Making a curves adjustment using Photoshop's "Quick Mask" function can help you even out the lighting in your photos. We've all had those images where the foreground is bright and the background is dark, or the background is nice and bright, while the subject (or foreground) is too dark. Using an overall curves or levels adjustment means that you are adjusting the brightness and tone over the entire image, not just the parts that are too dark.
For those of you who aren't sure how to make these corrections to .jpg images using the channels functions, an easy way to adjust a portion of your image is to use the Quick Mask in Photoshop to mask of the area you don't want to change.
Let's run through this very quick adjustment procedure. I wasn't able to find exactly the type of image I wanted but we'll use this one. The background on this snapshot is one that I'd really like to be white, not beige (which is the wall colour).
If I use the curves adjustment in photoshop, I can make it whiter, if I try to make it completely white it will also make everything else really bright too. To avoid making everything in my image really white I want to protect certain areas of the photo from being adjusted. In this case, I wanted to protect the metal hood and light bulb because those areas are mid-to-light tones. Without masking off the areas I need to protect, this is what I get.
To enter the Quick Mask Mode, click the little button that looks like a square with a circle in.
Select a hard paint brush of a size appropriate for masking off your area - smaller areas need a smaller brush. Begin painting the section you want to keep "as is". You'll notice the "mask" is pink or red. Even though you are "painting" directly on the image, this isn't an actual "paint" layer.
While in the Quick Mask mode, you can also switch to the eraser if you've gone over the edge, or painted on something you shouldn't have. Once you've got the "mask" completed you're ready to release it.
Now I'm going to adjust the curves so my background is white - the results are that while the background has been pushed up to white, the subject we've "protected" (here the silver metal and light bulb) is still it's original colour. The black is still black even though I didn't mask it off, and that's mostly because I adjusted the midtones and whites when using the curves.
Using the Quick Mask Mode you can also brighten areas in shadow, while leaving the background in it's original brightness, such as in the image below. These are the same image (one is just reversed). You'll notice the image on the left shows the face in shadow, while the image on the right has been brightened almost to the level of the background tone, yet the background colour is the same in both images. Lightening the background would have blown it out to almost white, rather than it's natural colour.
This is achieved simply - use the quick mask and paint the face and hair. When you release the mask for the selection, you'll want to invert that selection so the area selected for protection would be the background, rather than the shadowed face. When you use the curves or levels to lighten the face and hair, it won't lighten the background. To invert the selection, once you release the mask choose "select" from your menu, then choose "inverse" or "invert" before you begin making the changes to your image.