Summarit lenses
Arguably the most important product announcement by Leica in a decade!

For a very long period admirers of the Leica system have argued and even pleaded with the company to produce a range of lenses that would be more affordable for the Leica user. Geoffrey Crawley showed his exasperation in a leading article when he noted that the proposal to Zeiss and Leitz engineers to design lenses that would not occupy the Olympic heights of the best of breed met with the reply: do you really want us to design lenses less good than we could make them? The success of the Canon approach with a two-tier design of lenses within the FD series (the normal high quality designs and the additional L-designation for the truly excellent designs) was noted as an example of smart engineering. Leica in particular continued to design lenses with a cutting edge approach and every single optical design was and is created with the goal to provide the best performance that money can buy.  
Leica has always claimed that high quality optical designs with a special focus on excellent performance wide open need special glasses, special surface treatment, accurate mounts, very tight tolerances and a careful manual assembly with copious quality control and individually machined parts that can be matched pairwise. 
It is certainly true that it is more difficult to ensure extremely tight tolerances when the production runs are small and a high level of manual control is required. It is easier to maintain a high level of quality and consistency in large production runs, because the manufacturing process can be constructed around the mechanics of high volume quality control and the availability of parts that are machined to the required tolerance by dedicated machinery. In large volume manufacturing the laws of statistical quality control rule and the whole manufacturing process is adapted to these laws. In small production runs, the laws of statistics can not be employed as it would generate a too high failure rate compared to the total output. 
The current state of the art
During the previous decade Leica has produce some of the finest lenses for rangefinder photography in the history, but they sometimes seemed to tend to cater for the tastes of the collector and not primarily for the working photographer. I remarked in 2005:

It is sad to note, but the focus on the collectors side of the business, may be part of the trouble. Leica has the habit of producing on an almost annual basis some products for the classically minded customers. The special editions for rich buyers and the collector-users of the LHSA may bring some welcome cash flow, but it also alienates the working photographer to recognize its true mission: the manufacture of a photographer's tool that satisfies the highest demands for high quality imagery.
Cosina/Voigtlander showed the way that designs that suited the working photographer, had a lower price tag, but delivered fair performance could become successful products. Optically the Cosina lenses were no match for Leica, but the performance of some of the designs is really good. Cosina had to create a manufacturing process that could lower production costs and stay within an acceptable level of manufacturing quality. It is a matter of debate whether the quality and the haptics can satisfy the Leica user, but it is a simple fact that the lenses do sell.

The introduction of the Zeiss ZM lenses opened a new chapter in lens manufacture and design. Zeiss proved that it is possible to combine in one lens design  excellent optical performance and a very good manufacturing quality with a reasonable price level. Zeiss used their approach of design relaxation with the Cosina manufacturing approach of component sharing, and simple component assembly with parts that have been designed for non-selective assembly and ease of fit during during the assembly stage. The innovation here is to adapt large scale manufacturing techniques to smaller production runs to reduce costs.
In my review of the M lenses I concluded that the Zeiss lenses are able to challenge the Leica designs without reaching that high level of performance wide open, but Zeiss is ahead in some design domains, especially flare reduction and the use of normal glasses to obtain a high level of optical quality. I summarized the state of the art thus:  
Germany's political problems and its industrial problems (Siemens giving away its cell phone (mobile phone) production to a Taiwanese company) signal a larger and deeper problem: the transition to a global production system and an internet-style economy and consumer power is painful, but a swift action is needed. The political parties and the majority of manufacturers in old Europe have no clue what medicine is needed. Leica is firmly rooted in the culture of this old Europe too.
There is no need to throw away the baby with the bathwater, the manufacturing values of old Europe are fine and the new Leica lenses are superb examples of traditional craftsmanship mixed with high-tech design and manufacture. But Leica needs to reinvent itself to fit into the new business model for the 21th century. The lost years cannot be recouped, but the creative potential in the company should be exploited quite soon. 
Leica lenses continued to offer superior quality at a high price level and now they got additional competition form their own lenses in the secondhand market, where one can buy excellent Leica lenses for a quite low price compared to the list price of new lenses. But Leica cannot make money in this market and has to accept the competition from their own legacy lenses. 
Leica's production capacity for the introduction of new lens designs seemed to be restricted to one or two lenses in a year. This slow pace can be explained by two different causes: the design philosophy of the company and the risk- and investment avoiding style of the Cohn/Coenen period. 
The Summarit-M range of lenses
The basic specs can be downloaded form the Leica website. All lenses have the same maximum aperture of 2.5. Actually it is closer to 2.4 because Leica wants to make sure that the indicated aperture is what you get on film or sensor. Cosina lenses often have physical apertures that are a quarter or a third stop slower than the indicated apertures.
All four lenses have the same look and feel with some differences. The 75/90mm lenses have rubberized focus rings that we also know from the current R lenses. In fact the outward appearance of these lenses has a strong resemblance to the R-line. They are beautifully designed with clean lines and a very functional look and feel. These lenses are optimized for manual focusing. The 35/50mm lenses closely follow the design pattern of the current 28/35mm Leica M lenses.
The mounts have a solid feeling, are stable without the slightest slack, and move smoothly (the feeling is very close to that of the f/2 designs). The click stops of the aperture rings have a secure positioning with just the right amount of resistance.
Accuracy is on the same level as the other Leica lenses. It is amazing to notice that the combination of new production methods and higher volume production has not compromised the traditional precision engineering for which Leica is famous.  
We may wonder how long the Elmar-M 2.8/50mm and the Elmarit-M 2.8/90mm will stay in production.
The optical cells are different for every lens, but the mounts and some other parts are shared per pair of lenses. The new 2.8/28 lens has shown the direction that Leica is taking to provide high class lenses at a reasonable cost by employing new manufacturing and production techniques. (Leica did study the competition closely and there is nothing wrong with this: Leica has a historical record of being a great improver, more than being a great innovator).
For every lens Leica took the existing design of the Summicron designs as a starting point and optimized the design in relation to the smaller aperture (2.5 versus 2.0) and using standard glasses. It is well known in the optical literature that a small selection of the 200+ glass types available on the market can be used to design most lenses.
The assertion of Zeiss that it is possible to create excellent designs using this small selection of standard glass has been substantiated as is the claim by Leica that one needs extraordinary efforts to produce wide aperture top-class designs, like the Summicron and Summilux ranges. Exotic glass types do require exceptional surface treatments and very careful mounting and adjustment procedures, processes that are very expensive in time and material and equipment.
The really exciting fact is the production Standort: it is Germany, Solms and not Japan!
The difference in image quality between the Summarit and the Summicron lenses is not easy to detect. (The older Summarit 2.4/40mm for the compact Leica camera Minilux was as good as the Summicron 2/50mm!).
The Summarit range has been designed to provide a very low level of flare and secondary reflections, one of the areas where Zeiss could beat the Leica lenses. Tests are needed to support the claim, but it is significant that Leica has stressed the fact itself.
The lenses are designed for use with the classical 35mm format and per definition for the M8 too. The Summarit range can be used on all M cameras, including the M8 (with the reduced angle of view and no change in magnification as is still often claimed). The properties of the lenses are for use with the 35mm format and are not attuned to the reduced angle of view of the M8 image capture. And all four fall within the optimum range of focal lengths for the Leica rangefinder camera, as explained in my previous viewpoint. 
The differences between the Summarit and Summicron line.
It is indeed amazing to see how much effort is required (optically and mechanically) to deliver optimum performance when the maximum aperture is just a half stop wider.
Apart form the different assembly and manufacturing technology, Summicron lenses have a long list of high-end properties like apochromatic correction, aspherical surfaces, exotic glass types, floating elements, and can focus more closely. Build quality is also a notch better and the selection of materials is different to ensure durability and longevity and accuracy under all conditions. Here the Summarit line has to prove itself over time.
Weight and size differences are as follows: (length, width, diameter, weight, close focus range, price)
Summicron 35 = 34.5; 53; E39; 255 gr; 0.7 meter; Euro 2150
Summarit 35 = 34.0; 51.4; E39; 220 gr; 0.8 meter; Euro 1250
Summicron 50 = 43.5; 53; E39; 240 gr; 0.7 meter; Euro 1450
Summarit 50 = 33.0; 51.5; E39; 230 gr; 0.8 meter; Euro 1000
Summicron 75 = 66.8; 58; E49; 430 gr; 0.7 meter; Ero 2395
Summarit 75 = 60.5; 55; E46; 345 gr; 0.9 meter; Euro 1250
Summicron 90 = 78; 64; E55; 500 gr; 1 meter; Euro 2595
Summarit 90 = 66.5; 55; E46; 360 gr; 1 meter; Euro 1250
These differences can be gleaned form the specs, but the assessment of actual performance differences has to wait until comparison tests can be done. 
The introduction of a suite of four lenses for the M at one point in time is quite unique for Leica. The Leicaflex was announced with four lenses too at the same moment of introduction (35mm to 135mm), but that is as far as I know a once only event in Leica's history. Until now. The new methods of manufacture and assembly allow a lower price and a higher volume production. Manual labour is very expensive in Germany and it slows down production speed. 
The price level of the Summarit range must be set into perspective. For a true Leica lens it is really a bargain, but a price tag of around Euro 1200 will buy you a very decent DSLR with a wide range zoom lens. 
For a long time one would have assumed that a lower cost lens line would have to be fitted with a maximum aperture of f/4. But the Summarit range has a uniform maximum aperture of f/2.5, only one half stop less than the Summicron line. This small difference might pose a problem within the Leica lens system: the Summarit range might cannibalize the wider aperture lenses. A first analysis of the performance of the Summarit lenses indicates a quite high level of performance, but the fingerprint of these lenses shows enough differences with the Summicron lenses to warrant the side by side availability and use of both systems. The Summarit lenses are exquisitely suited for general use and for longer holiday trips as the set of lenses including one or two bodies (M8 and M7) fits into a compact bag.  For heavy duty use and for image quality with great finesse the Summicron range will be the premium choice. 
The announcement and availability of the Summarit range lenses indicates a major, if not a fundamental change in the design/production philosophy and marketing approach  of the Leica company. The Leica future looks bright at that is good news. 
Aperture facts
The true aperture numbers are as follows in a range of 0.3 and 0.5 stops
full stop: 2.0000
+ 1/3 stop: 2.2449
+ 1/2 stop: 2.3784
+ 2/3 stop: 2,5198
full stop: 2.8284 
Lens designs  

The Summarit name has been historically used for the 1.5/50mm standard lens, then a derivation from a Schneider lens design. Leica historical researchers differ in their assessment if this Summarit design is an original Leitz creation or just a renamed Schneider Xenon 1.5/50mm. This original Summarit design did not represent the state of the art of the day for high speed lenses. Leitz used the name Hektor for the 2.5/50mm design by Berek, but also for a 4.5/135mm lens. The Summarit designation got a new beginning with the Summarit 2.4/40mm and this lens design gave excellent performance. The new Summarit range may be positioned as the successor of the Elmar and Elmarit lens lines. Leica might settle for three ranges: very high-speed designs that represent the best performance in this domain and designated Summilux, high speed lenses with superior definition and designated Summicron and  all-round lenses designated Summarit. With the current quality of the 2.5 designs, there is no need for a 2.8 range. 
All four look familiar, especially the Summarit 50mm is a clone of the 2/50mm, but note that the lens elements around the aperture stop have a smaller diameter than in the Summicron version. The 35mm has the front and back lens groups reversed, loses one element (the negative curvature element). The 75mm too loses an element and the 90mm is a very close relative to the original 90mm lens from 1980, the Summicron III! 

Summarit-M lenses: philosophy, handling, MTF           
 In this, first, part of the Summarit-M lenses I will focus on design philosophy, handling and the MTF comparison. In the second part I will analyze the main aspects of modern lenses, definition and flare reduction (or preservation of contrast) and in the third part I will compare the Summarit range with the Summicron counterparts.
Design approach
Leica lenses are designed, manufactured and mounted to represent the ultimate in optical performance. This approach has its price. The type of glass that has been selected, is often very difficult to treat and after being very carefully grinded to exacting specifications, must be mounted and aligned with corresponding care. This lavish attention in the world of the micrometer asks for slow and time consuming quality control.  The final product is capable of outstandingly good imagery. The performance of Leica lenses can be exploited with slow speed films and careful processing in situations where one has full control over all variables. The roots of Leica photography are defined during the formative years of the Weimar Republic. In those days the inspirational dynamism of the compact camera allowed for a new vision on ordinary subjects and human activities. The quality of the image was defined primarily by the content and the photographer’s view and optical performance was highly appreciated but of lesser concern.
Over the years Leica has introduced many designs that were eminently suited for the working photographer. The exclusive research into and know-how of the basic issues of optical performance allowed gifted designers to shift the limits of optical quality into realms where hardly anyone was able to follow.
By defining this goal of producing the best lenses, Leica has put themselves in a niche market (by price and propensity) and for most photographers in the world this level of quality is not always needed or required.
It is indeed not always easy to see the differences between a top Leica lens and a somewhat lesser design.  Many Leica aficionados claim to see and appreciate the fine differences. But many photographers do not. And you need a discerning eye and technical expertise to find the true differences and fundamental characteristics and properties of these lenses. It is not good enough just to list obvious visible differences as many Leica users and commentators are accustomed to do.
The recently introduced Summilux-M 1.4/50mm Asph and the Apo-Summicron-M 2/75mm Asph. are good examples of top-quality designs where no expense has been spared to deliver the best imagery possible. The price tag is such that these lenses are not affordable for every Leica camera owner. A moment’s reflection does indicate that this route of development of Leica lenses will push the price tag to levels that even dedicated Leica users will have difficulty to accept.
 Leica has reflected on their main strengths in optical design and manufacture and have now concluded that a broadening of appeal is necessary to survive. The twin pillars of the classical Leica lens (superior optical performance and solid mounts assembled with precise manufacture) can be balanced in more than one way: using new manufacturing techniques and an optimized supply chain, costs can be reduced while holding the manufacturing quality. A fresh and careful look at and appraisal of the fine dividing line between optical excellence and photographic usability can bring a redesign of the optical components that allow for a new balance between optimum and sufficient performance.
 The Summarit range is the first fruit of this new thinking. I have always claimed that utilization of the superior performance of the Leica lens requires training and dedication. And many photographers around the world say that they love the fingerprint of the Leica optics, but do not want to stay and are not interested in staying in the lofty heights of the Olympic top of the performance mountain.
The Summarit range follows the thinking of Berek, who wrote in 1948 in the Leica Brevier, that he certainly could design lenses with better performance than was currently available, but that he also believed that the best lens for the Leica photographer is the lens that helps him to take satisfactory pictures with joy and pleasure.
The Summarit range is indeed a joy to use. The full set of four lenses, including the M7, weighs less than two kilograms and is easy to transport in a small camera bag. For this test report I selected the M7 and 100 ISO slide film. Additional comments, where appropriate, will be made when using the M8.  
The 35 has the looks and handling of the famous Summicron (I) 35 mm, that version with 8 elements. The 50mm has the same look and feel. The focus movement is very smooth and fast, with a relatively short throw from infinity to close distance. One finger focusing is fast and secure. The lenses are very compact and I would say that this is approaching the limit. The 35mm lens in particular has the focus ring and aperture ring very close together and when changing the aperture, one almost automatically moves the focus ring. The 50mm has a slightly more spacious layout. Both lenses fit very well in the hands of the photographer and are designed for fast shooting with manual focus. In the classical Leica tradition of the uninterrupted mental flow of vision and photographic capture, these lenses operate with great pleasure and exactitude.
The 75 and 90mm have a volume that is about that same as the Summilux-M 1.4/50 Asph. The broad rubberized focus ring gives a very good grip for two finger focusing while a third finger at the same time can move the aperture ring. With the M7 on automatic,  picture taking is very easy with excellent control over the precise location of the plane of best focus and the required depth of field for selective imagery.
Every time when I switch between the M8 and the M7/MP I am aware of the advantages of the full 35mm view and the large rangefinder with the precise focusing. I am also aware of the M8 benefits and gains, but using film is certainly not an obsolete or old-fashioned act in today’s photographic world.
The Summarit lenses are esthetically pleasing to look at and offer a sturdy feeling that is just short of the rock solid feeling of the other Leica M lenses.
Overall the lenses operate with the same haptic characteristics. This is fine from the viewpoint of getting accustomed to the use of the lenses. Some Leica users will complain that you no longer feel the difference between the type of a lens: most Leica lenses have enough difference in size to feel immediately whether a 75mm or a 90mm lens is attached to the camera or to feel in the camera bag which lens you are looking for.
MTF analysis.
The most interesting diagrams are for apertures till f/4. From 5.6 on the differences are generally quite small and may not be noticed in general photography.
The Summarit lenses have maximum apertures of 2.5, where the Summicron range offers an aperture of 2.0. This difference amounts to a half stop or a 2/3 stop depending where you set the true aperture (2.4 or 2.5).
Summarit and Summicron 35
The 35mm Summarit wide open has a high contrast central part of excellent definition with gradually decreasing definition to the edges of the image. The Summicron at f/2 has a somewhat lower overall contrast, but residual aberrations are better controlled over the whole image field.
At f/4 both lenses operate in the same league. The Summarit has improved reduction of spherical aberration, but also a higher level of distortion. The Summicron shows some focus shift of a non-rigorous character. At f/5.6 the Summarit offers tighter grain and therefore a quite crisp definition of fine detail. The Summarit performance is of a very high level and certainly equal to that of the Summicron.
Summarit 40mm.
Just for historical purposes I have included the MTF graph for the famous Summarit 2.4/40mm in the Leica compact camera.Wide open the lens has a lower overall contrast and definition of very fine detail is ot as crisp as with the 35mm lens. Edges are quite soft too and one needs to stop down to /5.6 to get really convincing imagery. Distortion is totally absent and again we see the trade off between contrast wide open and distortion.
Summarit and Summicron 50
Wide open the Summarit performance neatly falls between the MTF graphs for f/2 and f/2.8 of the Summicron. This is no surprise as both lens designs are very comparable. As with the 35mm comparison, the Summicron has no distortion at all and the Summarit as a distortion of 1%, not that much, but for really exacting work just visible. 
Graphs for all apertures are very close and one may expect the same fingerprint for both lenses. Overall differences are: Summarit has slightly crisper image, more distortion and smaller footprint. The Summicron has a bigger size, no distortion and somewhat lower contrast wide open, but at f/2.8 the score is even.  
Summarit and Summicron 75
Wide open the Summicron at /2 has better overall definition than the Summarit at /2.5. The Summicron offers a half stop advantage above the Summarit and holds this advantage till f/4.0 and from then on the Summarit has somewhat crisper imagery. The Summicron has very even performance till the far edges and corners, whereas the Summarit drops slightly in performance at this part of the image.
It is intriguing to note for all Summarit lenses that the behavior at apertures from f/5.6 till f/8 is different form that of the Summicron range. Where the Summicron line generally is at its top around /4 and starts to loose some definition at 5.6 and smaller, the Summarit range holds on till /8 and then losses contrast, quite visibly at /16.
Summarit and Summicron 90
It is quite a challenge to be a match for the superb Apo-Summicron-M 2/90mm Asph. The MTF diagrams indicate that the Summarit at /2.5 is reasonable close to the performance of the Summicron at /2. High overall contrast, coupled with outstandingly good definition over the whole image field is the result of rigorous aberration control that the classical design parameters of the Summarit cannot deliver. At /2.8 the Summicron still has a higher level of performance than the Summarit, but the differences are diminishing, but even at /4.0 the Summicron has a more even definition over the full image field. When using the M8 the corner fall off will be less visible of course.
Compared with the Elmarit-M 2.8/90mm the Summarit 90 is the better option and offers a wider aperture and smoother handling. The Summarit delivers excellent performance in a very compact size and is one on my favorite lenses for general photography. 
I made test-series with slide (Fuji Provia100 en Velvia 100) in the M7 and the M8 with ISO160 and the Raw files were developed with the new Capture One LE 4.0. There is need to use a standard raw developer for results to be comparable. In the world of silver halide emulsions, it is customary to use D76 as the reference developer. Capture One may not be the best developer, but as a reference it has much to be recommended. 
Definition and contrast
There is a certain artificiality in trying to detect the optimum sharpness plane and analyzing the possible detail definition at that plane. In order to find that plane with a rangefinder camera and manual focusing asks for a very high level of mechanical engineering and a keen eye. A slight defocus (caused by the operator or the mechanical-optical tolerances) will play havoc with the image quality. Real life subjects have a  three dimensional shape and in this case a slight defocus will simply shift the plane of best focus to a different part of the subject and there will be a sharpness plane in the photo, but presumably not at the intended focus point. Depth of focus and depth of field will help to mask any defocus effects and the photographer will see a sharp picture. It is not possible to ascertain from this one image whether the best quality has been captured, but as long as it is “sharp enough”, nobody will complain. This is the basis for the current Autofocus systems. Test after test it can be shown that the AF system is not reliable and accurate, but in most scenes there will be a sharpness plane and that is enough.
Accurate focusing of the rangefinder camera relies heavily on the operator experience and the accuracy of the camera. The M8 has some disadvantages compared to the M7, MP and previous models. The sensor surface does not allow for any depth of focus and the rangefinder magnification (at 0.68) is on the edge and some would even say over the edge for accurate focusing.
I created a test setup with a low to medium contrast lighting. In my view a high contrast target is not representative of many critical situations where the performance of a lens is really required. A sunlit scene with brilliant, sharp lighting is an easy test for most lenses and even a low-cost standard zoom-lens will perform commendably in these situations.   
During my testing I had to resort to focus bracketing to find the best/optimum sharpness plane, especially with the 75mm and 90mm lenses. I also noted again that not only accurate focusing is required but also a stable camera: slight movement of the camera during the exposure will ruin the maximum quality.
The results I am to present here then represent the best that is possible with the M8 and the Summarit lenses.
Summarit-M 2.5/35mm
On the testchart the 35mm lens at aperture 2.5 gave a definition with good contrast with 50 to 55 linepairs/mm over a large part of the image area. This is an outstanding result. This performance holds for straight lines in the vertical and horizontal direction. Diagonal lines were visibly smeared and show the limit of the sensor layout with straight rows of pixels at this level of magnification. At 1:5.6 the same quality can be seen at a slightly higher contrast.
There is no coma to be seen and the very small distortion value does not become bothersome in the slides. Straight lines at the edges of the frame show very slight curvature at the corner of the frame. Vignetting is visible wide open, however, but only on film when the full vield of view is being exploited. On the M8 vignetting cannot be detected.
As a comparison I used the Tri-Elmar 4/28-35-50 at the 35mm setting.
On the testchart the lens at 1:4 gave a definition with very good contrast with 50 lp/mm over a large part of the image area. As with the Summarit lens diagonal lines were less good resolved but contrast did hold. At  1:5.6 image quality dropped a bit below the performance of the Summarit 35.
Summarit-M 2.5/50mm
On the testchart the 50mm lens at aperture 2.5 gave a definition with good contrast with 55 to 60 linepairs/mm over a large part of the image area. This is an outstanding result. This performance holds for straight lines in the vertical and horizontal direction. Diagonal lines were visibly smeared, more so than with the 35mm version.  At 1:5.6 the same quality can be seen at a slightly higher contrast. But I had to refocus to get the best result. At the same distance setting the picture showed a lower overall contrast.
As a comparison I used the Summicron 2/50mm. At aperture 1:2.5 the central definition can be set at 60 lp/mm, a notch above the Summarit, but overall contrast was a bit lower. Diagonal lines were crisper delineated however. At 1:5.6 contrast became very high at the same level of definition.
Summarit-M 2.5/75mm
At full aperture 50 lp/mm can be clearly defined with quite good contrast. Very fine detail just at the limit of the resolving power showed color artifacts, a sign that the software algorithms had trouble making sense of the information. There is also a contrast difference between the vertical and  the horizontal direction of the bar pattern, a sign that the overall optical correction is not yet at its best. At 1:5.6 the contrast improves and the detail definition visibly crispens. Fifty-five lp/mm are easily resolved and even the fine diagonal lines are differentiated.
Below left: Summarit 75, right: Apo-Summicron 75

The obvious comparison is the Apo-Summicron M 2/75 ASPH.
At 1:2.5 the Apo version has a very clean and crisp definition of very fine image details:  60 to 65 lp/mm are  resolved with excellent clarity in all directions: diagonal, horizontal and vertical. As the magnification factor is lower than with the 35mm lens, there is less chance for blurring of the lines.
At 1:5.6 the same pattern of quality can be seen, but contrast begins to drop.
Summarit-M 2.5/90mm
At full aperture 55 lp/mm can be visibly defined with quite good contrast. There is also a quite visible contrast difference between the vertical and  the horizontal direction of the bar pattern, a sign that the overall optical correction is not yet at its best. At 1:5.6 the contrast improves slightly and the detail definition visibly crispens. Sixty lp/mm are just resolved and even the fine diagonal lines are differentiated.
Below left: Summarit 90, right Apo-Summicron 90mm

The obvious comparison is the Apo-Summicron M 2/90 ASPH.
At 1:2.5 the Apo version has a very clean and crisp definition of very fine image details:  65 lp/mm are  resolved with excellent clarity in all directions: diagonal, horizontal and vertical. This lens has at 1:2.5 slightly better performance than the Summarit at 1:5.6. At 1:5.6 the Apo version has the same pattern of quality as at 1:2.5, but contrast begins to drop.

Close distance performance.
Summarit-M 2.5/75mm
The Apo version has a floating element to compensate for loss of definition at close distances.
A test at one meter revealed the following facts:
At 1:2.5 the Summarit resolved now 45 lp/mm with medium-high contrast, but the edges of the barlines are now becoming soft. At 1:5.6 contrast improve visibly in the vertical direction but drops in the horizontal direction, a sigh that aberration control is less good. At the edges color fringes can be detected.
At 1:2.5 the Apo version resolves 50 lp/mm with high contrast in all directions, with crisp edges and no color fringes can be detected.  At 1:5.6 this performance holds but the contrast drops slightly for lines in the horizontal direction.

Summarit-M 2.5/90mm
A test at 1.5 meter revealed the following facts:
At 1:2.5 the Summarit resolved now 55 lp/mm with high contrast, but the edges of the barlines are now becoming soft. At 1:5.6 contrast drops in the horizontal direction. At the edges color fringes can be detected.
At 1:2.5 the Apo version resolves 55 lp/mm with medium-high contrast in all directions, with edges that show a shade of softness, but no color fringes can be detected.  At 1:5.6 this performance improves to 60 lp/mm but the contrast drops very slightly for lines in the horizontal direction. In the diagonal direction lines are defined with high contrast and crisp edges. The Apo 90 is not known for superior close distance performance as the aberration control has been focused on optimum performance at medium distance and long distance shots. Still the image quality is at least as good that of the Summarit 90mm, if not better.
For all lenses it is advisable to avoid very small apertures. At 1:16 you will generally see a significant reduction in overall contrast and definition will suffer too.
Flare and secondary reflections
The propensity to flare is a serious issue and one of the more important sources of image degradation. You hardly can detect flare on test-charts and MTF graphs and additional tests are required to analyze flare.
The two best approaches I know of are a test for flare where you take pictures of a small dark subject against a light background, for example branches of trees against the sky. The results give you insight into the amount of flare, that is the scattering of light over the image area due to interal reflections of the mount or the surfaces of the lens elements. The second test will find secondary reflections. In this case you take pictures of a scene where you have a dark part in the lower area of the film or sensor and position the camera such that the light source is just outside the viewing angle, but just on the edge of the front element.
These tests were done on film as the sensor area is itself a source of reflections and then I am unable to tell what is the real source of flare.
Summarit-M 2.5/35mm
This lens is for all intents and purpose flare free. Even shooting with the sun at oblique angles does not induce flare or veiling glare. At 1:2.5 branches against the clear sky have a faint dark grey color where stopped down you see an intense black. Secondary reflections are also absent, even with the sun shining directly into the lens. The Summarit in this respect is better (but not by a large margin) than the Summicron 2/35 asph.
Summarit-M 2.5/50mm
The Summicron 1:2/50mm (now one of the oldest designs in the Leica scuderia of lenses) can show a central area of evenly distributed illumination when you take pictures where the sky operates as a large dome of light. The Summarit-M does exhibit a very faint and small reflection f the light source. You need some dark part in the scene to be able to see it. Overall the Summarit-M exhibits at the wider apertures some flare, which is gone when using the mid apertures.
Summarit-M 2.5/75mm
There is no trace of secondary reflections, but wide open some flare is visible when analyzing back-lit scenes of trees with thin branches. This lens can be used with confidence under all lighting situations. The Apo version has an even tighter black (on slides) and exhibits less flare wide open.
Summarit-M 2.5/90mm
In behavior the 90mm falls neatly between the 50 and 75 Summarits. There is less flare than the 50mm has, but it is slightly below the performance of the 75. The Apo version performs on the same level and given the large front lens diameter this is a very good result.
Intermediate conclusions.
The Summarit 35 and 50mm are viable alternatives for the Summicron versions. You will hardly notice the differences. Especially the 35mm is very close in performance to the Summicron asph version.
The 75 and the 90 are excellent in themselves but compared to the apo versions they have some aspects to take account of. The basic issue in my view is the accuracy of focusing. The different geometry of the rangefinder curve on the lens may account for this behavior: the focus travel from 1 meter to infinity is 75mm for the Apo 90 and 52mm for the Summarit 90 and 52mm for the Apo 75 and 37mm for the Summarit 75 (distance from 0.7/0.9 meter to infinity).

Summarit lenses, part three (february 11, 2008)

The Summarit range of lenses is a most intriguing series of lenses. With this series Leica is walking a new line: producing lenses with high optical quality and new manufacturing techniques. Tis procedure may be called cost-cutting, but this is not the best way to describe the lenses. One of the few historical instances where Leica tried a cost-cutting approach was with the Tele-Elmarit-M 2.8/90mm (II). This approach did not stand the test of the times and Leica did not pursue it. 
The current Summarit designs do own nothing to that historical example. One of the main changes between the new Summarits and the current (Apo)-Summicron versions are the all-black parts in the rear segment of the lens.
This blackening should be helpful for the reduction of flare. Indeed the level of flare for the Summarit lenses is quite low. A chack was made for the 35mm Summarit. Measurements give a value for flare of 0.53% to 0.89% for 1:2.5 and 0.48% to 0.58% for 1:8. These vales are really excellent: compare this with values between 2% and 5% for a lens like the Canon 70-300 DO at the 300mm and 70mm  settings. Many observers complain about the flare at the 70mm position and do not complain for the 300mm setting. The Summarit 35mm has a much lower flare level than the Canon at the 300mm position. The comparison is made to give substance to the numbers: a value of 0.5% in itself does not signify much: in perspective it becomes an outstanding value. 
I have often claimed that high performance lenses cannot be evaluated and assessed without recourse to elaborate measurements. 
There is no lens that does not absorb some light when transmitting the light energy. The Summicron and Summilux lenses have been designed with glasses that have been carefully selected for their good transmission values. Transmission values for the Summarit lenses are very good and are as follows:
Summarit 35mm: 0.12 EV
Summarit 50mm: 0.08 EV
Summarit 75mm: 0.11 EV
Summarit 90mm: 0.09 EV.
Generally speaking we may note that all Summarit lenses loose less than one tenth of a stop in light absorption, which is also a good indication for a low level of secondary (outward bound) reflections. 
The color transmission of the Summarit lenses is close to neutral with a slight warm bias. With a slight exception for the 75mm lens, all Summarits have identical color transmission, which is very important for colorslide users.
Aperture blades
A good measure for the mechanical precision of the lens assembly is the accuracy of the size of the aperture. Measurements give the results below:
Summarit 35mm at 1:2.5 is within 0.02 EV from its nominal size (1:16 = 0.06 EV)
Summarit 50mm at 1:2.5 is within 0.06 EV from its nominal size (1:16 = -0.05 EV)
Summarit 75mm at 1:2.5 is within 0.03 EV from its nominal size (1:16 = -0.07 EV)
Summarit 90mm at 1:2.5 is within -0.07 EV from its nominal size (1:16 = 0.02 EV)
In normal parlance these figures tell you that the nominal aperture values are accurately set to two hundredth of a stop. This level of accuracy is not often encountered. 
An interesting result is the question which direction gives the most accurate values: going from 2.5 to 16 or the other way around. For the 35mm the result is: go from 2.5 to 5.6 and from 16 to 8 to get most accurate stops. For the 50mm you should go from 2.5 to 8 and from 16 to 11, for the 75mm you shold move from 2.5 to 4 and from 16 to 5.6 for best results. For the 90mm you can choose the movement as you wish: the accuracy of the stops is the same: that is selecting the mid apertures starting from wide open or starting from the stopped down position.    
This exercise should be seen as a study on micro level: the differences between the stops is at most 0.15 stop or hardly visible in practical shooting. If you wish to approach your equipment from a scientific view, these differences are informative for the excellent mechanical quality of the Summarit lenses. 
Overall verdict
I will publish a fourth part about the Summarit lenses, covering background unsharpness as created by the Summicron and the Summarit lenses.  I do not consider these characteristics as part of the formal lens review cycle, because of the high level of subjectivity involved. 
Having used the four Summarits for a longer period (I bought all four at the start), I would give the following general conclusion, based on measurements like the ones above and practical experiences with slide film, black white film and digital capture. 
The Summarit 35 and Summarit 75 emerge as the best lenses in the range, operating in close vicinity of the Summicron versions. I would even claim that the Summarit 35mm is better than the Summicron asph version. The Summarit 75 is not as highly color corrected as the Apo version, but in all other respects quite close. 
The Summarit 90mm is a very compact lens with good handling and excellent performance. The Apo version is at least one full stop ahead of the Summarit: the Apo version at 2 is better than the Summarit at 4.5. The Summarit is optically no substitute for the Apo-Summicron and in the ideal case one needs both. 
The Summarit 50mm brings to the M range a new level of portability for a standard lens. Now we are back in the classical period where one could slide the camera with lens into one's pocket. The Summarit 50mm is a true substitute for the collapsible Elmar-M with its compact dimensions and solid engineering. Optically the Summarit 50mm is a bit restrained by the drive for compactness: a classical double-gauss design may pose its own challenges when one wishes to create a short optical cell. The Summarit 50mm is my preferred lens for the film loading M7 when I am in snapshot mood. It is beautifully compact and in many cases that is the most important characteristic.