C-Biogon-T* 2.8/35 ZM: the king of flare reduction?


Introduction


Current lens design is focusing on convenience rather than on ultimate performance. Introduction of new prime lenses is relatively scarce. Zoomlenses are the norm nowadays and here the emphasis is on extending the zoomratio while preserving good quality at both ends of the range, on reducing weight and size and on even performance across the aperture range. With autofocus, image stabilization and in-camera and post-processing software many classical parameters are reduced to secondary or even tertiary importance. High quality at wide apertures is no longer required it seems (but still necessary as a means of image composition) because of the effects of IS and high ISO speeds available in current sensors. The lack of mechanical quality and stable mounts (accuracy of focus or even focus shift) are being compensated by AF and processing software. The recent announcement of Olympus and Panasonic about the new Micro Four Thirds format (MFT), emphasizing smaller and more compact lenses is an indication that consumer convenience is the new yardstick. Not without reason, let us e sensible here! To walk around with a huge body and a hefty lens system is not a joy forever. So the MFT introduction might indicate a resurrection of the classical Barnack vision (small format, big results) in a new disguise of a camera system with minute dimensions and modern conveniences.

The rangefinder scene is not yet infected by these developments. Here mechanical and optical quality, high speed lenses and a predominance of primes are en vogue. The original Tri-Elmar lens (three focal lengths, modest aperture) was not met with overwhelming enthusiasm, ignoring its excellent performance and convenience of use.
Basically there are three players on this stage: Leica, Zeiss and Cosina under the Voigtlander brand name. Traditionally Cosina has been good at delivering high value products, that is good quality and very low prices, balancing cost with performance. It would be unfair to fault the basic designs of the Cosina lenses which are very good, but is it fair to note that the production quality is uneven because of wider tolerance margins and older manufacturing equipment. Zeiss lenses are designed in Oberkochen, taking advantage of lens expertise from the cinematographic and micro-lithographic departments and of course the photographic heritage, but the production has been outsourced to Cosina. The injection of additional expertise in quality control in the Voigtlander manufacturing process has upgraded the overall mechanical quality of the lenses. Most recent Zeiss lenses now have a label saying that they are under Zeiss quality control and guarantee. Zeiss has always stressed the importance of high performance at medium apertures and design relaxation, meaning that optical performance is preserved by mutually compensating tolerances.
Leica's approach to lens design reuires very tight tolerances during manufacture and assembly, as this is a mandatory requirement for very high performance at wide apertures. To ensure this high quality a large investment in human labour during assembly is necessary. It may be surprising in his age of computer controlled manufacture, but a person with expertise can assemble and control tolerances on a one-micron level, where mechanized production can manufacture consistently at a five-micron level. Leica's challenge is to design equipment that can operate at this one-micron level consistently without too high an investment in costly human labour force.
Even Leica has to admit and acknowledge that prime lenses, even those that are delivering superior quality, cannot be sold at any price as price elasticity operates in the Leica world too. The Summarit range is Leica's answer to the challenge from the combined Zeiss/Cosina/Kobayashi attack by delivering very good quality at reasonably modest prices. Again we have to stress the relativeness of things. For one single Zeiss or Summarit lens with a price tag of about 700 to 1200 Euro, you can buy an excellent dSLR with a quite good zoomlens.

In this report I compare the new C-Biogon 35mm with the comparable Summarit 2.5/35mm.

The Biogon-T* 2.8/35mm ZM


The Zeiss C-Biogon T* 2.8/35 ZM and the Leica Summarit-M 2.5/35mm.
The basic specs for the Zeiss C-Biogon T* 2.8/35 ZM and the Summarit-M 2.5/35mm are listed below.
Left: C-Biogon 35, right: Summarit 35

diagrambiogon35summarit35

The C-Biogon 35 ZM has seven elements in five groups, the Summarit has six elements in four groups.
ZM dimensions are: 30mm (Length from bayonet flange) x 52mm (Diameter) and 200 grams (Weight), filter is M43.
Summarit dimensions are 33.9mm (length from bayonet flange) x 51.4mm (Diameter) and 220 grams (Weight), filter is E39.
The ZM has a slightly narrower angle of view or a somewhat longer focal length: 62 degrees versus 63 degrees for the Summarit. This difference can be seen in side by side picture comparisons. The Zeiss lens is closer to a focal length of 36-37mm (a 5% tolerance is just allowed when specifying the nominal focal length).
The total length of the ZM is close to 45mm as the optical unit is longer and protrudes farther into the body. The ZM lens design is symmetrical, being a true Biogon design.
The comparison is quite interesting as the Summarit range was designed and marketed partly in response to the introduction of the ZM lenses. The ZM lenses follow a hybrid approach: the optical design is from Zeiss as are the manufacturing specs and the manufacturing and the assembly are Japanese with Kobayashi fingerprint.
The Summarit lenses are Leica designs, manufactured and assembled in Germany and with a construction philosophy that incorporated aspects that adopted, adapted and improved on elements from the Kobayashi approach of manufacturing lenses.
Both lenses represent the best of German engineering philosophy: you do not question the basic values, but try to improve on the existing systems and do this on the highest possible level. Improving on a 35mm lens design with modest aperture would not be considered a worthy goal for Japanese optical companies that focus on innovative specifications. But German engineering is focused on precision, top quality and merit to be injected in proven designs.
Leica designers want to create designs that are as compact as possible, while preserving the best possible image quality. For most users, the idea of designing a compact lens, may seem to be a no-brainer, but for optical designers this goal creates a heavy headache. When you ask one of the current optical design programs to optimize a certain lens design, the result will invariably be a big lens. When constraining the optimization program to small physical dimensions, the image quality drops severely. To accomplish the twin goals of small dimensions and high performance, the lens designer's craft and genius are required.
Zeiss has a very strong heritage in cinematographic designs, being the purveyor to the Arri company. Arri by the way, has recently introduced a new set of lenses and a new camera for Super-16 movies, strongly believing that film has distinct advantages compared to digital capture in the video world. The most important characteristic for this type of lenses is freedom from flare and reflections. Zeiss claims that the ZM lenses inherit these characteristics in a homogeneous lens design adapted to photographic uses.
In this report I will study the topic of flare, which is more complicated than often assumed. You can measure flare as a drop of contrast in terms of percentage, but this number is not easily transposed to real pictures.
A general note: all pictures are totally unretouched images, directly from the Leica camera to the Capture One 4.0 software without any manipulations, other than a format change to JPEG, from the Raw originals.

Flare issues



The range of conditions under which flare (and secondary reflections) can be studied is quite extended. The angle of incidence of the light source, the position of the camera/lens, and the fact whether the light source is directly or indirectly shining into the surface of the front lens are instrumental for the occurrence of flare and secondary reflections. Flare (or stray light) originates mainly, but not exclusively in (non) effectiveness of the anti-reflection coatings,the blackening of the internal mounts and the surface roughness of the glass elements. Secondary reflections (or ghost images) have a complicated origin: reflections from the sensor surface, from the lens element surfaces, from the aperture blades and the effectiveness of the coatings all play a part, as is the number of air-glass surfaces in the optical train. The curvature of the lens elements may also be a cause of ghost images. Stray light and ghost images have partly the same cause, but they can occur separately. Basically one may state that the effectiveness of the coatings will reduce both flare and secondary reflections. The brightness of the ghost images is also an aspect that needs consideration.
Perhaps surprisingly the propensity to flare and secondary reflections increases when stopping down. See below two series of pictures with apertures at 2.8, 5.6 and 11. (Summarit lens)
All test pictures were made with manual exposure settings, based on exposure meter measurements (incident method). I noted that the camera metering would give false readings, when flare is present. Note especially the gradual reduction in overall contrast in the series on the right.


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Below you will find a classical type of picture with the sun obliquely into the lens. Below is the C-Biogon and far below the Summarit-M. Note that the Zeiss lens can differentiate the highlights more effectively
and that the Leica lens has a higher level of reflections in the bottom right corner. As a by-product you may see that the sensor has trouble capturing the overexposed areas.

biogon


summarit

A better view of the more effective flare control by the Zeiss lens can be seen below in these series of images. The C-Biogon captures more detail and the details are black without spilling over of the light into the shadows.
Below C-Biogon

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Below: Summarit-M
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More subtle effects of flare and contrast reduction can be seen below. This is a quite typical scene, where the light source is coming from an oblique angle
and is not shining into the lens. A most common situation where one would not suspect any trouble. Note the visual loss in overall and detail contrast in the Summarit-M lens
Below: C-Biogon

Below: Summarit


In the introduction I remarked that the number of parameters when studying flare is quite large and not always predictable. By now the reader may assume that the Zeiss lens is superior to the
Leica lens in a number of situations. But that is not always the case. Below is a situation ith which the Summarit-M copes best and the C-Biogona has a bit of trouble. C-Biogon is on the left.
Note too that the Summarit has a slightly wider angle of view as the un-cropped image shows.

Below’ C-Biogon


Below: Summarit-M



Conclusion:


The claim by Zeiss that the C-Biogon 35 ZM has been tuned and optimized for flare reduction can be corroborated by the results presented here. It is also evident that the Zeiss lens is not 100% flare-free. I made hundreds of pictures on tripod under a large amount of conditions. Some of the results accompany this article. But the real situation is quite complex. There are so many situations to control that you always miss one where the relative merits might be different. There are conditions were the Zeiss lens has the upper hand, but there are also situations were the Leica lens reigns. Statistically however, the score is 70 to 30. The Zeiss lens is particularly good at contrast improvement when the light source is at oblique angles from the lens, but less good when the sun is shining directly onto the surface of the front lens. The bright filter ring might be the villain here, but this is conjecture, not a fact.
Leica asserts that their lens hood is very effective and should be used wherever possible. All comparison pictures were made without a lens hood to study the performance of the bare lens designs.

It is clear from the results here, that for best contrast and best overall performance a lens hood is indispensable. The design of this little accessory may become very important as the fight against flare and secondary reflections will become more important as the final means to improve the image quality of a lens.

It is most pleasant to see that lens design still plays an important part in the imaging chain and that lens designers are aware that there is more under the sun than Nyquist limited resolution.
The Summarit-M 35mm lens offers a high performance profile and is an excellent choice as an all-purpose lens with advanced pictorial qualities.
The C-Biogon is a very interesting lens, and should be a serious candidate wherever a rangefinder user wants a high performance wide angle lens for reportage-type imagery under adverse lighting conditions.
It is a pity that the M8 sensor reduces the angle of view to that of a standard 50mm lens. See below.