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Return to the August 2009 issue of the SMS Register.
Hitting the highlights, lurking in the shadows
Have you ever opened up a copy of your magazine and been disappointed that the picture in print doesn’t seem nearly as bright and vibrant as the image you saw on your screen? Why is it so hard to get the “pop” you’re expecting from your images?
The biggest culprit is the fact that the range of colors it’s possible to produce on a printed page is pretty small — particularly in comparison to the range visible to the human eye. Although it varies greatly from one individual to another, the average human eye can discern an estimated 7 million colors. This range, from the brightest white to the darkest perceptible black, is called the “gamut of human vision.”
Computer monitors produce colors by mixing red, green and blue light. Because of this and other physical limitations, monitors have a gamut that is limited to a smaller area of the visible range.
The printed page, which is viewed by reflective light using the four process inks, has a gamut that’s about half the size of RGB.
Now take into account that paper is not perfectly white but rather grey with a slight cast to it, and that the physical materials used to manufacture the inks do not produce pure cyan, magenta, or yellow, and the actual range of reproducible colors shrinks still further. All of this means that the road to a realistic representation of an actual live scene is a difficult and bumpy one.
To illustrate how shrinking color spaces can affect image reproduction, let’s take a scene from the real world. Suppose the picture at right is an actual live scene we’re viewing.
Compared to the full range of colors the human eye can perceive, this scene does not come close to using the lightest, brightest perceptible colors (like staring at a 100-watt light bulb, for example), or the darkest possible details. The lightest areas of the scene are still rather gray, leaving a large portion of the visible spectrum unused. This makes sense, in that we don’t expect eggs to glow like lightbulbs.
When we take the live scene and reproduce it in the RGB color space for viewing on our computer displays, we’re forcing the image into a smaller color space. Because the original scene uses only a portion of the entire visible spectrum, the conversion to RGB (below left) forces the color information into the relative portion of the RGB color space.
The conversion from RGB to CMYK (below right) again forces the image into the relative portion of the still-smaller CMYK color space.
At this point we’re not only reproducing a live scene using a dramatically smaller color space, but we’re using only a fraction of the available colors within that small space. No wonder printed images don’t always turn out as expected.
Fortunately, there are a number of tools and tactics we can use to ensure that the information we want to communicate makes the trip from live scene to printed piece in the best possible condition. The first step is to find and reclaim the “lost” space in CMYK images by adjusting highlights and shadows.
Adjusting Highlights and Shadows
In examining our CMYK image, we find the lightest highlight to be 19C 13M 9Y 0K (#1) and the darkest shadow to be 245% total Ink (#2).
The image does not take advantage of any lighter and darker colors available; rather, all the color information is squeezed into the midtone ranges. This makes sense for the live scene, but our CMYK color space simply cannot make anything “glow.” It can only be as bright as the paper we’re printing on. So the question becomes “Are the lightest areas of the real eggs as light as the paper?” They’re probably at least that bright, so we’d better make sure our image takes advantage of all the highlights the CMYK color space has to offer.
With the Photoshop Curves tool, we’ll adjust the highlight end of each channel to the lightest possible value, while still retaining some detail information. Our goal numbers are 3C 2M 2Y 0K.
We’ll also adjust the darkest value to have a total of no more than 280% of all four ink values combined. This value roughly equates to the highest total ink an area can have and still hold detail information.
These moves result in the final densitomer readings shown above. Note that Sample #1 matches our goal for highlights, while Sample #2 matches our total ink goal.
These color moves adjust the starting and ending points for our image within the CMYK color space to make the lightest and darkest regions available to the image, and redistribute all the colors in between (the midtones) along the curve relative to these new points, thus taking advantage of the entire CMYK color gamut.
Below are the before (left) and after (right) CMYK images:
Clearly, Photoshop Curves is a powerful tool, capable of accomplishing most color corrections. We’ll get into further uses of the Photoshop Curves tool in future issues.
Download a PDF version of this article.
Return to the August 2009 issue of the SMS Register.
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