If you don't already have photo editing, you can use free software from Pixlr.
This is one of the adjustments to use and I would recommend at least starting here. On Pixlr, the Brightness & Contrast is located under 'Adjustments' which is also where it will most likely be located with other editing software.
Brightness & Contrast has two separate controls will allow you to lighten your image or "brighten" it and will also allow you to either add or take away contrast (the dark points).
Start with the Brightness and move it to the right (software may differ with the direction, so play around until you figure out what direction makes your image lighten vs. darken). As you move to the left you will see your image lighten. You need to be careful not to lighten it too much otherwise you could blow out parts of your image. If you look at the wave example below you will see that the more brightness I add, I lose detail in the waves.
Now you can move onto adjusting your contrast. Typically you will want to add contrast, not take it away, however they may be an occasion that you will lessen the contrast.
You will need to play around with both adjustments until you feel you have achieved a good balance of brightness & contrast.
Curves intimidated me when I first saw it. I didn't understand how it worked, it seemed confusing and complicated. It's actually extremely easy when you understand how to use it. Curves can be used to adjust colors as well, but for now, we will just focus on adjusting lightness & darkness.
In the box you will see the line that goes from the top corner diagonally across to the bottom corner. This line is a visible representation of your image. The top corner of the line controls the lightest points of the image and the bottom corner controls the darkest points of the image. All the other shades between are along the line. You can adjust any shade you want by clicking on the line & creating an anchor point & dragging it up or down. Remember, the top part is the light part, so if you want to lighten you image you will want to drag up not down.
To add contrast you might want to darken some of the dark spots, or if your image is really dark you may need to actually lighten your dark points. Just play around with adjusting different points on your line.
You will want to avoid creating flat spots in your line. This mean there is no contrast at all on those specific points of your image. It will completely ruin your image. See image below.
You will also want to avoid down hill angles, because it means that you have reversed the order of light to dark. If you look at the example below you will see that I have completely ruined this image. The best tip of all, if you add an anchor point that you don't like or want to take away just grab the anchor point and drag it all the down off the screen. This will make it disappear.
There is a lightness adjustment on Hue & Saturation, but I don't really recommend using this to fix a dark image. The lightness will lighten all points the same, so it will leave you with a washed out image if you are not careful.
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