It turns into an eyedropper icon. Click and find out where on the curve that value lies. The final tip for this section is related to tip 2 above. We have talked about maximizing the available tonal range for your image. Now we get a little more deliberate.
Many photographs have a subject and a background or a foreground.
Think about a portrait for instance. In most cases, we care more about the subject than we do about the background, so the effort should be to get as much snap and sparkle into the subject as possible. We've seen how the contrast is increased where the slope of the curve is steeper. Click and hold in the image window and move the curser around the subject, and make a mental note of the approximate parts of the curve the marker slides around. Steepen the slope of that part and you will gain a great deal.