05-19-2012, 09:35 AM
A lot of different things get attributed to purple fringing here in this thread already.
But not all fringes are "purple fringing", even if the fringe is magenta, purple, blue-ish or reddish.
First we have CA or lateral CA. This kind of fringing never occurs in the center, and stronger the more you go to the edge of the frame. This is caused by different wavelengths of light get bend more or less through the optics, projecting some colours bigger than others. This kind of fringing always comes in pairs or colours, although at times you only see the 2nd colour on closer inspection. Blue-yellow, red-blueish green, magenta-green.
Then there is LoCA or longitudinal CA. This appears on OOF areas, mostly with wide aperture lenses. It shows up as green and magenta fringing around bright areas and spots. It occurs when different wave length light does not get focussed on the same plane.
Then there is a normal transition in blur, which often gets mistaken with LoCA. It also shows up around edges between light and dark, and is just a normal darkening of the blue sky for instance, mixing with the dark subject colours like leaves and branches. It is no kind of aberration, just something our minds have trouble in identifying as we do not see it when we look at things with our eyes (because we only look at what is in focus with eyes, but on photos we can actually look at things that are blurry in the background).
In the example of the Sony 135mm STF we do see PF. And since we do not see that with every lens on the same Sony camera, it is not caused by the micro lenses only, the lens does need to have influence on it occurring.
Then there is purple fringing. It does not occur with film, it is a digital age thing. Like Klaus suggests above (micro lenses) is not correct, else we would see it with every lens with the same light intensity and focal length (and we do not see that). So it has nothing to do with the sensor all on its own. It is lens dependent. The odd thing about purple fringing is that it is well understood by camera and lens makers, but on internet almost no one seems to have a clue... kind of like how exactly lens and camera work together with phase detect AF.
Lens manufacturers have changed lens coatings to combat PF (big sign also that it is not micro lens related). Tamron and Sigma have actually put that change in their nomenclature:
Lenses with changed coatings from Tamron started to get the Di addition in their name (and then APS-C only lenses got Di II).
Lenses with changed coatings from Sigma started to get DG in their name (and APS-C only lenses got DC).
The reason why PF shows up has to do with light intensity. Very bright areas will show a magenta, blue-ish or even reddish glow around them. Stopping down helps, because stopping down lowers light intensity.
]PF is caused by the sensor surfaces being reflective. Light gets reflected back to the lens, and somehow gets again reflected back to the sensor, creating blooming effects. Exactly how the light behaves I do not know (I am not in the camera or lens industry). But the added coatings in modern lens formulations do get rid of the PF. Usually we only see PF with certain older lens designs or certain cheap-ish compact cameras.
But not all fringes are "purple fringing", even if the fringe is magenta, purple, blue-ish or reddish.
First we have CA or lateral CA. This kind of fringing never occurs in the center, and stronger the more you go to the edge of the frame. This is caused by different wavelengths of light get bend more or less through the optics, projecting some colours bigger than others. This kind of fringing always comes in pairs or colours, although at times you only see the 2nd colour on closer inspection. Blue-yellow, red-blueish green, magenta-green.
Then there is LoCA or longitudinal CA. This appears on OOF areas, mostly with wide aperture lenses. It shows up as green and magenta fringing around bright areas and spots. It occurs when different wave length light does not get focussed on the same plane.
Then there is a normal transition in blur, which often gets mistaken with LoCA. It also shows up around edges between light and dark, and is just a normal darkening of the blue sky for instance, mixing with the dark subject colours like leaves and branches. It is no kind of aberration, just something our minds have trouble in identifying as we do not see it when we look at things with our eyes (because we only look at what is in focus with eyes, but on photos we can actually look at things that are blurry in the background).
In the example of the Sony 135mm STF we do see PF. And since we do not see that with every lens on the same Sony camera, it is not caused by the micro lenses only, the lens does need to have influence on it occurring.
Then there is purple fringing. It does not occur with film, it is a digital age thing. Like Klaus suggests above (micro lenses) is not correct, else we would see it with every lens with the same light intensity and focal length (and we do not see that). So it has nothing to do with the sensor all on its own. It is lens dependent. The odd thing about purple fringing is that it is well understood by camera and lens makers, but on internet almost no one seems to have a clue... kind of like how exactly lens and camera work together with phase detect AF.
Lens manufacturers have changed lens coatings to combat PF (big sign also that it is not micro lens related). Tamron and Sigma have actually put that change in their nomenclature:
Lenses with changed coatings from Tamron started to get the Di addition in their name (and then APS-C only lenses got Di II).
Lenses with changed coatings from Sigma started to get DG in their name (and APS-C only lenses got DC).
The reason why PF shows up has to do with light intensity. Very bright areas will show a magenta, blue-ish or even reddish glow around them. Stopping down helps, because stopping down lowers light intensity.
]PF is caused by the sensor surfaces being reflective. Light gets reflected back to the lens, and somehow gets again reflected back to the sensor, creating blooming effects. Exactly how the light behaves I do not know (I am not in the camera or lens industry). But the added coatings in modern lens formulations do get rid of the PF. Usually we only see PF with certain older lens designs or certain cheap-ish compact cameras.