[Leica] Ir filter and film cameras, take it off

[henningw at archiphoto.com (Henning Wulff)]

At 6:00 PM -0700 3/18/07, Frank Filippone wrote:
>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.

Interference filters are rarely used for film, 
and then usually only for very narrow angle work, 
such as macrophotography. Similarly, the area of 
application that these 'hot' filters were 
developed for initially, namely CCD videography, 
deals with much more telecentric designs.

>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.

It is in this that I believe you are slipping up :-).

While absorptive filters will absorb more of a 
given colour when the angle of incidence is 
flatter, the effect on an interference filter has 
when the incidence angle is flattened is that it 
shifts the frequency of the absorbed light, thus 
changing its spectral response significantly, not 
just the amount of absorption.

>
>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


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    *            Henning J. Wulff
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