Joe said:
Conversion was in photoshop cs4 to B&W which is still an RGB 8bit image.
Its not converted as in mode convert to grey scale.
In Tim Grey's January 12th newsletter he recommended always working with B &
W in 16 bit mode. I have copied his reasons below, and while that may not
impact the color cast you may find other improvements to your image.
Gary
You're correct that I recommend working in 16-bit per channel mode as a best
practice, but I do fully acknowledge that for color images that don't
require strong editing you likely won't see any real-world benefit to doing
so. With black and white images, however, the issue is more significant, and
I very strongly recommend working in 16-bit per channel mode for black and
white images even if you're only working in 8-bit per channel mode for your
color images.
The issue relates primarily to the amount of information available for color
versus black and white images. With a color image in 8-bit per channel mode,
there are 256 shades (tonal values) available per channel, which translates
to over 16.7 million possible colors (256 X 256 X 256 = 16,777,216). While
16-bit per channel mode would offer far more colors (281,474,976,710,656
color values, to be exact), provided you aren't making particularly strong
adjustments you don't really need that extra overhead. I prefer to have the
overhead available in any event, but you can still get excellent results
with properly-exposed color images in 8-bit per channel mode.
Things change dramatically when we're talking about black and white images.
Even if you keep the image in the RGB color mode, you're only actually
producing the equivalent of a single channel. What that means is that for a
black and white image in 8-bit per channel mode, you only have 256 tonal
values available. Compared to over 16.7 million possible values for an 8-bit
color image, you can appreciate that an 8-bit black and white image simply
doesn't contain very much information. By contrast, if you are working in
16-bit per channel mode for a black and white image, you have 65,536
possible tonal values to work with.
The issue you will very quickly run into when working with a black and white
image in the 8-bit per channel mode is posterization, or the loss of smooth
gradations in the image. The most common visible indication of this problem
for black and white images is a sky that does not have a smooth gradation,
but instead had bands of different tones. This looks something like a
rainbow, except that instead of bands of different colors they are just
bands of different shades of gray. This is the most common place you'll see
evidence of posterization, but it can appear in a variety of places within
the image. This posterization looks very wrong in the final image, and
should always be avoided. Thus, from my perspective, it is absolutely
critical that you work in 16-bit per channel mode for black and white
images.
In the upcoming issue (Winter 2010) of Digital Darkroom Quarterly, I cover a
variety of techniques for producing black and white images using both
Photoshop and Lightroom. If you're not already a subscriber and you would
like to receive this and future issues, you can sign up through my online
store here:
http://www.timgrey.com/store/index.php?app=ecom&ns=prodshow&ref=DigitalDarkroomQuarterly