In praise of AA filters

Started Oct 23, 2013 | Discussions thread
rovingtim
Veteran MemberPosts: 8,241
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Detail above Nyquist in MTF
In reply to Lee Jay, Oct 27, 2013

Lee Jay wrote:

rovingtim wrote:

Lee Jay wrote:

rovingtim wrote:

Lee Jay wrote:

rovingtim wrote:

This is a bird's folded wing surrounded by feathers. The bold diagonals should be plain, but a couple of them have become 'ropey' with a pattern that matches the feathers above them. Why?

That's the point - there's no way to know.

I'm agreeing with you here, Lee.

You can't recover aliased information.

No, but software can 'guess'.

It doesn't. It has no idea what information is aliased and what isn't, and it can't create data above Nyquist because there's no place to put it even if it had that data.

Nyquist is 2x the sampling frequency.

Nyquist is 1/2 the sampling frequency.

On a sensor, that means a single pixel is undersampled.

Yes, if the optics and scene have the ability to provide spacial frequencies higher than half the spacial frequency of the pixels.

Thus the need for an AA. Take away the AA and print a dot for every pixel as if it has all the information and you are effectively imaging beyond Nyquist.

No, you are not.

So, if at 100% everything is crystal clear before you sharpen, then you know that what you are looking at is beyond Nyquist. I can list a number of cameras that do this.

There's no way to do this as it requires a non-causal filter.

How does the camera do this? I'm guessing software invention, but what would you say?

It doesn't do this.

Then how is DPReview recording 'detail' above Nyquist in their MTF charts?

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