Hot to correct colors with a gray card

I wonder what can be the most appropriate way to correct a picture

color based on a gray card photographed in a different picture.

I take slides: first shoot the subject, then put the gray card close

to the subject and shoot again. I then scan the slides.

I would like to correct colors in the first shot, based on the gray

card taken in the second shot. I am using Photoshop CS, but also

suggestions related to other applications (e.g. PWP) will be

insightful.

Also, do you believe that taking the two shots with the same exposure

(time and aperture) is important?

Here what I am doing know:

- open the second shoot (with the gray card);

- open the levels or curves tool, or a layer, and use the gray

eyedropper to set the gray point from the gray card

- save the levels/curves

- open the first shoot (with the subject), open the levels or curves

tool (or a layer), and then load and apply the previously saved

levels/curves

Does it make sense, and any better way to proceed?

I'm interested in this as well. One (bullet-proof?) variation:

Cut a small triangle off a corner of your grey card, place it in the frame with the subject, albeit tucked away in some busy, random background area, and resign yourself to cloning it out of the picture.

Francesco

Ok this gets a little complicated so if you have questions e-mail me.

First make sure your color space in PS is set to adobe RGB 1998 and make sure that you are capturing images in adobe RGB (if you can).

Second at the very least get a black gray white card from either kodak or gretag macbeth.

Shoot the black gray white card in the first shot. Bring up the first shot in PS. Press control M (PC) to bring up curves. Next click the black dropper in curves and then click the black area on the card. Do the same for white then gray.

To use this curve on the next photo just open it and press alt control M. Which applies the same curve to the next photo.

There are other ways to do this but this is by far the easiest.

Good luck

worlds best greycard and colorcards(no joke)-best and affordable.

here is an expert of many years.

no more guessing at what angle to put the unique greycard with dot-pattern. but there is also a very small, very thick and stable 7x10cm-version.

small company and very successful. the asians are crazy for these cards/solutions

http://www.fotowand.de