Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Djon"
<westsidemaurice@y...> wrote:
> 
> Some of the best 6 and 8 MP digicams suffer significant purple
fringe
> at high ei. What's the impact on B&W conversions? Obnoxious?
Vanished?

Funny you should ask. I recently worked on a RAW image of a winter
(snow) scene taken with my 5 mp Canon S50. After converting from RAW
and then doing a B&W conversion, I began to notice some strange
lines-- "ghosts"-- in the image: these pale lines ran alongside
(parallel to) various high-contrast edges throughout the image
(wherever there was snow against dark background. 

I suspected that this was an artifact from the initial RAW conversion,
so I repeated the RAW conversion (with slight changes) and then
started to redo the RGB-to-monochrome conversion. I was using Channel
Mixer for obtaining the grayscale, and I realized that I had been
using a relatively large percentage of the blue channel. That's when I
had a "duh! moment" and started backing off on the blue level. I could
clearly see those "ghost" fringes begin to disappear. A close look at
the RGB image showed that there indeed was a small amount of chromatic
abberations (purple fringing) along those same edges. But in color, it
actually seemed less noticeable than the corresponding pale "ghosts"
in the B&W version.

So--go easy on the blue channel (which is common practice anyway
because of noise issues).

Phil

Phil