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Curves, Curves, Everywhere Curves

Around every curve in the road you are liable to come across another imaging application that contains some implementation of our current topic at hand. Once you understand how curves work, you'll find your newly acquired adjustment skills handy in any number of programs, not just Photoshop and Photoshop Elements (if you're an Elements user, you have added Curves support, haven't you?).

Nikon Scan does likewise for images scanned from film. So does SilverFast, a popular scanning program. I prefer to do most of my work in Photoshop, but it's nice to see Curves becoming so universal. Paint Shop Pro's support for Curves is somewhat limited, but other alternative imaging programs such the Gimp and Picture Window Pro provide full Curves implementations. Their usefulness has made Curves an all but required feature for serious image editing.

Curves in Nikon Capture
Curves in Nikon Capture
Curves in Nikon Scan
Curves in Nikon Scan
Curves in SilverFast
Curves in SilverFast

There are those though who decry the use of Curves as an abomination, claiming that they produce unnatural effects by altering the relative lightness values within an image. Many digital darkroom techniques are often criticized in similar ways and I want to take a moment to respond to this charge. Just because computers are powerful and are capable of gross image distortion doesn't mean that they are always used in that way.

Tone curves have been a part of photography since the very beginning in fact. Human vision sees different wavelengths of light as colors. Our eyes are most sensitive to green which lies in the middle of the rainbow and less so to the colors towards the ends of the spectrum. We also respond better when brightness is within a certain range and have a harder time distinguishing color and detail in dim or overly bright light. Every film has its own response curve which may or may not mimic our own that closely. Films such as Fuji Velvia exhibit exaggerated contrast and have a fairly steep response curve in the mid tones. On the other hand, films like Fuji Astia have flatter response curves. Pushing film during developing also exaggerates contrast while pulling it lessens contrast. Techniques such as pre-exposure have also been used by photographers for years to make lower contrast images. Ansel Adams extensively altered his developing chemistry as well as how long an image was kept in each pan in order to emphasize detail as part of his creative process. Indeed, he generally previsualized the entire development process and factored it in before he ever pressed the shutter release. All of these are, at their heart, nothing more than curves.

Curves the Gimp
Curves in the Gimp
Characteristic curve for Fuji Velvia
Characteristic curve for Fuji Velvia

Human vision can see between 15 and 30 stops of brightness contrast from black to white, depending on the circumstance. Film and digital capture see no more than five to seven stops and printed output is capable of even less. If we merely pulled five stops out of the middle of an average outdoor scene, we'd end up with a large amount of blindingly burned out whites, deep, unfathomably black shadows, or both. We need some way to compress the brightness range found in nature to fit within the medium we are working in. Simply squishing everything in towards the middle is one approach of course, but this would generally result in rather flat, low contrasts images that lack impact. What we want to be able to do is to make use of the limited contrast range available to us in ways that will come as close as possible to conveying what it looked like to be there.

Speaking the language of 8-bit imaging, we only have 256 discreet brightness values to work with from black at one end to the pure white at the other. That's not much. To maximize the creative potential of a medium with such a compressed dynamic range, we need to be stingy, doling out contrast only where it will do the most good. Squandering some of our 256 values in areas of an image with little of no important detail is simply wasteful. Yes, curves are somewhat magical sleights of hand, allowing us to shift contrast around to where it will do us the most good, but if they are used as intended (rather than to create bizarre pop art), they are an incredibly useful tool, not an abomination.

Now that we've taken time out to justify our existence (or at least that of Curves), we'll proceed next week with how to use Curves to get the most out of those 256 values.

Date posted: January 9, 2005


Copyright © 2005 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Curves (and Other Goodies) for Photoshop Elements Return to archives menu Next tip: Stealing Contrast with Photoshop Curves

Related articles:
Velvia: ISO 50 or 40 ... or How About 100 Speed ?
More Than a Bit of a Difference: 8-bit Versus 16-bit
Curves (and Other Goodies) for Photoshop Elements

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