Flaws in the new Leica M8 spark colorful debate.
Gero Furchheim, Division Manager Corporate Communications, Leica Camera AG, responds to concerns about the Leica M8 digital rangefinder:
The glass cover of the image sensor of the LEICA M8 is a combination of IR barrier filter and specially coated protective glass. The transmission in the red and infrared region of the spectrum can be controlled by the layer thickness of this filter. The comparatively small thickness we have chosen has proved ideal in the case of the LEICA M8 which is a very compact system. The short back focal length is the base for the compactness and the high quality of the standard and wide-angle lenses. Anyhow the resulting oblique angle of the incident light on the sensor requires special adaptations of the filter.
Absence of color fringing / Image Resolution
Due to the particularly thin layer, disturbing color fringing at the corners of the image is avoided. This phenomenon, which is also known as astigmatism and is frequently encountered with digital SLR cameras, is not a problem for the LEICA M8 due to the thin glass cover on the image sensor. This feature, plus the particularly high imaging quality of Leica M lenses, is the reason for the high corner-to-corner image resolution.
Rendering of black synthetic fibers
The elimination of color fringing and the improvement of image resolution have the side effect of higher IR sensitivity. Some synthetic textiles are therefore rendered in an artificial-looking purple color.
If the higher IR sensitivity has a disturbing effect in certain applications, e.g. fashion photography, LEICA Camera AG offers its customers a special IR barrier filter. This is screwed on in front of the lens and is an ideal combination of IR, UV and protection filter.
The use of the additional IR filter in front of the optical system has big advantages as the filter does not create reflections inside the optical system and thus enables the reproduction of finest tonal values even in shadows.
The filter is supplied as accessory with a special firmware adjustment which will be available shortly after the planned market launch of the camera at the end of November 2006.
The IR/UV filter is only suitable for use with digital M cameras and 6-bit coded lenses.
(Furchheim later added the following clarification: The IR filters do not generally require a 6 bit coded lens as I wrongly stated in my last message. We only recommend that lenses are 6 bit coded in the case of wide-angle lenses from 16 to 28mm to eliminate possible colour shifts.)
The high IR transmission may also be a creative advantage for applications in the infrared photography.
As clearly noted by Gero, the cause of the problem stems from the use of a thin IR cutoff filter (located between the Kodak-manufactured sensor and the rear lens element, see photo). Leica apparently chose a 0.5mm thick filter (mentioned in other press material) to prevent astigmatism and color fringing that might occur with a thicker IR filter. It should also be noted that the Leica M8 lacks an optical low pass filter (OLPF) used on most other digital cameras to prevent moiré patterns in highly detailed areas (more on this here). Forgoing the low-pass filter actually helps improve the resolution of the camera, albeit at the cost of increased moiré patterns.
Why don't thicker IR cutoff coatings -- and additional OLPFs -- cause noticeable color-fringing problems with digital SLRs such as the similar-priced Nikon D2Xs? The difference in the distance between the rear lens element and the sensor is the key. The Leica M8 lacks a flip up mirror required by a DSLR to redirect light to the viewfinder, but instead uses a rangefinder located to the side of the lens to aid composition. This arrangement has allowed Leica engineers to design a thinner camera body with smaller lenses for its famous line of 35mm film cameras, resulting in a far shorter distance between the rear lens element and either 35mm film or an imaging sensor. However, this shorter distance exacerbates problems with CCD or CMOS sensors, as rays of light coming in from the corners of the lens are at an oblique angle to the pixels and may not be detected without the help of microlenses located on each pixel. (For more information on this topic, click here.)
The Kodak sensor used in the M8 has specially offset microlenses located towards the edge of the sensor that optimize its sensitivity to angled light. A good illustration of this is found here.
Leica failed to mention that dark black hair and other black items (besides synthetic textiles) might also be problematic. And high IR sensitivity has been known to cause skin defects hidden below makeup or veins below the skin to appear in photos, although we have yet to test for this problem in the M8. However, Leica assured us that the increased IR sensitivity will not affect the camera's white balance system.
The solution Leica has proposed -- an optional IR filter for digitally coded lenses -- may not go over well with customers. Apart from colored filters for black-and-white photography, most Leica shooters shy away from optical filters or even protective UV filters, having already gone the extra mile to purchase some of the sharpest (and most expensive) lenses available. To use the M8 for, say, street or fashion photography, you'd need to buy a separate IR cutoff filter for each lens (at an estimated price of $100 to $150, given current street prices for third-party IR cutoff lenses), plus pay at least $100 per older M lens to have it digitally coded to work optimally with the M8.
On the bright side, the M8 could be a useful camera for capturing IR images (using opaque IR filters to block visible light) due to its increased IR sensitivity. But forget hand holding the camera or using it for IR street photography, as the built-in IR cutoff filter forces you to use longer exposures, higher ISOs, and even a tripod to get a good exposure. There are far cheaper and more sensitive solutions available for IR photography, including Fujifilm's new S3 Pro UVIR DSLR (see our Field Test).