Dealing with a 'Colour Cast'
Clive R. Haynes FRPS
notes have a specific reference to dealing with a colour cast within a scene where
the shift in colour balance is caused by the lighting being 'out of balance' with
the film type or camera setting (DI camera 'white balance').
notes are not intended to correct for a colour cast that may be encountered during
the printing process - the sort of 'differential' between monitor and printer.
'Gray Point Eyedropper'
examples below show the 'Levels' dialogue box but the operation is the same for
To use the 'Gray Point Eyedropper', simply click on the one you need, then take the cursor into the image and click on the point you wish to set as absolute black and/or peak white. You'll see a change in the shape of 'Levels Histogram' and if using an individual Red, Green or Blue 'Channel', the black & mid-grey or the white & mid-grey settings for that channel will shift.
The scene below is an example of the sort of problem.
This is a picture of yours truly giving a talk to a group of people under ordinary tungsten room light. As you can se there is a severe warm colour cast.
What can be done to bring the colour back into something like the correct balance?
This is where the 'Grey Eyedropper' is of great use.
preferred route is via an 'Adjustment Layer' - as the 'Mask' facility within the
Adjustment layer allows you to selectively erase the corrected areas should you
Creating a 'Levels'
From whichever dialogue box you prefer ('Levels' or 'Curves' - although the examples
below are for 'Levels') click on the 'Set Gray Point' Eyedropper and look carefully
at the image.
Note: The area of image that should be grey must have been
included in the original scene - it has to have received the same lighting (or
mix of lighting) as the scene. It's no good generating a neutral grey afterwards!
Above: In the scene there were, fortunately, at least three possible neutral grey points to choose from - the shirt (chosen), the 'laptop' computer surface or the front face of the data projector unit. Yes, I did try all three and the shirt gave the best tone.
|The Levels histogram may appear not to alter in 'RGB' but go to a 'Channel' (R, G, or B) and the shift will be observed. Similarly for 'Curves', the Curve 'line' will not alter in 'RGB' but the change(s) can be observed when visiting the individual R, G, or B 'Channels'.|
quite right? Some tweaking required?|
Having fixed the problem some fine tuning may be necessary.
Photoshop features several controls that can help in manually shifting the colour bias of the image either generally or targeted to a specific area. The most usual routes are accessible via the Image > Adjust menu, or more preferably via an Adjustment Layer and are:
Hue & Saturation (adjust the Hue slider)
Variations (not via an Adjustment Layer)
Anticipate the Problem
- Plan Ahead
A Reminder: After
loading/scanning the image into Photoshop, make an Adjustment Layer and choose
'Levels' or 'Curves' then with the 'Set Grey Point Eyedropper' activated, click
on the Grey Card in the scene and the correction is instantaneous!
the settings for the corrected scene could be 'saved'.
To discover more about transferring Adjustment Layers from one image to another - click on the link below.