# Having the anti-aliasing filter removed(?)



## MrLogic

Has anyone here had the anti-aliasing filter removed from their camera (and having it replaced with an IR cut-off filter)? If so, how do you feel about it now?


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## Garbz

I haven't, but I'm an EE who's done quite a bit on Digital Signal Processing and Analogue to Digital Conversion so let me add something.

The whole point of having the filter there is to allow proper operation of sensors which have a colour grid array like a beyer pattern. If you have a Sigma camera with a Foveron sensor then yeah maybe remove the filter, or if you're taking ONLY black and white pictures again maybe a good idea. 

However basic rule of digital sampling is that aliasing is bad, and when the filter is removed from a beyer array CCD you may gain the tiniest bit of sharpness but at very great expense to the colour accuracy of the details you resolve. Sure you may see a tiny bit of extra detail, but who cares if your picture is covered by a huge rainbow. 
It's much the same argument as people who swear black and blue that the CD players of the late 80s sounded better than today's versions despite clear measurements indicating their inability to properly reproduce high frequencies. 

Birefringent AA filters aren't cheap, and are part of your camera for a reason. If you don't like the idea of the loss of sharpness from Anti-Aliasing then buy a camera without a Beyer sensor which doesn't interpolate the final data like a Sigma SD14. 

Otherwise what you really are doing is replacing one small problem for one potentially much bigger one, and on that topic, do you already have a lovely collection of the best possible glass money can buy? Or would I look at your resulting image and say wow nice detail, pity about the CA, barrel distortion, edge sharpness, etc?


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## MrLogic

Garbz said:


> However basic rule of digital sampling is that aliasing is bad, and when the filter is removed from a beyer array CCD *you may gain the tiniest bit of sharpness*



You get about 15% higher effective resolution according to Thom Hogan:

Re: Remove AA-filter?: Nikon D3 - D1 / D700 Forum: Digital Photography Review

He also implied that it could make sense for certain Nikon D3X users (depending on what they shoot) to have the AA filter removed, which I thought was interesting.


But your points are well taken.


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## musicaleCA

Er...The AA filter is corrected for quite easily with sharpening in post. That's what the algorithms are designed to correct for, after all. Well, that and the loss in the A/D conversion.

Besides, what the heck is the gain, really? 3MP is honestly enough to make sizable prints. We end-up downsampling most of our images anyway now because of these insane resolutions on camera sensors anyway. Heck, many lenses now can't even provide sufficient LW/PH to satisfy these sensors, and LW/PH is only visually relevant up to 2150. Beyond that we're reaching into absurdity.


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## Garbz

Yes and no on both of you.

You do get higher resolution, but at the cost of aliasing. Lets for a moment assume you have an absolutely TAC sharp lens that can resolve those extra 15%. Typically the sensors outperform most lenses. This goes doubly for a D3x. The Aliasing filter is tuned to bifract the light over a small grid of red green blue green pixels. Every pixel on an image is actually an interpolation of the 4 surrounding pixels. So the AA filter on a D3x is notably finer than say a D3. Each filter is tuned to the sensor design so it makes no more sense for one camera over the other.

So each single point resolved by the lens gets spread over the 4 coloured pixels. This way the camera can determine what the colour of the point was. Now suppose you remove the AA filter and have a really sharp lens, that one point now will land on either a red, blue or half the time a green pixel. That point now can no longer be accurately determined since each pixel on the sensor represents one of red, green, blue or green. Post interpolation if you zoom out and end up with just the right pattern to resolve to just the wrong pixels you get an amazingly ugly rainbow effect across the image.

So again, sharpness yes, but at a great expense that certain of your now resolvable detail will just look horridly wrong. Example from the first site I googled:





Source:

Take note of the rainbow effect on the brickwork (in this case monochrome (Foveron sensor?, or just purposeful disregard of interpolation when downsampling), but also take note of the fine detail that is now resolved to single pixels on the staircase on the left. The image is so sharp that the detail resolves to just one pixel, and the lack of blurred lines cause an otherwise smooth metal pole to be resolved to a computer game.


musicaleCA you can't create detail that is gone. Yes sharpening is the solution. But that makes the image appear sharp, quite different from recovering the detail by making the source image sharper.  But you're dead right. I have lots of pro lenses but I doubt they actually outperform my sensor, and if they do, then my photography would be incredibly dependant on technique (accurate focus, stability, etc) to resolve the detail.


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