Originally posted by Pepe Guitarra An
anti-aliasing filter is a filter used before a signal sampler, to restrict the bandwidth of a signal to approximately satisfy the
sampling theorem. Since the theorem states that unambiguous interpretation of the signal from its samples is possible when the power of frequencies above the
Nyquist frequency is zero, a real anti-aliasing filter can generally not completely satisfy the theorem. A realizable anti-aliasing filter will typically permit some
aliasing to occur; the amount of aliasing that does occur depends on how good the filter is and what the frequency content of the input signal is.
Anti-aliasing filters are commonly used at the input of
digital signal processing systems, for example in sound digitization systems; similar filters are used as
reconstruction filters at the output of such systems, for example in music players. In the latter case, the filter is to prevent aliasing in the conversion of samples back to a continuous signal, where again perfect stop-band rejection would be required to guarantee zero aliasing.
The theoretical impossibility of realizing perfect filters is not much of an impediment in practice, though practical considerations do lead to system design choices such as oversampling to make it easier to realize "good enough" anti-aliasing filters.
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can someone explain what this mean?
Nyquist Frequency is ideally taken at twice highest frequency you wish to sample (that is why CD's sample at 44kHz as one can hear to about 20kHz). The practical issue at hand is that if the repetition of the pattern on the sensor is the same order of magnitude as the pixel pitch, you will have strange banding (moire). The AA filter blurs this image to reduce moire. For MF cameras, the MP count is high enough so that a very dense pattern is needed to induce Moire. When the pattern is so dense, the effect of moire is too fine to notice. Nikon with the D800 has got to the stage of MP count that an AA filter is almost not required for patterned images. I suspect that 16MP is too low to dispense with an AA filter without introducing other problems. I'm keen to see a back to back comparison of the K5II and K5IIs.
What interests me from this is why we are still resorting to physical AA filters and not by means of optical post processing on the sensor / processor. If the physical AA filter is removed, surely more light could get to the sensor and thus reduce the high ISO noise (by improving signal to noise ratio).
(Sorry; this is a classic case that I think I understand enough for myself, but not enough to explain to others... It comes from extrapolating my understanding of audio sampling theory to video)