Computer Graphics with Photoshop - exact colour correction?

Author

exact colour correction?

Peter

2005-10-22, 10:14 pm

I'm working some shots and I've always eyeballed my color correction using
levels in RGB on a channel by channel basis. I tighten the highlights and
shadow arrows to the histogram also with a final tweak on the mid tones in
rgb. I use the alt key and stop when I'm going to lose info on highlights or
shadows. I don't know if its me but is there a better and more exact way of
doing this? I seem to have on occasion a miniscule but bothersome color cast
on close inspection, it passes to the layman but for me I really need this
to work. Consistency here is very important.

Any input please.

Pete

Mike Russell

2005-10-23, 3:14 am

"Peter" <pierre.stj@nospam.sympatico.ca> wrote in message
news:piB6f.14678$ns3.1416926@news20.bellglobal.com...
> I'm working some shots and I've always eyeballed my color correction using
> levels in RGB on a channel by channel basis. I tighten the highlights and
> shadow arrows to the histogram also with a final tweak on the mid tones in
> rgb. I use the alt key and stop when I'm going to lose info on highlights
> or shadows. I don't know if its me but is there a better and more exact
> way of doing this? I seem to have on occasion a miniscule but bothersome
> color cast on close inspection, it passes to the layman but for me I
> really need this to work. Consistency here is very important.

Hi Peter,

Your procedure is a good start. Relying on the histogram in the way you
describe improves contrast, but will introduce color casts in quite a number
of images.

One approach that addresses this issue is the shadow, highlight, and neutral
drill, outlined in Dan Margulis's books. If you have Windows Photoshop, and
are interested in learning how to do this, consider taking the Curvemeister
class, which is free and starts on the first Sunday of each month.

Many people use the Curvemeister demo for the class, and most of the
techniques that I teach in the class may easily be carried over to Photoshop
curves.
--
Mike Russell
www.curvemeister.com

Bob Williams

2005-10-23, 3:14 am

Peter wrote:
> I'm working some shots and I've always eyeballed my color correction using
> levels in RGB on a channel by channel basis. I tighten the highlights and
> shadow arrows to the histogram also with a final tweak on the mid tones in
> rgb. I use the alt key and stop when I'm going to lose info on highlights or
> shadows. I don't know if its me but is there a better and more exact way of
> doing this? I seem to have on occasion a miniscule but bothersome color cast
> on close inspection, it passes to the layman but for me I really need this
> to work. Consistency here is very important.
>
> Any input please.
>
> Pete
>
>
When you say "I seem to have on occasion a minuscule, but bothersome,
color cast on close inspection", do you mean the PRINT has the cast or
your MONITOR image has the cast?
If your inkjet print has a cast, you can probably adjust it out by
tweaking your printer driver in advanced mode.
Are you using Generic inks?
When the cast occurs, does the it occur with all papers?
The more info you present the more likely you are to get a good answer.
Bob Williams

Peter

2005-10-23, 6:14 pm

The cast doesn't appear on the monitor because I don't want to see it, but
with a print and a comparison I can see casts. When I go back to Photoshop
it's confirmed, my color correction is out. I missed it somehow. As for
printing I usually correct it somemore with soft proofing. I'm using the
epson r2400 and everything I use is epson, I've not ventured out with
different papers and such.

The problem really lies in my unscientific placement of the gamma sliders in
levels, I'm just eyeballing it and I feel that a more exact method is
required here.

Peter
"Bob Williams" <mytbobNoSpam@cox.net> wrote in message
news:6aG6f.90706$lq6.20810@fed1read01...
> Peter wrote:
> When you say "I seem to have on occasion a minuscule, but bothersome,
> color cast on close inspection", do you mean the PRINT has the cast or
> your MONITOR image has the cast?
> If your inkjet print has a cast, you can probably adjust it out by
> tweaking your printer driver in advanced mode.
> Are you using Generic inks?
> When the cast occurs, does the it occur with all papers?
> The more info you present the more likely you are to get a good answer.
> Bob Williams

JFV4492@att.net

2005-10-23, 6:14 pm

If there are true gray tones in the image, *and* if the gray tones are
not supposed to have cast, then monitor the Info palette values while
you color correct. In such areas, a true gray without cast should be
r=g=b. But true gray areas without any intended cast often do not exist
in an image. Think white sheet of paper lit but candle light.

Peter wrote:[color=darkred]
>
> The cast doesn't appear on the monitor because I don't want to see it, but
> with a print and a comparison I can see casts. When I go back to Photoshop
> it's confirmed, my color correction is out. I missed it somehow. As for
> printing I usually correct it somemore with soft proofing. I'm using the
> epson r2400 and everything I use is epson, I've not ventured out with
> different papers and such.
>
> The problem really lies in my unscientific placement of the gamma sliders in
> levels, I'm just eyeballing it and I feel that a more exact method is
> required here.
>
> Peter
> "Bob Williams" <mytbobNoSpam@cox.net> wrote in message
> news:6aG6f.90706$lq6.20810@fed1read01...

tacit

2005-10-23, 6:14 pm

In article <piB6f.14678$ns3.1416926@news20.bellglobal.com>,
"Peter" <pierre.stj@nospam.sympatico.ca> wrote:

> I'm working some shots and I've always eyeballed my color correction using
> levels in RGB on a channel by channel basis. I tighten the highlights and
> shadow arrows to the histogram also with a final tweak on the mid tones in
> rgb. I use the alt key and stop when I'm going to lose info on highlights or
> shadows. I don't know if its me but is there a better and more exact way of
> doing this? I seem to have on occasion a miniscule but bothersome color cast
> on close inspection, it passes to the layman but for me I really need this
> to work. Consistency here is very important.
>
> Any input please.

Use curves, not Levels, for color correction. Using Curves allows you to
perform much more precise color correction.

--
Art, photography, shareware, polyamory, literature, kink:
all at http://www.xeromag.com/franklin.html

Peter

2005-10-23, 6:14 pm

I've read up on your posts and have done some reading thanks to your
insight. As for curves, I will have to start using it, I've been putting it
off long enough, no doubt. However because I started this latest assignment
in Levels I'll keep it for this one. What I've now opted to use is color
correction by numbers. I've been picking out my highlight and shadow based
on the threshold option in the adjustment menu. I use the color picker to
identify these points. I then put in the readings from the info menu and add
it in the levels menu. This appears to kill the cast. Now as for highlights?
In rgb I slide the gamma to lighten my images (grey cloudy day). How do I
know when enough is enough and when does posterization become a factor. Now
the reason I ask how much is sufficient consistency is gold.

Many articles site to pick out neutral areas that are supposed to be or you
think are grey, I try to avoid guessing. This seems unrealistic to me cause
on many a day your monitor may be out or light in you computer darkroom may
affect the visual.....

Your input is greatly appreciated

Thank You Peter
"Peter" <pierre.stj@nospam.sympatico.ca> wrote in message
news:piB6f.14678$ns3.1416926@news20.bellglobal.com...
> I'm working some shots and I've always eyeballed my color correction using
> levels in RGB on a channel by channel basis. I tighten the highlights and
> shadow arrows to the histogram also with a final tweak on the mid tones in
> rgb. I use the alt key and stop when I'm going to lose info on highlights
> or shadows. I don't know if its me but is there a better and more exact
> way of doing this? I seem to have on occasion a miniscule but bothersome
> color cast on close inspection, it passes to the layman but for me I
> really need this to work. Consistency here is very important.
>
> Any input please.
>
> Pete
>