Correct an Underexposed (or Overexposed) Image
Your exposure determines how bright your image looks. An underexposed photograph appears dark all around and typically contains little to no detail in the shadow areas. An overexposed image appears bright and can lose details in the highlight areas. The more under- or overexposed a photograph is, the more detail you lose and the harder it’ll be to fix the file in postproduction.
If an image is slightly underexposed, you can fix it by using the exposure adjustment tool in your photo-editing software. Simply raise the brightness of the image until it looks the way you want it to.
For an image that’s significantly underexposed, you may notice as you brighten that you have little to no detail in the shadow areas. If this is the case, you’ll see a lot of digital noise in those areas as they become brighter and more visible. You may want to consider using the software’s noise removal filter to reduce digital noise.
If you’re working with an overexposed image, you want to lower its brightness with the exposure adjustment tool. If too much highlight detail was lost in the exposure (meaning there are areas in the frame that are pure white, with no information) then you won’t be able to bring it back no matter how far you decrease the brightness of the image.
Most digital photographers tend to expose an image based on their highlights, ensuring they have information to work with in those areas. In postproduction they can bring up the shadow areas, if need be. If you notice that a scene has too much contrast to capture the shadows and highlights in one exposure, then consider lighting techniques that will balance the scene out some.
Shooting in the RAW format enables you to have more postproduction control over your images. Your camera processes a JPG before it’s saved to the memory card. This means the color balance, and shooting style is saved to that image.
Your camera does not process RAW files, giving you control to change the color settings, exposure, contrast, saturation, and much more after the shot has already been taken. It's best to shoot in the RAW format any time you intend to perform postproduction enhancements to your images.