I took a photo that has what seems like chromatic aberration. It's a purple color at the left and bottom edges, especially at the bottom-left corner:
This is a 30-second exposure at ISO 3200 with the lens wide open (F2.8) on the Sony NEX-5R. I then took another photo with the same parameters, but with long exposure noise reduction, and the fringe goes away:
To see the difference, please open both images in new tabs, full-screen your browser, and switch back and forth between the tabs repeatedly.
This seems like chromatic aberration , but then why does long exposure noise-reduction eliminate it? I thought long-exposure NR corrects for ISO noise, not CA, which is a property of the lens, not the sensor as with ISO noise.
Reducing the ISO to 400 gets rid of the purple color. There's still a white fringe, but it has lost its purple color. And it's less bright than at ISO 3200. Further reducing the ISO to 200 and then to 100 progressively reduces the brightness of the artifact.
So, am I correct that this is chromatic aberration? And how is it related to ISO and long-exposure NR?
: This purple fringe doesn't seem to be ISO noise, either, because ISO noise usually doesn't have a color, and doesn't occur along the edges of the photo.