IR filter: is there a trade-off?

[hammam]

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It now seems almost certain that we will have to use IR cut filters to fight the magenta aberration. Okay. I'll live with that. However, I understand that Leica deliberately chose to put a very thin IR filter on the sensor, in order to get maximum sharpness. Apparently, the expected results are there, albeit with the side-effects of too much IRs entering and hitting the sensor. Now, isn't an IR filter in front of the lens going to defeat the purpose? Are we going to get a kind of «reverse trade-off», no more magenta aberration, but less sharpness?

[adan]

1) There is a big asymmetry between optical effects that happen between the lens and imaging surface (i.e. a built-in filter), and those that happen in front of the lens (i.e. an add-on filter). When I was in college a fellow student's slide show showed horrible blurring and "glowing" around shots with his 24mm Nikkor. After careful inspection we found a small scratch on the rear element, that would have been unnoticeable in pictures if it had been on the front element, yet ruined a bunch of shots just because it was in the lens/film light path.

 

This is partly an effect of scale - the distance between lens and sensor is measured in mm's, and the fine details the lens is projecting is measured in microns. At the front of the lens the subject distance and size is measured in meters or even hundreds of meters. So there is far less tolerance for aberrations behind the lens.

 

2) That being said, the front-mounted filters DO involve a trade-off - cyan tints in the corners of wide-angle pictures. This is presumably to be corrected by the camera firmware, IF the camera knows how wide the shooting lens was (via zebra-coding or other means).

 

3) The other point in favor of a front filter, is of course, that it can ALWAYS be removed for a given picture when corner image quality is more important than perfect blacks. Not possible with a thicker built-in filter.

 

In fact, in my case - and for B&W photographers as well - it will likely be a case of putting the filters ON when shooting an occasional color shot, and leaving them off most of the time.

 

Incidentally, my 15mm Voigtlander lens already shows a bit of cyan cornering even with the relatively "light" built-in IF filtration of the M8. If the internal filtering were stronger, it would be even worse.