The curves adjustment layer is, arguably, the most versatile of all the regular adjustment layers. It's commonly used to fine-tune image attributes such as brightness and contrast, since it allows users to selectively lighten and darken individual areas within an image based on the particular area's luminance values.
Additionally, the curves adjustment is well suited to the job of correcting or manipulating the colours within an image. In much the same way that lightness can be shaped, by independently applying a curve to one or more of the individual colour channels (red, green and blue) photographers can selectively alter the colour profile of specific areas within an image.
The trouble is, though, that when a tool is as powerful as the Curves adjustment layer is, it can be difficult to know exactly where to start when it comes time to edit your own images. In these circumstances, it can be helpful to use the curves Auto button and to allow the computer to take a stab at autonomously applying an adjustment or two.
While using the Auto button seldom results in a photo that looks as good as it could, and may even see you attracting a few frowns from 'those in the know', it is difficult to argue against the merits of getting a fresh perspective of an image when, today, the Undo button is always so close at hand. Furthermore, the Curves adjustment layer in Adobe Photoshop has some hidden auto-functionality that helps give advanced auto-users a little more street-cred.
By holding down the Alt key while clicking on the Auto button, Photoshop opens the, otherwise hidden, Auto Color Correction Options dialogue box. As is often the case with Photoshop, this dialogue box is characteristically unintuitive. However, for the purpose of merely getting a fresh perspective, all one need do is click on one of the four checkboxes in the Algorithms section to reveal a set of unique computer-generated curves adjustments.
Like the default Auto button, the four hidden auto algorithms are unlikely to represent a result in their own right, but they usually produce a diverse and thought-provoking set of interpretations. Some of the algorithms even permit users to tick the Snap Neutral Midtones checkbox which adds even greater variety to the collection.
In many ways, employing the algorithm options as I have outlined here has a hint of Instagram-filter thinking to it. It is a click-and-hope tactic that requires nothing in the way of intellectual capacity. However, unlike your typical iPhone app, once an algorithm has been applied to an image, all the auto-adjustments are transferred back onto the underlying curves adjustment layer. This auto-transparency gives photographers the opportunity to then hone the final product with far more control than any cellphone permits!