I recently wrote a few pretty decent posts on this. You can check them out here:
Those posts were a bit to read through, so I thought I’d write a condensed summary here, so here goes.
To apply a Levels adjustment layer in an effort to either brighten or darken an image, follow these instructions. This adjustment layer also helps with adding or removing contrast.
– Open your photograph in Adobe Photoshop.
– In the Adjustments panel, click the Levels icon.
– In the Properties panel that pops open, push the black, white and gray sliders under the histogram to the left or to the right until you see your desired result.
– Push the sliders attached to the black to white gradient bar underneath the previous sliders for additional results.
– Use the Auto button for a result that’s suggested by Photoshop.
– To adjust the Reds, Greens or Blues in the photo, click on the RGB drop-down and make your color selection. Then, push the respective sliders for those colors.
This is a nice short summary. For further instructions, please visit the blog posts.