Inside this issue
Lens Cast Calibration
Fixing colour casts (and dust!)
Amateur Photographer who plays with big cameras and film when in between digital photographs.
Last issue we visited Chris Ireland's "Universal Lens Calibration" tool that allows one to take a 'white wall' shot and then use this to remove dust, fix vignetting and also remove any colour shifts caused by wide angle lenses and digital sensors (oh and also to remove the effect of a graduated filter or just it's colour cast). In practise we didn't see too much colour cast apart from the the 12mm end of my 12-24mm lens which proved quite hard to use with the Calibration tool because of a fixed lens hood.
Well Robin Jones offered to bring a Leica M9 over with a 15mm Voigtlander lens to the office to see whether the the Leica system's lack of mirror and subsequent closeness of the lens to the sensor would make any difference.
The Leica system has a set of built in 'fixes' for many of the typical Leica lenses which seem to work quite well but with no fix for the 15mm Robin has been picking another close focal length and then using 'Corner Fix' to try to correct some of the colour shifts. And what colour shifts! Turning the Leica correction off (by applying a very long lens setting) the calibration shot looked like this.
Which when combined with a real photo gives something like this..
I've made a set of images at the bottom of this article that show the separate components of the colour and vignetting in this image but the following two show the interesting items as far as I'm concerned - the first shows the colour component on it's own and the second shows the results from our test image with colour and vignetting removed using Capture One.
The colour casts are quite dramatic and interestingly assymetrical at the far edges. I can only presume this is due to the assymetry of the bayer colours but can't think why. The final result is quite dramatic in cancelling out both the cast and the vignetting completely.
We also tested the free software 'cornerfix' available on Sourceforge. You have to convert files to DNG in order to be able to use this as it was designed for the Leica M8 originally. It works with any file though and has been reported to get excellent results with the Sony Nex and custom lenses.
The results from Cornerfix match the Capture One results quite well although you don't have quite the level of separate adjustment of the colour correction and luminosity correction as with C1 and you also don't get the dust fixing feature. What do you expect for free!
You can buy the Universal Lens Calibration tool from Direct Digital Imaging here.