Gerhard Fuernkranz wrote:
I think you should not overestimate small errors introduced by correcting distortions with an image editing program, particularly if you do it with 16 bits per channel, in a linear light space (raw RGB with gamma 1.0). When capturing a target with a camera, there are IMO other typical sources of error with a significantly larger contribution to the total error (for instance uneven illumination, natural and optical vignetting induced by the lens, still some residual glare).
Regarding that, how sensitive is argyll with respect to vignetting. During adding vignetting correction to my photo stitcher (details at http://hugin.sf.net/tech/ ), I noticed that most lenses/cameras have a light falloff of about 20-50% in the corners. Actually, since most light falloff is radial from the image center, it might be possible to add simple vignetting model (for example polynomial) to the profiling process.
Alternatively one could divide the image by a flatfield image, this would also account for uneven lighting, and can be used to correct the image in a preprocessing step. It will only work for "linear" (with gamma 1.0) images, though.
I have no feeling how strong vignetting will influence the profile, but if you buy a measurement device that varies its brightness measurement by 20-50% one wouldn't be happy. For now, the easiest way to avoid larger errors due to vignetting is to use only the center area for the image. It might be interesting to profile both with a target covering the whole image and a target covering only a quarter of the image. Comparing the profiles should give an indication how serious the influence of vignetting on the profiling process is.
ciao Pablo
