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Canon low pass filter explanation
Dennis Hissink : February 17th 2005 - 12:00 CET
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CanonCanon low pass filter explanation : Certain fabrics can have a very tight, regular weave. On close inspection, this can appear as a series of alternating stripes separated by dark shadows between the weave. When the pitch of this weave is close to a digital camera sensor's pixel pitch, light from the subject will only activate selected rows of pixels. The sensor will then produce false colours, which can change with rotation or lateral movement of the camera. These false colours appear as waves on the image, known as moiré. Subjects such as distant geometric patterns are also susceptible to producing this effect. To combat the effects of moiré, camera manufacturers sometimes utilize a low pass filter to blur incoming light.
But a single filter can only blur a pattern in one direction, which leaves many false colours uncorrected. Moreover, this approach effectively lowers the camera's resolution.

Canon's approach is to use a unique three-layer Canon low pass filter. The first filter polarises and separates incoming light vertically by precisely one pixel distance. In order to then split the light horizontally, the light must first have the polarisation removed. This is achieved with a second filter known as a phase plate. The third low pass filter polarises the light a second time, only this time separating it horizontally.

Incoming light is therefore spread by precisely one pixel in both the vertical and horizontal directions.

This deliberate blurring has the effect of removing moiré. Unless it is compensated for, however, it also has the effect of reducing image sharpness. Performing accurate compensation, however, requires an exceptionally complex series of processor hungry algorithms, one of the reasons why competing manufacturers have difficulty tackling the moiré phenomenon. One of the Canon's great advantages is the processing 'head room' of its DIGIC II processor, which is used to perform the most sophisticated of correction routines and ensure the benefit of sensor resolution without any trade off in camera performance.

The result is high resolution images with outstanding colour reproduction accuracy.




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