Using a Lens to reduce Moiré

Started Feb 9, 2013 | Discussions thread
Arky Regular Member • Posts: 433
Re: Using a Lens to reduce Moiré

DMillier wrote:

I've long had an alternative idea to the AA filter (which I now know isn't original, but I claim independent invention): use anti shake in reverse as a tunable bandwidth limiter. No one ever wants to discuss the idea which makes me think it cant work but what do you think?

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A few thoughts about your idea to shake the sensor to achieve a controlled blur.

The movement would need to be in both the vertical and horizontal directions. These vertical and horizontal movements would need to coordinated so that the maximum movement along one axis would occur when the movement is centered along the other axis. Coordinated properly, this movement would result in a circular movement of the sensor resulting in a nice symmetrical blur. If the horizontal and vertical movements were not perfectly coordinated it could lead to the sensor moving back and forth along a diagonal line instead of a circle. Moving the sensor in a diagonal direction would result in an asymmetrical blur.  It may be possible to reconstruct an image with an asymmetrical blur but probably only if the direction of the blur is known (top-left to bottom-right or bottom-left to top-right). If the timing of the movement can be controlled to a degree that the diagonal movement direction is known to be in a specific direction then it wouldn't be any more difficult to time the movements to move the sensor in a circular pattern instead of diagonally. I can't help but think a nice circular symmetrical blur would be less problematic to reconstruct.

The speed of the sensor movement would need to match the shutter speed or be an even multiple. If the shutter speed is 1/8000 second the sensor would need to move in a full circle in 1/8000 second. That's way too fast for the usual anti shake sensor correction movements. I'm not sure how much power it would take to vibrate something with the mass of a sensor at 8000 Hertz but tweeter speakers can do it easily, although they are usually pretty small and don't have much mass. At a shutter speed 1/4000 second the same 8000 hertz speed would result in the sensor moving in 2 full circles while the shutter is open which would be okay too. At a shutter speed of 1/6000 the 8000 hertz speed wouldn't work though, one side of the blur would be exposed twice while the other side is only exposed once resulting in an asymmetrical blur.

Although the frequency would need to be variable the amplitude of the movement would need to remain constant to give a controlled amount of blur.

A movement circle of about the width of 1 pixel would probably be ideal for anti-aliasing a 3 layer Foveon type sensor. If the movement circle were much bigger that one pixel, then when imaging a point light source (a star or a distant light) the pixel in the center of the movement circle would remain largely unexposed.

For a Bayer pattern sensor I think the movement circle would need to be slightly over 2 pixels wide to properly reconstruct the color and that would definitely leave a dark pixel in the center of the exposure pattern when imaging a point light source. A robust deconvolution algorithm could conceivably reconstruct the point light source but that would involve a lot of computational power and would probably result in a lot of undesirable artifacts in the resulting image.

With all that said, I think your sensor shake is probably technically possible but fraught with difficulties.

A 200-300 Megapixel sensor is probably no more technically challenging than implementing your sensor shake idea and would achieve it's anti-aliasing by over-sampling the Airy disk projected by even the sharpest lens.

That's my thoughts anyway, but as always I could be wrong.


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