Photoshop puts a really useful tool at your disposal that can automatically change the levels in an image, but most people seem confused by how it works. They’ve read various books and web sites that suggest that one of the first things you should do when starting to edit an image is to apply automatic levels to it and then in dismay they watch the colours in their image shift all over the place only to be left with a crappy looking image. So whats going on? I hear you ask, why has Photoshop decided to shaft all the colours? Well by default it will auto level each of the RGB channels independently rather than applying the levelling to the luminosity of the image.
You can see this effect in the image below where the left hand side has been auto levelled using the per channel method and the right by the monochromatic method. The image on the left has a colour shift towards green giving the skin a green tint, whereas the one on the right doesn’t, maintaining the natural colours in the image (ignore the fact that it was shot with auto white balance etc). So how do you fix this, don’t despair, it is simple to fix this from the levels dialog.
Open the normal levels dialog by clicking on the menu “Image > Adjustments > Levels…“at which point you should see the dialog below.
Click on the “Options” button and you will now see the “Auto Color Correction Options” the following dialog.
By default this dialog has “Enhance Per Channel Contrast” selected, which as the name suggests performs the auto level operation on each RGB channel independently causing the colour shifts. Select “Enhance Monochromatic Contrast” instead remembering to select the “Save as defaults” checkbox. If you don’t select the checkbox, Photoshop will forget the change! Click on the “OK” button and the “OK” button for the “Levels” dialog.
From now on when you perform an auto levels operation, Photoshop will apply the level operation to the luminosity of the image as a whole and won’t end up colour shifting the image.