Colour Correction - Indepth for print work

Colour correction can involve up to seven steps: setting Gray to correct color, achieving good contrast, balancing colour to remove colour casts, adjusting skin tones, saturation, sharpening and converting to CMYK.

Make sure info palette is open. Set (palette options) your first colour readout to grayscale and your second to RGB. Choose the eye dropper tool and set the sample size to 3x3.

1. Finding Gray to correct colour
Find a neutral gray in the image. K in the info palette should read 50%. If there is no gray, find an object that is somewhat neutral. Write down the numbers displayed in the RGB readout window. Add these values together and divide by 3 to get the average brightness of these values. (Equal RGB values will create a neutral gray with no colour cast.)

Image>adjust>curves
Leaving the pop-up set to RGB will only change the RGB in equal amounts. This is not the correct procedure.

Start by choosing the Red channel. Click anywhere on the curve to add a point. Click on the input number and enter the Red Value of the sampled colour. Click on the output number and ten enter the average number you calculated.

Repeat this step for the Green and Blue channels. Most of the colour cast should be removed after this step is complete.

If the image looks totally out of whack, you may have selected from a poor area. Try finding another neutral area on the image.

2. Achieving good contrast
If the image looks flat and lifeless you may need to optimize the contrast. The best way to adjust the contrast is to adjust the levels for each colour. (Adjusting the RGB slider may cause posterization effects in your image.)

Image>adjust>levels
Change the pop-up window to Red and drag the input sliders (left and right) until they touch the beginning and end of the main histogram. Ignore stray pixels and concentrate on the areas where the slope begins and ends.

Repeat this process for the Green and Blue Channels.

Your image now has a full range of contrast (0-255) for each of RG&B.

3. Colour Balance (removing colour casts)
Highlight, shadow and mid-point are three areas of an image that should usually be a shade of gray. The objective here is to make the brightest area of the image as bright as possible and the darkest as dark as possible without losing detail.

Pure white (255, 255, 255) settings will "blow-out" when the image is printed and pure black (0, 0, 0) will "plug" when printed.

To adjust for this you need to set up minimum and maximum ink limits. Click on the foreground colour to open the colour picker. Set the Saturation (S) to 0 and Brightness (B) 100 (white) and then click on the Brightness button. Move the slider down the grayscale bar until the Magenta and Yellow readouts indicate values to who are adjusting toward. (M 5%, Y 5%) The cyan value is usually higher.

When done, check the RGB values to the left. They will show you the value (they should be the same) to be used to achieve the minimum ink values in CMYK. Make note of the RGB value.

Repeat the process for the shadow areas.

Total ink limit is usually around or less then 300% (add the CMYK values). Make note of the RGB values here as well. For the midtones area, refer to the colour readout in the info palette. Add the numbers and divide by three.

Create a curves adjustment layer. Since you are attempting to balance out the colours, you need to work on the individual colour channels by adjusting them. Add an adjustment point to each curve (central location) Proceed to apply the values for the highlight, shadown and midtones by selecting the appropriate adjustment point and entering the input and output values.

For the mid-point area you will have to use the individual RGB channel adjustment.

4. Skin Tones

I'll do this later. The image now should be pretty good and this step may not be necesary.

5. Optimizing Saturation
Before adjusting the saturation, turn on the Gamut Warning.

view>gamut warning

Create another adjustment layer for Hue/Saturation. If no gray appears in your image you can increase the saturation until you start seeing small areas of gray. If you see large areas of gray (blotches), your image is over saturated. Decrease the saturation if this is the case.

After all these corrections, merge the adjustment layers with the image.

6. Sharpening the Image
Use Unsharp Mask

Filter>sharpen>unsharp mask

Adjust Radius: Radius controls how wide samples are on each side of an edge. A small radius creates a smaller halo. A general rule of thumb is to use a radius of output resolution divided by 200 (300ppi/200=1.5 radius). This is a starting points but dont vary too much from it.

Amount: Amount adjusts how intense the halos are or how much tonal differences are accentuated. Start at around 200% and work your way up or down as desired.

Threshhold: Low threshold values result in an overall sharper image. Try starting at 3-4. Anything about 10 exclude so many areas it is difficult to see any effect in the image.

7. Converting the image to CMYK
Convert the image to CMYK mode and then turn on the Gamut Warning and check to see there are no large gray blotches. The image is now ready to print.

Some good advice there Spectra.

I would like to offer a slightly different approach to your inital step of finding the mid grey and altering that first.

Add a threshold adjustment layer, this should reduce the image to a black and white posterised look.

You are aiming to find the black and the white point in the layer. To find the black point, move the slider all the way to the left and then gradually bring it back to the right until you find the smallest amount of black showing in the image. Mark this point with the colour sampler tool.

Repeat this process to find the white point and also mark it with the colour sampler tool.

Throw away the adjustment layer and you should be left with the 2 colour sampler tools marks.

Add a curves or levels adjustment layer and using your 2 marks as guides set the white and the black point with the relevant eyedroppers.

After this if neccesary I would then move on to adjusting the midtones, perhaps adding a small s-curve if the image needs more oomph.

Another sharpening techinque for your consideration.

Duplicate your layer and run a high pass filter of around 3-10 (depending on your image resolution and sharpening requirements)

Set the layer blend mode to soft light or hard light (soft light is kinder as a rule) and reduce the opacity of the layer to suit.