2 in a future firmware release.
The optics were designed by no other than Mr. Peter Karbe himself. Karbe is the head of optical design at Leica and has given the world such masterpieces as the 50mm Summilux ASPH, 50mm APO Summicron and 75mm APO Summicron for the M system. And, he's hardly a stranger to zoom lens designs. Earlier in his career he pioneered cutting edge zoom optics work which resulted in such legendary lenses as the 28-90mm f/2.8-4.5 ASPH and the 35-70mm f/2.8 ASPH for the R system.
...
I can verify that the lens is indeed a real Leica lens, a real Karbe design, and just really, really good. In my actual testing I couldn't find too many faults with the lens, if at all. The Vario-Elmar lens offers extremely solid quality with no visible distortion along with sharpness at every focal length and aperture setting. I don't have specs or MTF charts to back up this claim, but after taking over 1,000 shots I'm confident in my analysis. While some may deride Leica's choice to sacrifice max aperture in favor of optical quality, I'll take performance any day of the week. Ultimately, do you want a lens that looks good on paper, or one that produces stunning images? I'll take the latter, thank you very much.
Ian Farrel (Amateur Photographer): The X Vario's zoom optic has raised a few eyebrows, although not for the reasons Leica perhaps wanted. The most frequently made comment about the camera's lens design relates to its speed - or lack of it. A maximum aperture of f/3.5 at the wide end of the zoom is not going to set the world on fire, and f/6.4 at the 70mm end could be seen as pedestrian. However, it's worth stepping back for a moment and considering the challenge Leica faced when developing the X Vario.
The ideal zoom lens must be (1) small, (2) fast and (3) give great quality right across the image circle. In reality, though, only two out of these three ideals is achievable at any one time. Sure, you can get standard zooms that maintain f/2.8 throughout their range, but they are always large and heavy. Equally, we see f/3.5-5.6 kit zooms with entry-level DSLRs that are light and compact, but often leave something to be desired in terms of image quality. Given Leica's wish to satisfy its customers' desires for a more versatile X-series camera that still turns in great image quality, the company had no real alternative than to compromise on aperture size.
Perhaps f/6.4 is 1⁄2 stop too far, and f/5.6 would have resulted in less tutting and rolling of the eyes, but this is a camera that performs as well wide open as it does in the middle of its aperture range - and there aren't many DSLR zooms you can say that about. The X Vario is a pretty good high ISO performer, too, don't forget.
So, is the slow speed of the X Vario's zoom nothing to worry about? Well, it certainly might limit you if you like shooting handheld in low-light conditions, but if you are more conventionally inclined then don't be put off buying the camera because of its maximum aperture. It's an otherwise versatile high-quality camera.
Bigheadtaco: Another major plus for the Leica X Vario is the image quality. It all starts with a super sharp 28-70m equiv zoom lens. Yes the lens is not very bright at F/3.5 to F/6.4, so don't try shooting at night. During the day, and even when its cloudy, no problem. Also remember that the Ricoh GR V and the Nikon Coolpix A are both F/2.8, which is only 1/2 stop faster, so F/3.5 in real world shooting is negligible. However, at the long end of the lens, you're at F/6.4, so forget about BOKEH, unless you're really close to your subject, and the background is really far away. For street use during the day, I didn't find the small apertures prohibitive in most of my shots, since I shoot at F/5.6-8 most of the time anyway. However, if you decide to shoot at night, I would stick with shooting at 28mm and F/3.5.
Another point is that this is a real Leica lens, not a 3rd party rebadged lens by Panasonic or Olympus. This lens is sharp, sharp, sharp! For its size this lens performs very well (remember, this lens only zooms out 1/2", so it is a complex designed, compact zoom for it's class), and its definitely Leica quality. Could it have been faster? Perhaps, but I would assume either the quality of the image wide open would have suffered, or the lens diameter would have to be increased significantly.
Erwin Puts: The X-Vario shows a very good balanced performance at all apertures, focal lengths, object distances and over the full field of the image.
Mike Evans: The "Quad-Elmar" lens. The 28-70mm Vario-Elmar is the camera's crowning glory. This lens is a masterpiece and offers outstanding sharpness and contrast throughout the zoom range. Ergonomically it is perfect, a real Leica lens. It is an automatic electronic design, of course, but Leica has done a sterling job in simulating the feel of a traditional mechanical M optic. The zoom is smooth and quick (a rotation of about 90 degrees) and is clearly marked with the four prime-equivalent views of 28, 35, 50 and 70mm. This is a welcome nod to those prime addicts who think in terms of fixed focal lengths.

Lens not Retractable

Many compact digital cameras, like my wife's Ricoh CX4, have a retractable lens, which makes them compact and easy to put in a pocket. The main disadvantage of such a design is that it is delicate (the CX4 has a "wobbly" lens) and also that it is susceptible to dust (we had this issue already with my wife's CX).

My GXR Ricoh A16 camera unit has a different lens design, because it has an APS-C sensor. The lens is already fairly long so that it is definitely not "pocketable." It extends a little bit when turned on at the wide end (24mm equiv.), and extends more and more towards the long end, reaching nearly twice the initial length at a focal length of 85 mm (equiv.). Since there is only one moving part, I expect the design not to be susceptible to dust.

The Leica X Vario lens does not extend when the camera is turned on. It is shortest at a focal length of a little more than 50mm (equiv.), extends a little bit when turned to wide angle, and extends a little bit less when turned to tele. Since this lens also has a non-retractable design, it should not be susceptible to dust as well. Time will tell whether this is indeed the case.

Figure 3: The non-retractable lens design does not make the Leica X Vario "pocketable"...

On the other hand, both the Leica X Vario and the Ricoh GXR A16 cannot be easily put in a trouser or shirt pocket. This is the price to pay for an APS-C sensor camera with a zoom lens.

Lens Noises - What They Reveal

First of all, all the lens noises described here are OK - there is nothing wrong with the camera. In the following, I list the "usual" noises that the lens makes and provide explanations what they reveal.

Viewfinder Image Brightness

If the camera is turned "on" and you do nothing except for pointing the lens to different targets, you can here a "rattling" noise. It is created by the iris blades (or aperture blades - or by the shutter, because it is a leaf shutter...) that move to adapt the brightness of the viewfinder image. You can see the blades move when you look at the lens while moving the camera.

When you look through the viewfinder and move the camera, you can observe how the image gets lighter or darker, depending on the target. This happens rather smoothly, sometimes, however, with a slight delay. All this suggests that there is an electronic gain control involved.

People with very good eyes might even recognize that DOF changes. However, I am not able to recognize this...

Focus

When the camera is turned "off," you hear only mechanical noise when you turn the focus ring. When the camera is "on" and you move the focus ring (manual focus) you can hear the "working" of the step motor, which moves one small lens element. I put the lens cap on the camera when checking this so that there is no confusion with the first noise. In autofocus mode, you can hear the step motor work, when you half-press the shutter button for setting distance and exposure.

All these observations confirm that the X Vario lens uses a "fly-by-wire" mechanism for focusing, even though it may feel as if focusing were mechanical.

Zoom

According to colonel/harold1968, the Leica X Vario's zoom works purely mechanical. An indication of this is the fact that the lens changes its length (a little) already when you zoom while the camera is turned "off." However, when I turn the camera "on," put the lens cap on the lens to avoid confusion with other "noise sources", there is more to hear than when the camera is "off." It sounds like something is being moved when the camera is "on," sometimes even with a little delay. At the end of the zoom movement, you can also hear the "rattling" noise of the iris blade, because the camera adjusts aperture for the viewfinder after zooming.

Only lens designer Peter Karbe knows what exactly is happening...

In-Camera Correction of Lens Deficits

All lens designs are a compromise between different requirements and therefore have certain deficits - the X Vario's lens is no exception to this rule. In an The Leica Camera Blog interview A Look through the Elmar-Vario Lens, lens designer Peter Karbe states:

The design and construction of any lens is an attempt to create harmony among a number of sometimes contradictory requirements. It’s not good enough to optimize one single property of the lens; it’s much more a matter of achieving an equally high standard in many areas. I like to use a metaphor from the world of sports, comparing the process with the modern pentathlon, where only the best in all five disciplines can win.

Above, I cited "the five things" Leica "really strived to achieve with this lens" - here they are again:

Imaging quality equal to that of the best Leica M-Lenses
Constant imaging quality from the minimum focusing distance to infinity
The best conditions for fast autofocus
A compact construction
The familiar rugged and resilient construction of a Leica M-Lens.

So, there must be a downside of or a secret behind the lens design, but where is it? One downside is that the lens is fairly slow. Karbe comments on it, and many users did so, too - I do not want to join this discussion here. Another one is the fairly small zoom range of about 1:2.5 (18 mm to 46 mm). But what about the typical lens characteristics that are checked in camera tests? In such tests, the Vario-Elmar shows fairly low distortion, very little chromatic aberration, and little vignetting (see the comments on Chasseuer d'Image's findings below). But there is a secret behind these excellent values: They are the result of in-camera corrections performed in software. The newer Leica T does the same, and in the respective forums a controversial debate arose. Some users were deeply disappointed by this strategy, while others did not bother. Obviously, software corrections are common practice these days and have been implemented by Fuji, Sony, Canon, Nikon, and other manufacturers.

Distortion

Distortion correction may be easiest to recognize. But since it is already applied to JPG images and also performed automatically in Adobe Lightroom, you need a tool that can display DNG files without any correction applied. The following thread in the l-camera forum discusses distortion correction and shows a sample scene (the second sample does not have distortion correction): XVario lens distortion. Another example of distortion without correction (at 28 mm equiv.) can be found at the top of Erwin Puts's second part of the Leica X Vario review.

Chasseuer d'Image (No. 357, October 2013) finds a barrel distortion of about 0.3% at 18 mm; beyond 32 mm distortion decreases to 0.1-0.2% at 46 mm (also barrel distortion). They regard this amount of distortion as "in practice, invisible in the images." These values must, however, be electronically corrected ones.

Jim Fisher from PC Magazine finds much higher, probably uncorrected, distortion values: "Distortion was modest, but present; at its best it's about 1.2 percent, and only 2 percent at its worst."

Imaging Resource (Leica X Vario Review - Lens Quality - Summaries) summarizes that there is "low geometric distortion from the X-Vario's 18-46mm fixed lens in JPEGs, though strong distortion at wide angle in uncorrected raw files."

Details: For JPEGs, the lens produces only about 0.2 percent barrel distortion at wide angle (18 mm), which is much less than average and hardly noticeable in its images. Distortion at 46 mm is even lower at about 0.1 percent, and is also barrel-type instead of the usual pincushion (confirms the results of Chasseuer d'Image).

The authors converted raw files of the same test shots with dcraw, which does not correct for distortion. The actual barrel distortion at 18 mm that they found was quite high at about 2.3%, while barrel distortion at telephoto remained the same at about 0.1%.

Chromatic Aberration, Corner Softness

Chasseuer d'Image (No. 357, October 2013) asks "which aberration?" and concedes that it is difficult to decide whether aberration is corrected optically or electronically - but at least it is "quasi absent."

Imaging Resource (Leica X Vario Review - Lens Quality - Summaries) summarizes:

Chromatic Aberration: Low levels of chromatic aberration in JPEGs, though uncorrected raw files show higher amounts.
Chromatic Aberration Suppression: The Leica X Vario applies lateral chromatic aberration correction to its JPEGs, as uncorrected raw files show moderate levels of green and magenta coloration at wide angle, and cyan/red fringing at telephoto.

Details: Chromatic aberration in the corners is low at wide angle, and just a little higher at telephoto in JPEGs. ... As usual, color fringing gradually reduces in brightness and width as it approaches the center of the image, where it is almost non-existent. When stopped down to f/8, ... chromatic aberration is a bit lower.

Luminous Landscape: ..., no CA that I can see, sharp in the corners ...

Vignetting (Corner Shading)

Chasseuer d'Image (No. 357, October 2013) finds a decrease of 2/3 EV at the edges for f/3.5 and f/4 at the wide end (18 mm). For other focal lengths, the decrease is below 1/3 EV, which means that it is more or less invisible.

Imaging Resource (Leica X Vario Review - Lens Quality - Summaries) writes about Vignetting: Some minor corner shading ("vignetting") is also noticeable from the difference in brightness of the center versus corner crops above, and it appears the camera is applying some shading correction. When stopped down to f/8, ... vignetting (corner shading) also improves, but overcorrection at telephoto is still detectable.

Corner Softness

Imaging Resource (Leica X Vario Review - Lens Quality - Summaries) summarizes that corner sharpness is "very good to excellent sharpness in the corners."

Details: The Leica X Vario's 18-46mm lens produces slightly soft corners at wide angle when wide-open at f/3.5, and much of the softness is likely due to the strong distortion correction. Corner performance is very symmetrical (all four corners show similar sharpness), and the center is quite sharp (though sharpening halos are quite evident). Corners are sharper at full telephoto when wide-open, with excellent sharpness across the frame. When stopped down to f/8, corner sharpness improves just slightly.

These results are in line what Jim Fisher (PC Magazine) found (see above).

Additional References

Below, I list some more references to lens tests and other lens-related stuff :

Erwin Puts (The TAO of Leica): Leica X Vario, part 1 (www.imx.nl/photo/leica/leica-x-vario-part-1.html) • Leica X Vario, part 2 (http://www.imx.nl/photo/leica/leica-x-vario-part-2.html)
PC Magazine (Jim Fisher): Leica X Vario (Type 170) Review and Rating (www.pcmag.com/article2/0,2817,2422259,00.asp)