Anti-Aliasing Filter
From ProPHOTO WIKI
Contents |
Introduction
The Anti-Aliasing filter (aka Optical Low-Pass Filter) is an optical element found in most modern digital cameras. As most digital cameras only measure one colour at each pixel, small points of light may not hit enough pixels to get all the necessary colour information. When this happens, areas of the image with fine details can generate distracting rainbow patturns.
The anti-aliasing filter combats this problem by softening details that are small enough to trigger this effect. This ensures that projected points of light will hit enough pixels so that all three colours can be measured, thus reducing the effect of aliasing on the resulting image.
How it Works
On a sensor with no anti-aliasing filter (Fig.1 - bottom), a small point of light (1) projecting through the lens (2) may only illuminate one of thre three colours on the Colour Filter Array (4). The result is a distortion in the colour of the image projected on the sensor (5). When an Anti-Aliasing filter (3) is added (Fig.1 - top), the point is slightly blured to make sure that it hits sensors representing all three colours.
Side Effects
Unfortunately, the process of removing these fine details causes noticable softness in the resulting images. While this can be corrected to some degree in post processing, not all lost detail can be restored. As such, camera designers must tweak the strength of this filter to balance the benefits of removing aliasing artifacts against the loss of detail.
Sharpening
In-Camera
In-camera systems typically use simple sharpening algorithms that increase the contrast around edges. This process doesn't actually restore sharpness to the image, however it creates the impression of a sharper image when viewed by the human eye. For this reason, many photographers elect to switch in-camera sharpening off and perform this processing on the computer.
Deconvolution
Several newer software packages (including Photoshop CS2 via its 'Smart Sharpen' option) use a process called deconvolution to mathematically reconstruct the lost data. This process is computationally intensive, however modern computers have become sufficiently powerful to make it practical.
See Also
- Colour Filter Array - The filter which allows monochromatic sensor technologies to capture colour images.
- Camera Technology - A summary of the various different technologies involved in modern digital cameras.
- Technology - General summary of the various technologies involved in all areas of digital photographic workflows.
