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12-08-2009, 01:28 AM   #1
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Anybody had their AA filter removed?

Just curious if anybody has tried removing the AA filter on a K10 or K20 ... and what the results where.

Can't aliasing/moire be removed in software? Correct me if I'm wrong but isn't it just a blurring filter. Can't we use a selective gaussian blur or something? It'd be much cooler to selectively blur only where needed and keep the other bits super crisp.

12-08-2009, 01:51 AM   #2
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Simple answer is no (or yes at low quality).
Basically you need to blur before you sample.
Sampling with lower rate than actual detail results in random readings.
Simple filtering can only smear random readings worse than precise blurring.
To digitally filter this randomness correctly you would have to reconstruct the image pattern to its full detail, and resample it - something like that.

What do you expect to gain. K10D already has very weak AA filter (i wouldn't be surprised if it actually uses software filtering). Some cheap cameras and mobile phones for example doesn't address those issues at all, so you can clearly see the effects - moire patterns and bayer artefacts.

Physical filters often blur the image just enough for the resolution to match the one of sensor (even if they should blur even more to match single bayer filter block for maximum effect).
I don't know about Pentax cameras, but sometimes they come together with hot mirror (ir filter) - which is a dielectric coating on top of the AA filter. If this is the case, all IR modded cameras should be without AA filter.

Last edited by ytterbium; 12-08-2009 at 03:10 AM.
12-08-2009, 11:59 AM   #3
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I don't know - As I understand it, an optical AA filter just spreads the incoming light over a larger area. The fact that it is basically a low-pass optical filter is just due to the fact that it's a very weak filter. So where a line might randomly jump between two columns or rows of pixels (or Bayer patterned sensors), the AA filter "smears" them across both, just like a 1-2px gaussian blur would.

An AA filter always removes detail to quite a bit below what the sensor can resolve.

The result in removing an AA filter would be to get much sharper images with more detail - but only in situations where aliasing/moire is less evident. Since moire is most evident when shooting man-made objects (because of the repeated patterns of straight lines) and I mostly shoot landscape - I think this could benefit me.

furthermore, I don't mind simple aliasing so much because none of my photos are printed at pixel-peeping level. It's the patterned moire that drives people crazy.

I understand that for an APS-C sensor with relatively low resolution (compared with 30MP+ MF), it would be a horrible decision to ship a consumer camera without the AA filter. The thing is, an avid hobbiest or pro with the time, patience and desire to learn and use the software might be able to do much more without the AA than a consumer who uses their camera on auto/jpeg.

I just saw this interesting discussion over at Luminous Landscape: Luminous Landscape Forum > Anti-aliasing filter effects demonstrated

Thanks for your reply! It made me understand a bit more about what goes on. If I were to write a program to auto-remove the moire, I might use a Fourier transform or DCT to break it down, then filter only the hi-freq data out (drop it) and rebuild the image from the left-over component amplitude/frequencies. If I were really smart, I'd add some kind of detection for exactly which frequencies to drop, probably using a Markov model. The main problem is probably the excessive computational power needed. However, I'd only use the algorithm on small feathered patches rather than the whole image.

Not that I'm smart enough to write that software right now. I'm sure it must exist "out there" and is probably even patented.

Mainly, I'm interested if anybody out there actually removed the AA filter on a relatively modern Pentax camera so we can see if there is any perceived benefit.

here are some cool examples using the Nikon D200: Nikon D200HR Of course that company removes AA filters so they might sharpen their sample photos to cheat you... but it's cool none the less.
12-08-2009, 12:17 PM   #4
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I almost did it(this close(pinching fingers)) with a Fuji S5 Pro, we owned 'til about a year ago.

But we sold it instead.
The problem wasn't only in moire as you mentioned but also in the demosaic'ing(pardon my french) as well. The output seemed really bad in some color ranges and correcting it looked like quite a chore and moire removal is not easy as well as it ads quite a bit of extra work after the fact.

PS. we replaced the S5 with an A900 and haven't looked back since

Last edited by JohnBee; 12-08-2009 at 12:32 PM.

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