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can anyone give me the idiots guide to what the AA filter does. why it is used etc. have done a fair bit of googling but the answers arent sinking in.

This is probably the simplest explanation I have seen:

The anti-aliasing filter essentially blurs the image slightly in order to reduce resolution to below the limit of the digital sensor

Basically, when the lines in the picture get close to the resolving limit of the sensor, Artifacts known as moire patterns start to appear. By blurring the image near this limit, these artifacts are eliminated or substantially reduced.

You can see moire patterns by looking through two pieces of fly wire spaced a little bit apart. This is what the AA filter is trying to eliminate.

Follow the link for the full article and some examples.

Mr Darcy wrote:This is probably the simplest explanation I have seen:

The anti-aliasing filter essentially blurs the image slightly in order to reduce resolution to below the limit of the digital sensor

Basically, when the lines in the picture get close to the resolving limit of the sensor, Artifacts known as moire patterns start to appear. By blurring the image near this limit, these artifacts are eliminated or substantially reduced.

You can see moire patterns by looking through two pieces of fly wire spaced a little bit apart. This is what the AA filter is trying to eliminate.

Follow the link for the full article and some examples.

ok, thats pretty good. why do some cameras have a stronger AA than others ? if the resolving limit of the sensor is the determining factor in the AA, then shouldnt all 12mp say sensors have the same AA filter ? or do manufacturers apply the filter differently according to preference ? and do DX cameras have a stronger AA filter than FX ?

the reason i ask is that having been using the d5100 almost exclusively lately, i find the images dont have the same bite and clarity as the d700, not even close actually. i was talking to a guy about it and he said alot of that is due to the AA filter. he did go on to explain a little but i was totally lost.

Short answer. I don't know.
Longer answer. My guess is that there will be a call by the designers as to what is optimum, so each design team will come up with a different AA filter for any given sensor.
Given that the sensor cells are bigger(Bigger sensor. similar Mp) on the 700 than they are on the 5100, Then I would expect that the AA filter on the 700 to be stronger ( cut in at a higher level) than the one on the DX sensor. However, it is all relative to the cell size. We are back to the designer's choice.

While the anti-aliasing filter reduces moiré, there's also the infrared blocking effect which may be logically different but as far as I know is the same filter.

Murray Foote wrote:While the anti-aliasing filter reduces moiré, there's also the infrared blocking effect which may be logically different but as far as I know is the same filter.

Taking a simplistic approach at viewing this (as I always do), these are two very different things though. The IR filter will surely be due to wavelength filtering properties imbued within the glass, whereas the AA filter (properties) are more of an optical defocus effect, that affects a much broader spectrum; one that is not wavelength based.

Chris, what Greg has said sounds about right. Also bear in mind that even cameras that ostensibly use the same or very similar sensors may have, at their heart, a physically different sensor size (compare width and height across the Nikon DX range to see what I mean) and thus the internal componentry may not be quite what what we expect in terms of overall sameness across the broad spectrum of the camera range.