Viewing EXIF data in the Properties panel
The Properties panel displays your camera's EXIF data, and also supports Adobe's XMP standard. To view metadata in Picasa, click the Show/Hide Properties Panel button. You can also right-click (Windows) or Control-click (Mac) a photo and select Properties (Windows) or Get Info (Mac) to open the Properties panel.
Double-click any photo to see a histogram of the image located below the editing panel. A histogram is a chart showing you how the pixels in your image are distributed between dark and light. When you look at the histogram chart, the left side represents dark pixels and the right are bright pixels. Picasa shows you a histogram for each of the primary color channels that make up a typical photograph: red, green, and blue.
The histogram tells you how your edits affect the pixels in your photographs and the overall quality of your image. If you brighten your image to the point that you see pixels moving out of the range of the histogram then you are losing detail in your highlights. Conversely, if you darken shadows to the extreme left of the histogram you are losing shadow detail.