15 Dec The A to Z of Photography: low-pass filter

Source: Tech Radar

Low-pass filters have long been used on camera sensors to prevent moiré or interference effects when you photograph subjects with fine, regular patterns. You may have seen this effect on old TV sets with presenters wearing striped shirts or check jackets – those horizontal or vertical lines clash with the rectangular array of photosites on the camera sensor.

Today’s sensors have much higher resolutions, so it takes a much finer pattern to produce this moiré effect, but in theory it could still happen when photographing woven fabrics, for example, or fine rectangular patterns in man-made objects.

It’s not just the rectangular grid of photosites that’s the problem, but the color filter array that’s placed over them to capture full-color images. The most common layout is the ‘bayer’ array, with a regular pattern of two green photosites, one red and one blue in a repeating 2 x 2 grid. This can cause color fringes and artefacts if the camera captures ultra-fine lines that don’t straddle all four photosites; it’s rare, but with a super-sharp lens and the ‘wrong’ subject, it can happen.

What does a low-pass filter do?

The solution is the low-pass filter. The technical description is a filter which allows lower frequencies to pass through (coarser detail, in this case) but attenuates or blocks higher frequencies (finer detail). In a nutshell, a low-pass filter slightly blurs the pixel-level detail to reduce or prevent moiré effects. This slight softening can be disguised with a little extra image sharpening later…

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