View Single Post
Dorian Gray
Veteran Member
 
Join Date: May 2005
Location: Paris, France
 
2012-02-06, 04:21

Low-pass filters (e.g. anti-aliasing filters) are essential for digital sampling without artefacts. But, many photographers seem to like some artefacts in their photos (notably, ones that increase the perception of sharpness or detail).

If a perfect optical low-pass filter existed, I don't think there would be much debate on this. The filter would attenuate to zero all spatial frequencies above the Nyquist frequency, while having no effect on frequencies below the Nyquist. The full MTF of the lens would be captured to the limits of the sensor resolution, without introducing moiré and other artefacts.

The problem is that such a filter doesn't exist. If a real filter is tuned to cut most of the signal above the Nyquist frequency, it also undesirably attenuates the signal below the Nyquist. As long as it doesn't attenuate the signal to zero anywhere below the Nyquist frequency, you could theoretically reproduce the full signal strength by appropriate sharpening in software. In practical cameras, this is very nearly the case. The cameras resolve detail down to very close to the Nyquist limit, and that detail can be made as visible as you wish with suitable sharpening (example; and that's not my house, gee!).

There is an obvious downside to this need for sharpening: it introduces noise.

Photographers also find it hard to determine the appropriate sharpening level and apply it to best effect. But that's a knowledge or tools problem, and can be fixed by reading this excellent book.

We all know that removing the anti-aliasing filter allows moiré to occur. The particularly nasty colour moiré is also common if the camera uses a Bayer-filter array. However, all details in the scene are subject to artefacting, and do suffer from artefacts; you just can't see these artefacts as easily as moiré, because they don't repeat in patterns across large swathes of the image.

Ironically, it's this false detail that landscape photographers seem to enjoy, since it fills a field of grass, for example, with lots of sharp edges. Many of those edges never existed in the original scene, but who's counting blades of grass? The perception of a field full of sharp blades of grass may trump an accurate representation.

Therefore I suggest that the removal of the anti-aliasing filter does not make sense from an information theory perspective, but may make sense from an artistic-intent perspective.
  quote