Lily Galib, Production Associate, Image Quality Control, has written a three-part post on the ins-and-outs of light value adjustments. Part 1 covered histograms, part 2 covers working with Photoshop’s Levels Adjustment Tool and the Curves Adjustment Tool, and part 3 concludes with color.
Adjusting with the Levels Adjustment Tool:
Levels is a fairly basic tool for making light value adjustments. When adjusting with Levels, you have three set adjustment points laid out on top of a histogram: black, white, and a midpoint adjuster. You make changes to the histogram by sliding these three adjusters along a horizontal line at the bottom of the histogram. Moving the black and white adjusters sets the black and white points and moving the midpoint adjuster redefines the midpoint. Sliding the midpoint adjuster to the left stretches the values between the midpoint and the black point (0) and compresses the values between the midpoint and the white point (255). This lightens the midtones and increases the contrast in the darker half of the histogram, while decreasing it in the lighter half. As a result you will see more detail in the darker midtones and less in the lighter midtones. Sliding the midpoint adjuster to the right does the opposite. Once you set these points, Photoshop will interpolate the light values accordingly and even out the histogram. The black and white points that you set become the ends with the midpoint once again centered between the two.
Levels may be all that you need to make adequate light value adjustments, but in many instances finer adjustments will be necessary and for that you need to use the Curves Adjustment Tool. Curves can be intimidating at first, but once you’re accustomed to working with it you’ll find that it’s an incredibly useful tool for making both light value and color adjustments, which I will discuss in a separate section.
Adjusting with the Curves Adjustment Tool:
When making adjustments with Curves, you’re still using a histogram as you are when using Levels. Curves is just a different way of working with it. Instead of having three predefined adjustment points that you can slide to the left or right, you place anchor points along a diagonal line and you adjust the light value by pulling these points. This allows for far greater flexibility and much finer adjustments than are possible using Levels.
Two very basic adjustments which will help familiarize you with Curves are the “S curve” which increases contrast and the “inverse S curve” which decreases contrast. More specifically, the “S curve” increases contrast in the midtones, which gives the appearance of an overall increase in contrast. This comes back to what I mentioned earlier about moving contrast around. The “S curve” increases contrast in the midtones, therefore decreasing contrast in the highlights and shadows. You will see this effect as a loss of the fine details in the highlight and shadow areas of your image as the shadows become darker and the highlights become lighter. The “inverse S” does the opposite: it increases contrast in the highlights and shadows, thus giving more detail in those areas, but it does so at the expense of the midtones which begin to appear flat or washed out.
Practicing with these two types of adjustments will give you a basic understanding of how the Curves Adjustment Tool works. Once you become more familiar and comfortable with it you’ll see that there is so much that you can do with Curves because it allows you to place anchor points anywhere along the line and make very specific adjustments. Personally, Curves is my favorite adjustment tool because of the greater flexibility it allows, but I do use Levels to quickly bring in the white and black points so that I have the full tonal range from 0-255 with no gaps. In keeping with ARTstor’s philosophy of maintaining integrity of the image, I do this only to the point where there is no clipping in the highlights and shadows, i.e. I wouldn’t set the black or white point further in than the ends of the histogram. I generally use Curves to make adjustments in the midtones. I always use adjustment layers when I do my corrections instead of working directly on the image. This allows me to go back and make changes to my work as I go along and also allows me to use masks if necessary. I’m not going to go into detail about masking here, but it basically means obscuring part of a layer so that the corrections are only applied to a specific area of the image. Once I’ve finished with all my corrections I’ll flatten the layers.