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Camera Lens News No. 9
NAB 2000 edition |
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Professional digital photography – are Zeiss lenses fit for the task?
Capturing images on digital receivers has much in common with using conventional silver-halide film. So conventional high quality taking lenses should prove suitable for digital photography. However, there are significant differences between the two techniques. Therefore ideal lenses for digital photography – ”digital lenses” – should address the following issues:
1.
Digital image receivers are to a higher degree flat and even/plane – which is not the same – than silver halide film. The light sensitive layers in conventional film can have a thickness of 10 or more microns (1 micron = 0.000 001 meter, 1 thousandth of a millimeter). The surface may be slightly uneven depending on influence factors like temperature, humidity, geometry of camera mechanics, geometry of 35 mm film cartridge mouth, properties of backing paper of 120 mm films, time elapsed since last film transport, or even, under unfavourable conditions, film warping in the film gate of the camera.
Due to this three-dimensionality of the image receiving film surface it is rather questionable to put special effort and cost into redesigning lenses to achieve even higher flatness of field or reduced secondary spectrum. Digital image receivers, on the other hand, can be made to achieve and maintain a higher degree of flatness. For lenses used in digital photography it therefore pays to have well corrected flat field characteristics, like the Zeiss Planar (the name actually means plane field lens), Biogon, Sonnar and newer Distagon and Tessar designs.
2.
Digital photography is in wide-spread use today in product photography for catalogues and brochures. In this kind of work it is crucial that lenses transfer important fine detail like fine print lettering on labels for packshots with accuracy and clarity.
This can be achieved only when the lens produces images which are free from severe color fringes over the entire frame. This requires lateral chromatic aberration of the lens be well corrected. (However, even with a well-corrected lens color fringes may occur since one-shot digital receivers do generate color fringes under certain imaging conditions as explained in CLN 6.)
3.
Demanding digital photos are often taken with digital line scanning systems which can require a minute or more to complete an image. Contiuous light is mandatory for this kind of work as opposed to flash light usually used in photo studios. Some scanning cameras require HMI metal halide lighting, a quite costly light source widely in use with television and motion picture production. In order to limit the cost of the HMI lighting equipment, digital photographers will want to work at lower illumination levels, and as a consequence of this use larger apertures, like f/8 or even wider. However, conventional large format lenses are usually corrected for apertures of f/22 and smaller only. Their performance and detail resolution is clearly lacking at f/8. Many Carl Zeiss lenses, on the other hand, have offered detail resolutions of 200 lp/mm at f/8 – way beyond the capabilities of today’s digital receivers used in professional photography.
4.
Digital image receivers can detectradiation outside the range that is perceived by the human eye or normal films. Photographers usually do not include this radiation in their image creation ideas – simply because they cannot see it, so they cannot control it. But the light sources normally used in photography do emit such radiation – ultraviolet (UV) in case of the sun, many HMI luminaires and some studio flash tubes, vast amounts of infrared (IR) with tungsten lamps of all kinds. Digital image sensors for general photography apply special protective fil-tering to block out these parts of the spectrum.
However, there are applications where these frequency bands are to be used as is the case in scientific and forensic UV and IR photography. In these cases special lenses with extended correction are recommended, like Apochromats or even Superachromats for IR applications and special optics including elements made of quartz and fluoride to provide sufficient transparency for UV radiation. Zeiss has been a pioneer in developing and marketing these exotic lenses for many decades.
5.
Today’s image receivers come with pixel sizes ranging from 12 to 4 microns. Using pixels of this size range, resolutions from 40 to 125 lp/mm can be achieved. The resolving power of many Zeiss lenses is considerably higher, so 4 micron pixel size is in no way a problem for Zeiss lenses. On the contrary it can be advisable in some cases to even reduce the resolution of the image created by the lens to prevent aliasing effects.
Summing it up: Increasing usage of digital image receivers in professional photography has shown shortcomings of many lenses, mainly from the field of large format photography. This has caused their makers to introduce new enhanced optics, which they recommend for digital photography. Most Zeiss lenses, on the other hand, have offered the required performance for decades. This explains why the Hasselblad camera system with its Zeiss lenses has been the preferred platform for virtually every manufacturer offering digital backs for professional usage.
Zeiss has been developing and producing high performance digital cameras for scientific and defence applications for many years, guaranteeing that Zeiss lenses fulfill these needs to the fullest. The expertise from the defence field is transferred into Zeiss camera lenses, both for film and digital applications, since they become in many cases the readily available optical basis for demanding defence cameras. Zeiss’s leadership in each field reinforces our position in the others, establishing new levels of performance across the board. | |
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