The limits auf automatic longitudinal CA correction
2011/01/01 Leave a comment
Isn’t the current world of photo tools fantastic? Just one click in modern applications like Capture NX2 and artifacts like color aberrations are magically eliminated. At least, that is what we are made to believe. Those tools have some big disadvantages – they did not accompany you while you captured your photo, did not get an explanation what your intend was, did not recognize the objects in the scene. That’s why they are sometimes limited in their utility or even worse, counter productive. Let me share an example.
Two months ago, Europe experienced heavy snowfall, which impacted air travel in the northern part. I got stuck at Berlin Tegel airport. Finally, after 6 hrs delay and 2 re-bookings, we were allowed to walk to the Air-Berlin plane around midnight. I took a couple of images with the AF-S 35mm/1.4 mounted on the D700, while waiting for boarding over the staircase.
I took the image below and was really surprised when I developed the photo via CNX2 and wanted to save the final image on disk. This was not the image I recorded originally. It was significantly different. All the letters were framed in black borders. The red “Air Berlin” letters on white background, as well as the white letters on the red background on the engines.
Triggered by this abnormal behavior, I started to investigate for the root cause. Finally the culprit of this strange thing was found. Turning on longitudinal CA removal in CNX2 “created” this black frame around the letters. Turning it off, removed it. It is a repeatable thing. Weird.
I don’t have insight into Nikon’s software algorithms, but based on the behavior of CNX2 with my image, I would think that the way how this algorithm works is based on edge detection with certain color values and halo ranges. By selecting the “remove this” button, the software replaces the CA halo with darkened pixels, i.e black. To me it looked like the company colors of the Air Berlin plane was not in line with the automatic algorithms of Capture NX – hence the letters were framed black.
It is easy to reproduce with this image. If your are interested to try it for yourself – click here to download the original NEF file.
This the overall image
This is the image, when longitudinal CA removal is not used. (This is the way it should look like)
This is the image, when longitudinal CA removal is turned on. All letters are framed with a black border.
By the way, did I say I love the AFS 35mm/1.4 G ? I love it ….. (Taken from the staircase upon entering the plane)