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Author Topic: Avoiding aliasing shooting screens (TV, monitors, smartphones,...)  (Read 262 times)

Guillermo Luijk

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Whoever has taken pictures at a screen (TV, monitor, smartphone) probably has suffered terrible aliasing, moiré or even colour changes. The reason is we are sampling a discrete image with spatial frequencies in the same order of magnitude as our sensor:

For a full HD TV set we have 1920*3=5760 LEDs in the horizontal direction, while a 24Mpx camera (6000x4000) has 6000 Bayer photosites in the same direction. Trouble is assured, specially if the sensor has a weak AA filter.

I have tested three atialiasing strategies on my A7 II comparing with the standard perfectly focused shot at f/5,6:

  • Diffraction: using f/22 Fresnel circles as AA filter
  • Camera shake: unorthodox and not very reproducible
  • Defocus: using slight defocus as AA filter
100% crops:

With this kind of artifacts (periodic patterns), it's important to check if they prevail once the image is reescaled. Photoshop is not very cautious regarding aliasing when downsizing, gaining some extra sharpness in exchange. 800px resizing:

The first two images have important aliasing artifacts. Camera shake improves this quite a lot, but still displays banding in some areas so as the highest sharpness loss. Defocus seems to be the best option. Extra output sharpening would increase aliasing artifacts in the first images and would compensate for the defocused version drawbacks.

Experiences or thoughts?

« Last Edit: May 29, 2018, 10:28:05 AM by Guillermo Luijk »
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