In the Levels dialog you can create a manual brightness curve which provides very detailed control over brightness levels for any photos.
To create a manual curve just click or drag on the histogram area, and you will see a control point added and a curve. Right click on a control point to delete it.
The Curves control is used in combination with the other Levels controls, such as the central gamma control, and the black and white input and output levels. This means you can have both a manually entered curve, and still make general adjustments such as adjusting the gamma, or even the contrast on the main Photo tool Infobar.
This example shows a manually entered curve with three control points. In this example the mid to dark shades have all been lifted (lightened), but the brighter shades of the photo (right side of the curve) have not been altered at all.
The histogram graph shows both the original histogram (dark red) of the unmodified photo and the modified histogram (pale red overlaid) that is the result of the adjusted curve.
Note: The two methods of altering the brightness levels (the new curves 'blue line' and the original brightness/contrast type controls) can be used together - the final histogram is the cumulative results of both methods.