Archived posting to the Leica Users Group, 2007/01/10
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>From the LFI German Website...... ++++++++++++++++++++ Leica M8 - Trouble in paradise? For countless followers of rangefinder photography the M8 is a dream come true. However, the celebration was soon to be interrupted, with complaints homing in from every angle of the World Wide Web. So what happened? After experimenting with the camera, several customers posted onto the Internet their discovery of peculiar magenta tinges in the reproduction of black synthetic material. Soon the problem was identified; evidently the M8 was reacting overly sensitively to infrared light. But then there was more: other M8 owners were uploading images showing highlights, mostly light sources, barred with bright stripes. This effect is known as banding and tends to occur at higher ISO settings. Leica now promises to put an end to this nuisance by offering some explanation - and relief. We shed a light on the technological correlations while also sketching out the official fixes. Aside from the two very real problems of infrared and banding, forum members were fanning the flames of another hot topic, ignited by the question of whether the M8 exposed with a colour depth of 16 bits, as stated in the datasheet, or 8 bits, as suggested by the camera's DNG file type. The answer is ... both. +++++++++++++++++ The article continues in print only. I do not get the print edition. A bit more search produces this interesting item..... Brian and all you candle mavens, note the last paragraph.... +++++++++++++++++++++++++++++++++++++++++++++++++ Why do digital camera sensors require band elimination filters for infrared light when the sensor cells already have filters for red, green and blue light? Is it because the infrared may overheat the sensor? If there wasn't such a filter, we could also use the camera for infrared photography. CCD-Sensors are more sensitive to the wavelengths of the near infrared, starting from 700 nanometres, than to visible light; but near infrared has nothing to do with heat radiation (far infrared), where wavelengths are substantially longer. The individual sensor cells are covered with colour filters to the Bryce E. Bayer standard, enabling the camera to reconstruct colours in addition to brightness information; unfortunately, as to which wavelengths are allowed to pass and which not is a little more complicated. For example: the red colour filters block green and blue but permit infrared to pass nearly unimpeded. Surprisingly, this is also true for the blue filter, despite the blue wavelength-band being far away from the near infrared. The green filter in turn blocks a sizeable percentage of infrared light. The sensor's red, blue and - to lesser extent - green-sensitive cells would therefore also register infrared light, a distortion that cannot be undone by simply altering the camera's white balance. Different materials reflect infrared differently. Rarely can this be predicted, and colour has very little to do with the phenomenon. Green foliage reflects a lot of infrared, causing the photographic reproduction to be particularly bright; this is because the plant cells reflect the infrared in the same way that soap bubbles reflect visible light. In other cases, green objects may just as easily turn out black in the infrared exposure. Two similarly coloured fabrics can have completely different reactions to IR light if they consist of different material or have been processed differently. Without an effective IR band elimination filter the resultant colour shifts are unpredictable and hard to correct. The second problem that comes with the sensor's susceptibility to infrared light is sharpness falloff. Depending on their wavelength, rays of light are refracted by different degrees of force. The consequence is the so-called chromatic aberration, which has to be corrected for the colours of visible light in order to prevent the contours in the periphery of a photo from dispersing and leaving colour fringes. Infrared light is refracted even weaker than the red light, and this deviance remains uncorrected. Older lenses feature a red marking opposite the range scale, indicating to which extent the focal plane shifts in infrared photography. If the infrared percentage of light is not filtered out, then a blurred, slightly larger IR picture will overlay the sharp picture formed by the visible light. ++++++++++++++++++++++++++ Frank Filippone red735i@earthlink.net