Wednesday, December 1, 2010

Curves & Auto Curves Image Adjustments by Paul Baldassini

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Greetings everyone! It's been over a month since I've posted as I have been very busy with my graphic design and photo restoration "day job" work. It's quiet again so I'm back to painting and writing for my Blog. My newest painting is also moving right along and I will be posting in-progress updates on this Blog very soon.

Meanwhile, here is Part 3 of my Image Adjustment series, Curves & Auto Curves — actually just the first page is posted below — it's way too much to put up on this Blog. You can download the whole Curves Studio Notes from my website — along with all the Studio Notes — which are  available as .pdfs on the RESOURCES pages. Click this link to get there fast:

Curves & Auto Curves
All images can be significantly improved by performing three simple and painless adjustments: Shadow Highlight, followed by Curves, followed by Sharpening, preferably in that order. The Shadow Highlight adjustment was already discussed in detail in a previous post/.pdf, and this discussion is about using Curves to adjust the tonal range of an image.

Much has been written about image adjustments using Curves, and along with Levels, they are Photoshops most powerful tools for controlling the tonal range of an image. Levels are much easier to learn and use and do an adequate job of improving images, but Curves are more powerful. For years I used only  CMYK Curves (and occasionally RGB Curves) to adjust the tonal range of an image since I was preparing images for use in print media only. I adjusted the curve for each individual channel and produced images that would reproduce well in print. I made sure that shadows would not plug up, that there would be some tone in the highlights (except for specular highlights) and that the mid-tones were neutral to compensate for colorcasts. If I were to describe in detail here how all of that is accomplished and theory behind it all, then this little discussion would soon become a little book and you would most likely not make it past the first couple of pages. If you are interested in that kind of knowledge than I highly recommend reading books such as Professional Photoshop, Makeready, or Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace by Dan Margulis; Real World Photoshop or other books by Bruce Fraser; Photoshop Color Correction by Michael Kieran; or Adobe Photoshop for Photographers by Michael Evening. Online, a Google query of something like “using Curves in Photoshop” will result in a great many interesting and informative hits.

My posts are directed towards artists trying to get the most out of their digital images for use as quality source material for paintings. These reference materials may or may not be in addition to sketches, plein air field studies, or memory. But whatever the reason, the mostly likely end use is probably going to be either an acceptable inkjet print mounted next to your canvas for easy viewing or an on-screen image, or more often, both.

I use the Curves tool in two different ways, one a bit more involved than the other but both perform a necessary improvement to digital images. To access the  Curves dialog press Command-M (think Modify) and you will be presented with the Curves default dialog. If you have been using   Levels up to now, the major difference between the two is that although both adjustments let you adjust the entire tonal range of an image, Levels uses only three adjustments (white point, black point, gamma/brightness), whereas   Curves allows for multiple adjustment points throughout an image's tonal range from shadows to highlights. Many points can be plotted on a curve, but it’s the   Shadow, Highlight and  Midtone points that are the most important to learn how to use. Although it’s more complicated (and very confusing if you’re new to adjusting images this way) you can also use Curves to make precise adjustments to individual color channels in an image. And, like other image adjustments, the settings made in the Curves dialog box can be saved and loaded to apply to other images.

Photoshop Curves Dialog showing location of Highlight, Midtone & Shadow Points.  Highlighted settings at right need to be changed from default settings.

To adjust the tonal range of an image you need to click on and move a point on the Curve. The default state of the Curves dialog contains two endpoints — you have to click to add other points such as a midtone point. So, for example, if you want to adjust the Shadow point, you would click on that point and move it manually or by tapping one of the 4 keyboard arrows, which allows for very precise positioning of the points. Moving a point upward and/or to the left lightens a particular range of tones, and moving a point downward and/or to the right darkens a particular range of tones. Thus, to lighten the shadows, move up a point near the bottom of the curve; if you want to darken the highlights, move down a point near the top of the curve.

Changing the shape of the curve alters the tonality and color of an image. Bowing the curve upward lightens an image, and bowing the curve downward darkens it. Steepen the curve (move the Shadow point to the left) and you increase the image contrast; lower or flatten the curve (raise the highlight point) and the contrast will decrease and the tonality will flatten.

Curves can have different shapes, the two most common being the  “Z” Curve or the  “S” Curve. The simplest Curve adjustment is the   Z-curve which acts much like the  Levels Adjustment in that only the darkest shadows and lightest highlights are adjusted (the top and bottom points moved left or right respectively) making for a “punchier” image with increased overall contrast. The midtones remain unchanged. And often this just is enough to improve the image from its original settings.

By adding additional points (usually not more than one or two, but up to 14 are possible) somewhere on the default 45° curve and moving them up or down, an   S-curve is created. This typically adds contrast to the midtones, since that is normally where images will need improvement, and suppresses detail in the shadows and highlights. All images have to be assessed for their unique characteristics and “deficiencies” so that an appropriate curve can be built. There's no precise formula to follow, and the exact appearance of the curve will depend on the content of your image.

Go to  for the complete Curves & Auto Curves Studio Notes.