Expert: Scott Valentine - 12/1/2007

Question
Dear Scott,
   my question is when adjusting black and white photos into Photoshop No. 7 if I want to lighten the dark shadow areas how can I do this using curves.  When using levels I am somewhat disappointed with the results.  To sum up my question, would you be kind enough to given me step-by-step directions on the use of curves.  I am using Photoshop No. 7 and Epson 1280 printer and scanning my slides on a Connon scanner at 4000 dots per inch.  Respectfully yours stewart

Answer
Thanks for your question, Stewart. While it's outside the scope of this forum to provide step-by-step details on this topic, I'm more than happy to give you some general approaches.

First, you are on the right track in choosing to deal with Curves. As with all adjustments, you should consider applying Curves as an adjustment layer, rather that directly on an image layer (Layer > Adjustment Layer > Curves).

Next, consider using Lab space for your tone corrections. Dan Margulis' excellent book on using this color space also spends a lot of time with discussions on Curves, and is a great resource for digital imaging. You can find a description of the book here:

http://www.peachpit.com/store/product.aspx?isbn=0321356780

You can also search online for additional information on using Lab space, and what it means for your images to convert between spaces. These will be important issues when you go to print, so be sure you understand the color space implications.

Essentially, a Curves adjustment is a remapping of tonal values along a contour. This is different than Levels which merely compresses image information to a parametric range. Curves adjustments will actually change the relationship between neighboring colors in a more controlled way.

There are some basic techniques for applying Curves, depending on your goal. Creating a lazy "S" shape increases contrast, while an inverted "S" will reduce it. Also, you can drag in the end points on the top and bottom on individual channels to increase saturation, or drag points down the sides to decrease it. Moving the curve away from the center will shift the colors themselves, which can be useful for correcting unwanted color casts or fine-tuning shadow and highlight colors.

In summary: for RGB space you can adjust all channels globally to change contrast and brightness; in RGB and Lab, you can adjust individual color channels for shifting or other corrections; in Lab, you can adjust the L (Lightness) channel independently to increase contrast without changing the relationship of colors.

I hope this helps get you on your way. Curves can be a complicated topic, so I recommend sitting down with a book or a few online tutorials to help understand the variety of options. I use Curves on almost every image, and have abandoned Levels to special cases only. I think you'll find the time well spent.

Let me know if you have further questions or would like some clarification.

-Scott