image processing - Why is a physical anti-aliasing filter still needed on modern DSLRs?

I understand the purpose of the anti-aliasing (AA) filter is to prevent moire. When digital cameras first emerged an AA filter was necessary to creat enough blur to prevent moire patterns. At that time the power of in camera processors was very limited. But why is it still necessary to place an AA filter over the sensor in modern DSLR cameras? Couldn't this be accomplished just as easily by the algorithms applied when the output from the sensor is being demosaiced? It would seem that the current processing power available in-camera would allow this now much more than even a few years ago. Canon's current Digic 5+ processor has over 100 times the processing power of the Digic III processor, which dwarfs the power of the earliest digital cameras. Especially when shooting RAW files, couldn't the AA blurring be done in the post processing stage? Is this the basic premise of the Nikon D800E, even though it uses a second filter to counteract the first?

Answer

Aliasing is the result of repeating patterns of roughly the same frequency interfering with each other in an undesirable manner. In the case of photography, the higher frequencies of the image projected by the lens onto the sensor creates and interference pattern (moiré in this case) with the pixel grid. This interference only occurs when those frequencies are roughly the same, or when the sampling frequency of the sensor matches the wavelet frequency of the image. That is the Nyquist limit. Note...that is an analog issue...moiré occurs because of interference that occurs real-time in the real-world before the image is actually exposed.

Once the image is exposed, that interference pattern is effectively "baked in". You can use software to some degree to clean moiré patterns up in post, but it is minimally effective when compared to a physical low pass (AA) filter in front of the sensor. The loss in detail due to moiré can also be greater than that lost to an AA filter, as moiré is effectively nonsense data, where slightly blurred detail could still be useful.

An AA filter is just designed to blur those frequencies at Nyquist so they do not create any interference patterns. The reason we still need AA filters is because image sensors and lenses are still capable of resolving down to the same frequency. When sensors improve to the point where the sampling frequency of the sensor itself is consistently higher than even the best lenses at their optimal aperture, then the need for an AA filter would diminish. The lens itself would effectively handle the necessary blurring for us, and interference patterns would never emerge in the first place.