I’ve used a slow shutter speed to capture an indoor scene without using flash, so I can preserve the existing atmospheric light sources. The photo is well exposed on one side but a touch too dark on the other. How will I be able to brighten up the problem areas without altering the correctly exposed sections?
Bobby Cameron

Here’s a technique that will work in any version of Photoshop. It enables you to quickly brighten up underexposed sections of the shot without burning out correctly exposed areas.

01: Open your source file
In our example, the left-hand section is underexposed while the correctly exposed right-hand side is much brighter. This is due to an off-screen light source at the right of the shot. Click on the Create New Adjustment Layer icon and choose Levels.

02: Change levels
Drag the Midtone slider to the left so more of the image’s pixels have a midtone value instead of a darker shadow value. This brightens the image’s overall midtones. To stop the darkest pixels looking washed out, drag the Shadow slider to the right a little. Click OK.

03: Preservation
By brightening the midtones you might also overexpose the highlights. Click the adjustment layer’s mask, hit the Gradient tool and draw a black-to-white gradient from right to left. This preserves the correctly exposed pixels on the right and brightens up the darker ones on the left.