Purple fringing, chromatic aberration and digital photography
A play script where the actors are:
– A beautiful Leica summicron IV 50mm f2 lens
– A Leica M3 and a diapositive film, Velvia 50
– A not less beautiful Sony NEX7
Now here’s what happened. I took a first picture with that lens on the Leica M3 and the velvia 50 film. The picture as such is not very interesting, but one thing is interesting: the photo shows some grain, maybe is a bit out of focus but other than that, no problem at all. But then, I took a picture with the same lens mounted on a Sony Nex7. Oh surprise ! I had chromatic aberration, also called purple fringing.
Anything wrong with the lens ?
No, other than that this lens mounted on a very recent 36 megapixels-body.
So why the purple fringing on the digital picture and not on the film ?
Colors are wavelengths and lenses are designed in such a manner that several wavelengths focus in the same manner. The point is that a digital sensor registers more wavelengths than film, slightly beyond what the human eye sees – we humans see everything between infrared and ultraviolet, i.e. everything from red to violet. So does film. So a lens designed in the years when only film existed was not designed to focus on anything beyond ultraviolet and infrared. But put that lens on a digital camera that registers beyond ultraviolet and you’ll see that the lens is simple not designed to focus wavelengths that film doesn’t register. That is chromatic (« χρῶμα » is greek for « color ») aberration (« abberatio » is latin for « distance » or « diversion » – dunno why in english the double « b » was replaced with a double « r »).
The phenomenon is very well explained here : http://en.wikipedia.org/wiki/Purple_fringing.
This also explains why most (all?) camera brands have reviewed their lens range to fit digital photography. At least Leica did this. Digital photo software contains plugins in order to remove purple fringing.
It therefore also is not necessary to criticize a Leica M9 body that has purple fringing since this simple is how a sensor works…
Here are the 2 details of the photos I talked about, the first one with a Velvia 50 and the second one with a Sony Nex7.