Does aperture change how a camera sensor’s anti-aliasing filter affects sharpness?
Asked 10/23/2013
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Dust on a sensor becomes much more visible at small apertures like f/16–f/22, but is often nearly invisible wide open. I understand that this is because stopped-down light forms a narrower cone, so dust casts a more defined shadow.
That made me wonder about the sensor’s anti-aliasing (AA) filter, which also sits in front of the sensor. Does changing aperture alter how the AA filter affects image sharpness or other image qualities?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
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Not in the same way as dust.
Dust becomes visible at small apertures because it casts a sharper shadow when the light cone is narrow; wide apertures produce a broader cone, so the shadow is softer and less noticeable.
An AA filter works differently. It slightly spreads incoming light to reduce aliasing/moiré, typically using birefringent layers that split the image by a tiny amount horizontally and vertically. Its behavior depends mainly on the angle at which rays strike the filter, not simply on whether the lens is used wide open or stopped down.
So stopping down does not make the AA filter suddenly act like sensor dust or dramatically change its effect. The AA filter always slightly reduces sharpness by design. In general, that softening is most noticeable when the image reaching the sensor is otherwise very sharp; if the image is already softened by other factors, the AA filter’s effect is less obvious.
In short: aperture strongly affects dust visibility, but it does not meaningfully change AA-filter performance in the same direct way.
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