Leica U.S.A. response to magenta issue and WB.

Started Nov 9, 2006 | Discussions thread
Joseph S Wisniewski Forum Pro • Posts: 34,130
Comments on the Leica statement...

Stovall wrote:

Here it is.


I don't like that "official statement", at all. I see it as containing many inaccuracies. Now, I imagine the original statement is in German, and it's not only possible, but quite common, for accuracy to suffer when a technical document is translated from one language to another, but I see this statement as going beyond this. I consider it to contain a couple of outright lies.

"The transmission in the red and infrared region of the spectrum can be controlled by the layer thickness of this filter. In the case of the Leica M8, which is a very compact system, the thickness of the filter, 0.5 mm has proved to be ideal."

The first sentence is only correct if you consider the "layer thickness" to mean the thickness of an interference filter, alternating layers of metallic and dialectric materials deposited on glass. And in that case, even the thickest coatings, having hundreds of layers of exotic metals, are microns thick, and have no significant effect on the total thickness of a half millimeter thick filter.

The second sentence talks of the thickness of the glass. If the filter is done only by chemically dying the glass, then the statement is only true if they are dealing with near saturation glass chemistry, and in an infrared blocking filter, this is not the case. If they're not near saturation, then making the filter thinner just means making the dyes stronger (and yes, I'm also a glass worker and have formulated my own colored glass).

So no, no matter what kind of filter they are using, restricting glass thickness to 0.5mm has nothing to do with either the cutoff frequency, slope, or depth of the filter's IR response.

"The short back focal length is the base for the compactness and the high quality of the standard and wide-angle lenses."

The filter lives between the shutter blades and sensor surface. Leica has said they're using an R metal shutter, that leaves enough room for a thicket filter, whether or not it's used with wide angle lenses...

"The extremely thin layer of the filter, 0.5 mm prevents color fringing at the corners of an image. This phenomenon, which is also known as astigmatism"

No, it's known as chromatic aberration. Astigmatism is a monochromatic fringing. We'll put that one down to translation...

"and is frequently encountered with digital SLR cameras, is not a problem for the LEICA M8 because of the thin glass cover on the image sensor."

I've never seen an image from a DSLR that had either astigmatism or color fringing that could be traced to the thickness of the filter pack in front of the sensor, even for sensors with a four layer pack.

"The elimination of color fringing and the improvement of image resolution results in higher IR sensitivity. This causes some synthetic textiles to appear an artificial-looking purple.

The problem is a lot bigger than "some synthetic textiles".

It causes most black materials, whether synthetic and organic textiles to appear purple. The purple effect is caused by the transparency of many colored organic compounds to IR. You'll see it on black human hair, a black fur coat, black cat or dog fur, and black dyed fabric. You'll see it even if the fabric is naturally black, like black casmier. The only relatively common black pigment that does not suffer from IR contamination is carbon. This is used in some (but not all) black cosmetics, but not in fabrics.

IR contamination also causes many green materials (in particular, a rare and uncommon green plant called "grass") to appear yellow. It causes red flowers to lose saturation, and often acquire violet tints.

And it causes human skin to appear "blotchy". Areas with less subsurface blood appear unusually light, areas with increased subsurface blood appear as darker blotches. (Such irregularities in blood concentration can be due to circulation problems, or to quite normal physiological reactions that constrict or dialate capillaries). There's also a tendency for surface veins to be unusually prominent, especially in fair haired people.

"The filter is supplied as an 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."

Odd, because I've used dichroic filters on a Nikon D100 and an Epson RD-1 without "special firmware" or "6-bit coded lenses".

"White balance ensures neutral rendition of color in any light. It is based on the M8 being preset to reproduce a particular color as white. ...

For the best image quality, Leica recommends the use of the RAW mode (DNG). When working in the DNG mode, customers have the option to change the Kelvin temperature to their desired WB. For best results, it is further recommended that the specific lighting situation within the WB setting is selected." (ellipses mine)

Unfortunately, IR contamination is a "many to one" problem. You can't fix it with white balance: try to set WB to get rid of yellow IR grass, and green paint, green cosmetics, or a green dress suddenly turn blue. Try to WB out the magenta tint on the black dress, and black eyeliner turns green (and skin gets much less healthy looking).

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Ciao! Joe


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