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I recently came across the phrase 'excited aliases.' The phrase seems to be related to lens sharpness in some way. While a web search turns up the phrase a few times, I am unable to locate a decent definition. What does it mean to excite the aliases? Is it a good thing or a bad thing? Something worth worrying about or watching for?

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up vote 6 down vote accepted

Aliasing is an artifact that occurs when detail in an image is near, or exceeds, the resolution capability of the sensor. If the detail is finer than the pixels the sensor resolves, then inaccurate patterns (like Moire) can occur.

To avoid such artifacts, cameras have anti-aliasing filters, which slightly blur the fine detail to a level that can be resolved by the sensor without these artifacts.

Exciting aliases means that you have a lens sharp enough to resolve fine enough detail that it pushes the limits of the sensor to the point that you start seeing these artifacts.

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"Aliases" are artifacts that appear during an anti-aliasing process when for example a thin black line on a white background is smaller than the pixel itself. Ideally, the software in the camera will convert the pixel into a gray pixel to average out the image between the two colors or brightnesses. In extreme cases, the camera might decide that the pixel on the left should be black and the one on the right should be white. When taking pictures of a fine grid (door screens, chain link fences from further away) The regular shift of the pixel brightness to one direction will cause wrong colored pixels, moirée patterns - "aliases".

The issue here can be boiled down on the fact that structures of the image taken are finer than the resolution of the sensor. If the lens is not sharp enough, this might not happen since the blur of the structure will spread the item displayed across several pixels. A sharp lens however might have this issue in return.

The second factor is then the strength of the anti-aliasing software that might either simply soften the contrast or create too strong contrast or color variations that then become apparent when zooming in on the details of the photo.

You will have to be careful in case your camera tends to create those patterns when you take images of very fine structures. Some cameras will cause issues, and some will not, also depending which lens you use and how sharp the item is on your photo.

More information here.

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