Fixing an Underexposed Photo in Photoshop
Don't be disappointed if you get home from a day of shooting and a few of your photos are underexposed. Underexposed images can be fixed while overexposed ones cannot. You actually want your images to be slightly underexposed and free of any blow outs. Underexposed images are incredibly easy to brighten in Photoshop. The process is very quick.
If you are going for the underexposing to avoid blow out strategy, then your camera might be able to help you. Some cameras have something called Zebra mode. Zebra mode creates animated zebra lines on overexposed areas in your frame so that you can be super aware of them. If you see some zebra lines, then close your aperture to compensate (as long as you're not hurting the depth of field).
Fixing Overall Underexposure
Start Photoshop and open the file that is your photo. At the top of the screen, the 'Image' selection offers a few ways to bring the levels up in your photo. Probably the quickest way to get there is to select 'Auto Contrast'. This will balance the dynamic range of your image.
If you want more control, then go to 'Image'->'Image Adjustments'->'Brightness/Contrast'. Here, you can have more precise control over your whole image. Play around with the settings until you find a look you like. Remember, you can always undo any changes that you don't like.
Fixing Underexposed Areas
If only small areas of your image are underexposed, then you can't raise the overall image bright/contrast because you'll wash out part of the image. The only option you have is to isolate your underexposed area with the 'Magnetic Lasso' tool.
Click on the 'Magnetic Lasso' in your tool box. Click on the edge of the area you're going to isolate and slowly move it around the underexposed part. You need to be careful that you don't lasso any areas you don't want to alter. Once the lasso circle is complete go to the 'Image Adjustments'->'Brightness/Contrast'. You can make your adjustments to match the rest of the image.
Another way to change the overall exposure of your image is the 'Shadow/Highlight' feature. Go to 'Image'->'Image Adjustments'->'Shadow/Highlight'. Raising the shadow levels adds brightness to the image. Changing the highlights alters the contrast.
Another option to have great control over the image is the 'exposure' adjustment. Go to 'Image'->'Image Adjustments'->'Exposure'. The slightest adjustment has huge effects on your photo, so move slowly until you find levels that make your image look great. Experiment until you're happy.Popular P&S Cameras for High Quality Photos: