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I noticed a while back that with a lens stopped right down (say f16-22), any dust on the sensor becomes much more noticeable and "focused", and wide open (say f1.4) it is practically invisible. This is presumably due to the convergence of light from a single point (f22) or a wide area (f1.4) blurring the shadow cast on the sensor by the particle.

So this got me thinking, the AA filter is also mounted in-front of the sensor (but closer than the IR filter which has the dust on it) Therefore; does the convergence of light through the AA filter when shooting wide or stopped down affect sharpness (or anything else) ?

(edited to remove the micro-lens reference)

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2 Answers 2

The appearance of dust when stopping down is due to the light cone being very narrow, causing the dust to throw a clear shadow onto the sensor, (vs. a very wide light cone which illuminates under the dust for a soft shadow).

AA filters are implemented with a pair of birefringent crystals which have a different refractive index for different polarizations. The first crystal casts a pair of images separated very slightly horizontally, and the second crystal does the same vertically. As it's based on refraction the effect of the filter is dependant on the ray angle (this causes problems with rangefinder lenses with steep ray angles) but not the aperture used.

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But surely the larger aperture, as you say gives a larger "light cone", will introduce a more VARIED "ray angle" per site than a very small aperture? thus interacting with the aa filter differently? –  Darkcat Studios Oct 23 '13 at 17:59
    
@DarkcatStudios The problems with RF lenses occur with ray angles of 40 degrees, much wider than the light cone of an f/1.4 lens, but I suppose there will be a slight but probably undetectable aperture dependence with the AA filter. –  Matt Grum Oct 24 '13 at 8:33
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But there would be a difference, at least purely theoretically, wouldn't you agree? –  Michael Clark Oct 24 '13 at 9:53

The "dust spots" on your sensor you see in photos taken at higher apertures are actually the shadows of that dust. Just as a small, "hard" light source will cast a well defined shadow and a large, diffused light source will cast a very soft shadow so the definition of the dust's shadow is determined by how collimated the light casting the shadow is.

The Anti-Aliasing (AA) filter, on the other hand, is a system that is designed to slightly blur the light falling on it. It has the greatest effect on tightly focused light because when you spread out softly focused light you are basically just redistributing light that has already been softened. The AA filter results in slightly decreased sharpness of the light passing through it. This effect is most noticeable for the most focused light reaching the sensor/filter assembly. One reason manufacturers have begun making cameras without AA filters is to increase the sharpness the system (camera+lens) is capable of at the risk of producing moire, or repeating patterns in the image.

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The AA filter is not a system of microlenses, and doesn't actually blur the image, but instead produces a series of 4 very tightly spaced subimages, which only appears slightly blurred. –  Matt Grum Oct 23 '13 at 17:02
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Conceeded that the AA filter has no microlenses, but producing 4 tightly spaced subimages is blurring, since you are spreading focused light over a larger area. –  Michael Clark Oct 23 '13 at 17:06
    
I suppose it is a blur with a non-continuous (discrete) PSF (point spread function). –  Matt Grum Oct 24 '13 at 8:31
    
When the smallest discernible unit of the sensor's output is the size of the pixel pitch and the spread is below or at that threshold, how is the blur non-continuous and discrete? There's no way to detect the difference between continuous and non-continuous when the separation is smaller than the the smallest dot produced by the camera. –  Michael Clark Oct 24 '13 at 9:40
    
I'm speaking purely theoretically :-) –  Matt Grum Oct 24 '13 at 9:47

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