Scaling images for output changes their overall brightness. Downsized images become more dense, i.e., slightly darker, while upsized images tend to become slightly brighter. Photoshop’s interpolation algorithms (Bicubic and its cousins) do not account for this effect.

You can compensate for this effect by applying a simple RGB curve to your image and moving its center point slightly upwards to compensate for downsizing or downwards to compensate for upsizing (Figure 8-139). You will have to experiment to find out exactly which value suits the size and resolution of your particular image. Shadows often need to be brightened more than the rest of a downsized image, so it’s necessary to make the bottom left end of the corresponding curve a little steeper Figure 8-139).

Figure 8-139. You can compensate for the differences in brightness caused by heavy scaling by adjusting the center point of your image’s RGB curve.