Do IR filters themselves cause cyan corners ?

[freecitizen]

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It has been stated in various threads here that if the angle of incidence of light onto an IR filter becomes too steep, this alters the wavelength which is filtered. The M8 sensor coverglass incorporates just such a filter, although a relatively weak one which does not filter out all the IR light. An angle of approximately 34 degrees has been suggested as the point at which some of the red end of the visible spectrum starts being chopped. This appears to lead to increased cyan ( as red is removed ) in the corners of the image where the angle of light on the sensor is at its steepest.

 

Now let's consider a lens such as a typical 21m lens with approximate angle of view of, say, 90 degrees. Wider angle lenses will have even more coverage, maybe more than 110 degrees in some cases. Light entering such a lens must surely be at quite a steep angle at the edges. I would guess that at the extreme edges of a lens with a 90 degree angle of view, light rays would have to be falling on the lens at about 45 degrees, otherwise the lens could not have 90 degrees of view. So what happens when an IR cut filter is placed in front of this lens ?

 

My question is this .... does it behave the same way we are being told a similar filter behaves when at the other end of the lens and used as a sensor coverglass ? In other words, will placing an IR filter in front of a wide or very wide angle lens cause cyan corners by itself because of the steepness of the angle of light coming into the lens from the scene at the edges of the image captured ?

 

The cyan corners only seem to appear when an IR filter is placed in front of wide angle lenses. Is it possible the filter in front of the lens is causing this .... rather than putting the entire blame on the angle of light hitting the thin sensor coverglass filter ?

[sean_reid]

I'm doing in-depth testing of this aspect right now. In short, angle of view is only one of several factors that govern the degree to which one sees a cyan drift in files made with the 486 filters. Yes, the filters are the cause (beyond a certain AOV they start to block red as well as infrared in the outer zones) but there are interactive effects (with respect to the lenses) as well.

 

Cheers,