06-15-2011, 11:31 AM
I really don't get why LoCA is ruled out as a source of this type of purple fringing.
Referencing figure 5 at http://toothwalker.org/optics/chromatic.html I suggest the following thought experiment which could apply in achromatic, apochromatic, or superachromatic cases:
At the nominal focal point we supposedly optimise the luminance (green) for best focus. The red and blue will then be slightly defocused. Red and blue combines to give purple.
At a point in front of or behind the nominal focal point, then the red/blue channels will be either more or less defocused than green. This gives purple or green-ish fringes depending on the direction. There may be further colour skew depending on the degree of colour correction and the spectral composition of the light source itself.
So why don't we see them everywhere all the time? I believe this is because the correction is fairly decent, such that the fringe would be subtle enough not to be detected under most normal circumstances. Only in case of very bright areas, such as blown highlights, does it get strong enough to become visible in neighbouring darker areas. The key point here is that this does provide a mechanism for purple fringing to occur at the nominal focal plane as well as off it.
Note in the above, I am not ruling out the possibility for there to be other mechanisms for producing purple fringing, but this one seems to fit best and I haven't heard of anything else that explains it as well.
The UV/IR one is interesting but needs some more thought. I think UV is unlikely to be a cause as modern digital cameras and lenses are very poor at passing it. Do look up UV photography and see how hard it is to get an image even if you want to. IR is more possible, after all that is why they put the IR cut filter on the sensor. This mostly ends up in the red channel of the sensor though. Due to the longer wavelength, it would likely have a different focal point than for visible light but I'm not sure that would result in purple necessarily, my gut feeling is it would be more red than purple.
Referencing figure 5 at http://toothwalker.org/optics/chromatic.html I suggest the following thought experiment which could apply in achromatic, apochromatic, or superachromatic cases:
At the nominal focal point we supposedly optimise the luminance (green) for best focus. The red and blue will then be slightly defocused. Red and blue combines to give purple.
At a point in front of or behind the nominal focal point, then the red/blue channels will be either more or less defocused than green. This gives purple or green-ish fringes depending on the direction. There may be further colour skew depending on the degree of colour correction and the spectral composition of the light source itself.
So why don't we see them everywhere all the time? I believe this is because the correction is fairly decent, such that the fringe would be subtle enough not to be detected under most normal circumstances. Only in case of very bright areas, such as blown highlights, does it get strong enough to become visible in neighbouring darker areas. The key point here is that this does provide a mechanism for purple fringing to occur at the nominal focal plane as well as off it.
Note in the above, I am not ruling out the possibility for there to be other mechanisms for producing purple fringing, but this one seems to fit best and I haven't heard of anything else that explains it as well.
The UV/IR one is interesting but needs some more thought. I think UV is unlikely to be a cause as modern digital cameras and lenses are very poor at passing it. Do look up UV photography and see how hard it is to get an image even if you want to. IR is more possible, after all that is why they put the IR cut filter on the sensor. This mostly ends up in the red channel of the sensor though. Due to the longer wavelength, it would likely have a different focal point than for visible light but I'm not sure that would result in purple necessarily, my gut feeling is it would be more red than purple.
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