Talk:Purple fringing
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[edit] Microlens explanation
I'm thinking of adding the following explanation, but it could do with some improvement:
The proposed explanation based on microlenses goes as follows. Each CCD cell in the photodetector is covered with its own microlens. The microlens pulls more light onto the tiny sensor area, and also effectively reduces what would otherwise be a gap between the cells. The microlens is far from optically perfect, in particular for off-axis light there can be reflection (scattering) at a shallow angle such that the reflected light is detectable by nearby cells. This causes blooming at high-contrast edges because light from the brightly-lit cells is scattered and picked up by nearby cells which are less brightly illuminated (the scattered light is also of course picked up by nearby brightly-lit cells but it makes little or no visible difference to those).
The characteristic purple colour of the fringes arises because there are three different colours of cells, namely cells detecting red, blue and green light. Each cell is comprised of microlens, coloured filter and photodetector. In the standard Bayer colour filter mosaic array, there are as many green-detecting cells as there are red and blue combined. The lenses over the green cells scatter purple light, therefore a bright or overexposed white highlight scatters more purple light than any other colour; that scattered light is picked up by the "wrong" sensors nearby. In unusual circumstances (for example, strongly coloured highlights, or purple fringes combined with chromatic aberration in the main lens or elsewhere) the fringes seen can be of other colours than purple.
Purple fringing can be reduced using narrow (slow) lenses and narrow apertures, and generally by composition (avoiding bright and over-exposed areas with high-contrast edges). Adjusting the lens geometry through lengthening the respective distances between aperature, lens and sensor will also help to cut-down the off-axis light entering the sensor, although the ideal setting will vary from camera to camera and lens to lens. The fact that lens geometry has an effect on purple fringing causes some people to attribute the problem to the lens rather than the CCDs, when in fact it is caused by the CCD microlens array.
In principle a camera with a larger CCD sensor will have less purple fringing (both because the shape of the microlens is less extreme, so less scattering, and because the scattered light will cover fewer nearby cells because cells are more widely spaced). Different (non-Bayer) CCD designs may have different fringing effects or no purple fringes.
Link to article about microlens arrays and filters (in the context of digital microscopy rather than photography, but the principles are the same): http://micro.magnet.fsu.edu/primer/digitalimaging/cmosimagesensors.html
Link to image produced by Fujifilm showing a side view of the microlens array and filters (in the SuperCCD configuration which is similar to Bayer) http://www.dpreview.com/news/0301/fujisuperccd/srstructure.jpg
'CMcD' 19 September 2006
[edit] Reference quality
The Pictureline article is held by many in forum discussions I've seen to be not the highest-quality reference ... basically its "other causes" of purple fringing are all sensor effects, so I listed them as such. OTOH, it's a commonly-linked reference and one of the top Google hits on "purple fringing" - David Gerard 14:31, 10 April 2006 (UTC)
