Jez wrote:
> In the book "Photoshop for Photographers" concerning color
> balance there is an interesting suggestion that you photograph a
> grey card (ideally a Kodak 18 0rey reference) and then use the
> dropper tool to sample some pixels on the scanned image. The R, G
> and B components should all have the SAME numerical value if
> there is no color cast - and presumably should equate to 18%. So
> by tweaking levels on the color channels they can be equalized
> and then by tweaking brightness and contrast the image can
> presumably be 'calibrated' to the gray card.
> And then the rest of the photos on the same film should be correct.
>
> Anyone tried this?
>
Sort of !
I wouldn't use only an 18 0ray card, try something like a Kodak Gray scale
("Kodak Color Separation Guide and Gray Scale (Small) Q13 CAT 152 7654 - 20
steps from "white" to "black"). When photographing these backet your
exposures and check the negatives/transparancies to ensure that you can
separate the 20 steps. Scan the best negatives/transparencies and see if
you can resolve the 20 steps on the scan. Check the colour balance at
several points on the gray scale - I know my scanner doesn't have the same
colour balance over the full density range, possibly the film doesn't either
! Basically, a single gay card image will only give you 1 reference point -
you will not be able to "calibrate" from this 1 point.
Wayne Harridge
Ivanhoe, Victoria, Australia
http://members.optusnet.com.au/~w_harridge/
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