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Old 18-04-2008, 13:17   #8 (permalink)
silkstone
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Location: Silkstone Common, Yorkshire, UK
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Re: Basic question on lenses

The angle at which light is entering the camera at long focal lengths is much less than at wide angle, so you don't get the same internal refraction and diffraction problems. So, all other things being equal, it's easier for the optics to produce sharp images across the whole of the frame at longer focal lengths.

Wide angle is a severe test of the optics, especially with digital sensors which tend to be more susceptible to aberrations than film because of the physical characteristics of the sensor itself.

Yes, the red and green edges are one form of chromatic aberration (CA), caused by light of different wavelengths not focusing in exactly the same place on the sensor. Another form of CA is 'purple fringing' which is very common when shooting something like tree branches against a bright sky - especially if the sky is overexposed.

CA is mainly down to the lens, but some sensors can introduce it too. Better lenses have less of it. The easiest way to get rid of red/green fringing is by using a RAW converter or image editor that has a CA reduction feature. Basically this shifts the red and green channels progressively towards the edges of the frame, so they are in alignment. I've used the CA correction in Bibble Pro and Lightroom, and they are extremely effective - it's easy to eliminate R/G fringing altogether. Purple fringing can be more tricky, and sometimes the only way is to selectively desaturate purples/magentas.
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