Color Correcting By Numbers

Color correcting images on screen for digital output is one of the most problematic issues in digital imaging. What was a task left to pre-press professionals in the past and something of no concern with the general public using film cameras, has now become a must for anyone using a digital camera.

Over the years there has been a number of companies offering color calibration hardware and software to match what is seen on screen to the actual printout. Soft proofing (the ability to see an image on the monitor and being able to make a pretty good judgment about how it is going to look when printed) is definitely of big help.

This said, the only sure way to be able to precisely color correct is done by numbers. To make a comparison, take a pilot for example: when visibility is low pilots rely on what the instruments say to navigate. No matter if it is day or night, they can navigate without having to see where they are and where they are going.

The same can be applied to color correction. Depending on a monitor to apply color changes, especially when subtle modifications need to be made, is not the best way. All computer screens shift in color from the moment they are turned on. Numbers do not change.

Photoshop makes it very easy to see what the color measurements are for any given area in an image. The eye picker will read the quantity of red, green and blue when passed over the chosen area. For the purpose of this article we will stay with the RGB primary color space. When looking at a pure black the reading will be R=0, B=0, G=0. A pure white will be R=255, B=255, G=255, 255 being the maximum amount in the scale. What is between pure white and pure black? Grey, which can be any equal amount of these 3 colors, would be for example 122, 122, 122. A lighter grey is 180, 180, 180.

Why is this important? Simply put, very! Let’ s go into something practical: You are working on a picture of a toddler taken with that high-resolution camera you got for Christmas. It looks good on the screen but when printed the skin tones make your child look like he is blushing under 110 F heat. When going over the skin with the picker the reading is R=200, G=140, B=139. Immediately the red reading of 200 pops out as a lot compared to the fairly equal and lesser quantities of green and blue. Without looking at a proof you can see how that skin will be primarily red.

It is OK for skin tones to have a mix of colors and for a Caucasian complexion to have a higher level of red. A difference of about 60 between red and the other two primaries may be excessive. Also, a warm skin tone requires a little bit of yellow to be present. In our case the initial reading for B and G is about equal, meaning they neutralize each other and make the red stand out as the preponderant color. To add a little yellow we need to decrease the blue (Blue and Yellow are opposites). In this case a good mix for such skin tone could be R=190, G=140, B=130.

These numbers are not to be taken literally but as a guide to create a pleasing balance. Practice taking color readings in different areas of an image. Look at the printed proof. You will soon learn to correct by numbers and be free from monitor shifting.

Finally, keep in mind inkjet printers do not use red, green and blue inks or pigments to print. They instead employ Cyan+Magenta+Yellow+Black (CMYK). Others add a light Magenta and light Cyan to the mix (New printers are actually coming out with even more colors). Without going into details about why this is done, the result is that these machines cannot reproduce a lot of the bright, saturated colors seen on the monitor (which uses RGB phosphors).

Photoshop has something called “gamut warning”: it is a way to soft proof the image on the screen as it would appear when printed.

As you can see, there are quite a few tools right in your software that can help to preview and correct photos before being printed and without unnecessary waste of time and money.

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