You may come across a digital image whose values are inverted, or perhaps you scanned some film negatives and didn't use the right scanning mode. Changing an image from negative to positive is can be done in just one command with Photoshop. If you have a colour film negative that has been scanned as a positive, getting a normal-looking positive image is a little more challenging because of its inherent orange colour-cast.
Open your file by clicking "File," then "Open" or "Ctrl-O" and choosing it from its folder.
Click "Image," "Adjustments," "Invert" or "Ctrl-I." The image turns to positive, and back to negative if you click the command again. This process changes the values as well as colours to their complements.
Click "Layer," "New Adjustment Layer," "Invert" if you would like to perform this action as a separate layer that can be removed later.
Open the Color Balance tool by clicking "Image," "Adjustments," "Color Balance" or "Ctrl-B" to open the Color Balance window.
Drag the sliding scale for "Cyan / Red" all the way to the right toward Red. Do this for the Highlights, Shadows and Midtones by checking each of these Tone Balance choices at the bottom of the window and repeating the sliding scale action.
Drag the sliding scale for "Yellow / Blue" all the way to Yellow with Shadows selected. The image should start to look a little more balanced at this point. Further smaller adjustments may be needed to remove any blue and cyan casts that remain from inverting the orange-tinted negative.
Avoid having colour cast problems with your colour negatives by scanning them in "Negative" mode on your scanner.