On 5 Jul 2005 at 18:28, Joseph Tainter wrote: > Does everyone agree with P. J. that such purple "bloom" (or whatever it > is called) will be seen normally with any lens (emphasis) on a digital > sensor? Even "digital" lenses will do this? Should one never shoot a > dark subject out of focus against a light background?
Purple or green tinges are generally sensor bloom. But only for very poor lenses CA (transverse chromatic aberration) should be non-existent at the dead centre of the projected image and should become progressively worse towards the edges of the frame. http://www.vanwalree.com/optics/chromatic.html Bloom occurs when saturated sensor wells spill into adjacent empty wells, I'm not sure why it tends to render as purple or green but it's likely due to a combination of bayer sensor lay-out, CA and bayer decoding. So yes if you want to minimize edge bloom stay away from subjects that you would envisage could promote it or learn to deal with it. I use several post processing actions that isolate and then desaturate the offending colours. Also I find that in camera and ACR sharpening tends to worsen the effect so when I have a particularly badly affected image I don't sharpen at all until I've dealt with the bloom. An interesting thread on bloom reduction: http://www.photoshoptechniques.com/forum/archive/index.php/t-6939.html Cheers, Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
