To bring up the levels window, go to Image ▸ Levels (⌘L). Pro Tip: Work on a copy of your image when experimenting with levels to avoid making permanent changes to the original.
Levels are a different way of looking at an image. Instead of the picture being displayed in its normal viewable state, it is shown as a bar graph called a histogram. The histogram is essentially a count of how many times each color shows up in the image. In the image to the right dark colors and midtones are showing up a lot, as you can see by the high spikes in the graph at the left end and just to the right of the middle. The image is lacking very light colors and white as you can see by the absence of peaks on the right.
To correct or modify levels, simply drag the triangular slider controls on the left, right, and center at the bottom of the histogram. Doing so will instantly change the image on your canvas, hopefully for the better. The left slider controls the black point, the right slider controls the white point, and the middle slider controls the midtones. To improve this image, leave the slider on the left alone since it is already under the color peaks. Pull the right slider in towards the left until it rest under the start of where the closest peak tapers off. Then play around with dragging the midtones slider to the left to darken the midtones a bit. Every image is different, so look at your changes and make sure they are to your liking before you hit 'OK.'
Before Levels Adjusted
After Levels Adjusted
Below the histogram is a slider with two control tabs. These control how black the blacks are, as well as the whites. If you bring both tabs in towards the middle you will decrease the contrast of your image. Try experimenting with them to get the hang of it. If you have the 'live histogram' box checked, you will see how your histogram changes when you change the output slider positions.
Reset, Auto Levels, and Visibility
To restore the original values in the histogram and output slider, click on the rotating arrow button located on the left. Acorn can do the levels adjustment for you with 'Auto Levels'. Click on the middle button and the sliders will automatically move to where Acorn thinks they should be located. If you want to quickly see what your image looks like with the levels adjusted and without, click on the visibility button located on the right.