Sven Sturm / Product management photo Solms, April 15, 2004
To our agencies
Are Conventional Lenses Suited for Digital Photography?
Presently, no other subject concerning photography is being discussed nearly as extensively and controversially. Photographers are unsure whether they should replace their lenses by "digitally suitable" ones, or if they can take equally good pictures with their existing equipment.
As the following comments will show, the question cannot be answered by a simple yes or no, instead other questions must be answered first.
What are the Differences Between the Demands of Digital and Conventional Photography?
As with photography on film, the aim of digital photography is to "catch" and preserve fleeting moments, which in turn are then reproduced as (printed) images. In order to so, the lens refracts the light rays coming from the subject in such a way that a two-dimensional image of it is created. This image is then recorded by a lightsensitive sensor (formerly the film’s role), and stored permanently (also the film’s role in conventional photography).
Whereas the viewer of digital images can simply and easily create any degree of enlargement on her/his monitor, the very expensive enlarged prints from film were only rarely produced by specialist labs. In digital photography this popular technique only takes a few seconds and mouse clicks. This immediately reveals even the slightest flaws in the rendition quality to the viewer. Even though the digital photographer may only have the same 4 by 5 print done in the end, for which an average quality lens and an average quality film would have fully sufficed, the demands on lenses in digital photography have risen significantly - simply because every user can create enlargements at random and free of charge.
This makes it obvious that lenses of average potential which are well-suited for average-standard conventional photography immediately reveal their shortcomings when used with high-resolution digital sensors. It must be stressed that these shortcomings have always become equally visible in conventional photography with highresolution films and strong enlargements. At this point I wish to underline the fact that high-quality images, be they of digital or conventional origin, call for the same prerequisite: High-performance lenses. So, one can expect that a lens excellently suited for conventional photography will not disappoint the digital photographer as long as certain circumstances are ensured. These circumstances are discussed in the section "Which Particularities must be Considered in Digital Photography?"
What Distinguishes a Good Lens?
Every lens has a difficult task to perform. It must collect and refract the light rays coming from the subject in such a way that a sharp and exact reproduction is created on the sensor or film. In order to do so, glass lens elements are usually used. Each of these elements causes a number of aberrations, and every one of these aberrations is an obstacle on the way to creating a high-resolution and true-to-nature image of the subject. With the help of the most advanced software for simulation and optimizing, an experienced optical designer can succeed in selecting and positioning lens elements of different material, radii, and refractive indices at differing intervals so that the individual elements’ aberrations cancel each other out. The higher the demands on the image quality, the higher the challenge for the designer as well as the necessary efforts and experience will be. The optical system’s design is only one part of the way to a good lens, though. Production of the lens elements and the mechanics is the other decisive aspect. If the lens is expected to resolve structures just a few µm in size
(1µm = 0.001mm), the individual elements must be produced and positioned with the appropriate precision. At Leica, e.g. the allowance of a maximum tilt deviation from the optical axis of a mere 0.0033° is not an anusual demand. The positioning tolerance for certain elements is less than 5µm = 0.005mm. Since these extremely tight tolerances are difficult to achieve even for the experienced precision mechanic, some lenses are corrected
piece by piece. In these cases, individual lens elements or groups are displaced and then fixed so that the tolerances within the system counter-balance each other.
To allow for this technique, the foreseeing designer must arrange the lens system so that this displacement of elements and groups does not lead to new aberrations.
When the lens system is finally assembled and secured by the mechanics so that it meets all of the described demands, it must be able to take the partly harsh everyday use. It must withstand temperatures like those in a desert or the arctic. It must withstand the jolts of being carried in a rucksack or transported in an all-terrainvehicle as well as the occasional bump. And still, the elements must remain exactly in place, otherwise the outstanding image quality so many professional and amateur photographers rely on in a Leica lens would be jeopardized. It soon becomes completely clear that only the best of materials such as aluminum and brass stand up to the needs of the mechanical mount for a critical lens system.
Which Demands must the Camera meet?
In order to allow the capturing of high-resolution images on a film or sensor, the receiving medium must be mounted precisely plane, i.e. without any tilting, and at exactly the right position. Even the slightest deviations lead to reduced resolution. Since a camera consists of innumerous single parts, this demand is nearly impossible to fulfill. The individual parts’ tolerances can add up to considerable amount compared to the defined demands. This is where the Leica Camera AG turns to a special trick: The film guides in the LEICA R9, which are also the aligning plane for the sensor in the LEICA DIGITAL-MODUL-R, are only finished at the end of the assembling process. For this, the camera is mounted by its bayonet, the exactly defined interface to the optical system, and then the film guide surfaces are milled by a precision machine. This compensates the individual parts’ tolerances, the remaining being only those of the milling machine.
The LEICA R9’s film guide rails being milled
Which Particularities must be Considered in Digital Photography?
Like a film, a sensor needs a high-resolution lens if the highest of image quality is expected. But there are also specific aspects that must be considered in digital photograph
The Protective Filter:
For example, the sensor carries a protective glass filter which was not taken into account while designing the lenses. In the case of the LEICA DIGITAL-MODUL-R it is made as thin as possible for two reasons: Firstly, the sensor, which protrudes into the frame window in the LEICA R8/R9, must not obstruct the shutter’s movements. And second, this minimizes the adverse effects on the Leica R lenses’ performance. The following diagrams show how minute the thin protective glass filter’s effect is. Within the pairs, the left diagrams represent the performance without -, those on the right with the protective filter. In the lens crosssections, the filter position is immediately in front of the sensor. For easier comparison the image height (15.8mm - corresponds to half the frame diagonal) in all of the MTF-diagrams represents the sensor size in the Digital-Modul-R, while in the lens cross sections the ray paths correspond to the respective formats, i.e. to the 35mm film format on the left and the sensor format on the
LEICA SUMMILUX-R 19mm F2.8 Elmarit & 50mm f/1.4 Summilux
Leica 100mm F2.8 APO Macro Elmarit ROM
General comments on the MTF diagrams (Modulation Transfer Function)
The MTF is indicated both at full aperture and at f/5.6 at long taking distances (infinity). Shown is the contrast in percentage for 5, 10, 20 and 40 lp/mm across the height of the 35mm film format, for tangential (dotted line) and sagittal (solid line) structures, in white light. The 5 and 10 lp/mm will give an indication regarding the contrast ratio for large object structures. The 20 and 40 lp/mm records the resolution of finer and finest object structures.
Looking at the MTF-diagrams, the following becomes obvious:
The addition of the thin protective filter influences the image quality so minutely that it will not be
recognizable in the pictures.
The lenses’ imaging characteristics are not changed by the protective filter, i.e. especially powerful lenses that deliver the best images in conventional will also do so together with a sensor. Specifically the highly
corrected apochromatic telephoto lenses should be pointed out in this respect.
Micro Lenses:
The so-called micro lenses are another of the particularities concerning digital sensors. Since there are areas between the individual pixels for the respective electronic connections and controls, the pixels (the lightsensitive elements) do not cover the complete sensor surface. Therefore, the effective, i.e. net light gathering area is relatively smaller than the comparable part of the film area.
This drawback in sensors can be compensated by the so-called micro lenses. Tiny lenses are arranged in front of every pixel to gather even those light rays that would otherwise only reach the areas not sensitive to light between the individual pixels. Nevertheless, light rays arriving at too strong an angle cannot be handled by the micro lenses.
Schematic drawing of a micro lens arranged in front of a pixel
The path of a lens’ light rays reveals that the rays at the edges hit the sensor at an angle.
Ray Paths in the LEICA ELMARIT-R 19mm f/2.8
Note: Compared to a super wide angle lens’ angle of view, even the rays at the edges arrive at the sensor at only a moderate angle.
If the micro lenses for the pixels at the sensor edges would be placed exactly centrally in front of the pixels, this would lead to light fall off towards the edges caused by the stronger angle that the rays reaching the micro lenses arrive at.
Withthe LEICA DIGITAL-MODUL-R though, this effect is comparatively minor for two reasons:
All too strongly angled light rays do not come into
effect in the first place, because the sensor is smaller than the
film format the lenses were designed for.
In order to minimize the already lower light fall off towards the edges in the Leica R system, micro lens shifting is employed, i.e. the micro lenses near the sensor edges are positioned slightly off-center of their respective pixels.
Schematic drawing of a shifted micro lens in front of a pixel near the sensor’s edges
The micro lenses’ off-center position for pixels near the sensors’ edges allows the angle of acceptance to be tailored to the rays paths typical for Leica R lenses. Therefore the degree of vignetting in digital photography equals that of conventional photography. Since the sensor area is smaller than the 35mm film format, vignetting is in fact even less with the LEICA DIGITAL-MODUL-R than with conventional photography. In normal sensors, complete lines or columns on the imaging chip, the so-called transport registers, are used to read out the image data. This reduces the effective surface, as these lines and columns that are not sensitive to light cannot be used for the initial purpose. This loss can be compensated by strongly refractive micro lenses that collect the light rays of a larger area and direct them to the pixels. But, the stronger the micro lenses’ refractive power, the more limited their angle of acceptance becomes. This results in strongly refractive micro lenses working only with the nearly perpendicular light rays.
The LEICA DIGITAL-MODUL-R employs a sensor without transport registers, allowing a larger effective lightsensitive area and in turn less refractive micro lenses with a larger angle of acceptance that can direct even the light rays arriving at a very strong angle to the pixel. On the other hand, sensors without transport registers are read out using different algorithms and therefore cannot supply the "live" image that is customarily used for the viewfinder image in many compact digital cameras. Since this is not possible in SLR-cameras due to technical reasons anyway, it does not lead to any disadvantage for the LEICA DIGITAL-MODUL-R.
Summary
The top priority while designing the LEICA DIGITAL-MODUL-R was unrestricted compatibility with the existing Leica R system.
In digital as well as conventional photography, a lens with excellent optical performance is mandatory for achieving excellent pictures. The strong point of Leica lenses is their more than ample resolution. Thanks to this they display their outstanding image quality in digital photography even with the popular technique of zooming up while replaying.
The particularities of digital photography such as the protective filter and the micro lenses’ limited angle of acceptance were specifically taken into account while developing the LEICA DIGITAL-MODUL-R. The protective filter was made as thin as possible, the micro lenses are positioned off-center of the pixels towards the edges (micro lens shifting), and the critical peripheral areas do not come to bear due to the somewhat smaller sensor size compared to the 35mm film format. Even so, the extension factor of 1.37x is comparatively low.
We at Leica are therefore convinced that the high-performance Leica R lenses represent a perfect match for the LEICA DIGITAL-MODUL-R.