[Ryderscope (Rodney)]

As part of attempting to be more precise in matching my Canon EF200 lens to an ASI6200 camera, I've been puzzling about the effect of the UV/IR cut off filter in the DSLR body. I've developed a working theory on this and want to test it here if I may.

The Canon EF200 EOS lens has a stated set back distance of 44.0mm. This is with the lens mounted on the camera body. So when we go to mount this on to an astronomy CCD or CMOS camera, we target the set back distance as 44.0mm. However, I am now wondering if this is correct because it ignores the possible effect of the UV/IR cutoff filter in the DSLR.

I am going to make a couple of assumptions here. The first one is that the UV/IR cutoff filter will affect the length of the optical path in the same way as the filters that we use for monochrome imaging. So if this assumption is correct then, based on traditional rules of thumb, the length of the optical path will be lengthened by an amount equal to 1/3 of the thickness of the glass in the UV/IR cutoff filter.

I have been unable to find any specifications for the thickness of the glass in the UV/IR cutoff filters so the second assumption that I am going to make is that they are 1mm thick. The increase in the length of the optical path due to the filter then is 1/3 of 1mm or 0.33mm. If anyone has any detailed knowledge of this please share.

So then, if the EOS lens is mounted on a DSLR body, the UV/IR filter is adding in an extra .33mm. If we then remove the EOS lens from the DSLR body to use it on another camera, the actual set back distance expected by the lens is 44.00mm - 0.33 mm = 43.67mm.

So, therefore, if I am using my EOS lens on a CCD or CMOS camera, my target set back distance should be 43.67mm NOT 44.0mm.

I would be happy if others could pull apart my assumptions and analysis above and comment accordingly.

Clear skies,
Rodney