Archived posting to the Leica Users Group, 2007/03/18
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I'm not completely convinced..... It is true that the response of an absorptive filter depends on the thickness of the coating, and at more oblique angles, the thickness IS more than at 90 degrees. So there is a component of angle of view to spectral response. However, if true, then someone in the past would have noticed the effect using some kind of film with some ultra wide angle lens on some format including the same lens in question here and a Leica camera. I have never heard of this in the film world. Not that I am all knowledgeable or all knowing. But this fact missing gives me reason to doubt. The B+W 486 filter I reviewed, had a reflective coating. That means that instead of absorbing the frequencies of disinterest, it reflects them back and forth between the coating layers. The rays of light never go through the filter, except those in the pass band. This type of filter is significantly less susceptible to angular issues than the absorptive type, whereby all the rays go through the complete thickness and therefore are affected more. Taking this the next step, the only rays we capture on "film" IS the pass band, so these rays do go through the filter and may be affected by the angular issues already mentioned. There is another issue... as I said when I corrected someone on the filter type, the 486 filter does have a rather strange response curve in the pass band. It allows a bit more blue light to pass than say a UV filter. So it may just be that the specific filter makers specs may be the source of the problem..... BTW, your eye can may not see the areas at the edge of the pass band and especially the areas that might be causing the problem: 390nM and 690nM. The human eye is good for around 400-700nM. Some eyes go 380-780nM. This is the write up of the B+W 486 filter........ B+W Digital UV-/IR-Blocking Filter 486 DIGITAL-PRO SLIM This B+W Interference Filter has a completely colorless glass carrier coated with a number of extremely thin, partially reflecting layers with precisely computed thicknesses, similar to MC coating. The B+W Filter 486 does not block by means of absorption, but by interference of the unwanted UV- and IR radiation that is repeatedly reflected between these layers, affecting the wavelengths on both sides of the visible spectrum with a steep cut-off. It is used mainly on digital- and video cameras with CCD sensors without an integrated IR protection filter, because the IR sensitivity of the CCD sensor would otherwise cause color changes and unsharpness. That unsharpness results from the chromatic aberration of the lenses that are only corrected for visible light. In the visible range, the transmission curve is very high and straight. This filter is completely clear and it requires no increase in exposure. Its filter factor is ?. Frank Filippone red735i@earthlink.net It has nothing to do with film or the brand of filter. The problem is the color response of the filter at extreme angles. If you take an IR/ UV filter in your hand. Look through it straight on. The color through the filter is quite normal. Then slowly rotate the filter so you are looking through it at an angle. The color through the filter will turn cyan. The sharper the angle, the more cyan. Len