Photoshop Levels versus Curves: Which Should You Use?
I've written previously about both Curves and Levels in Photoshop, and readers that are familiar with both are no doubt aware that there is a reasonable degree of overlap between them in terms of capabilities. Since both can adjust white point and black point as well as overall brightness, which should you use?
Levels affords a great deal of accuracy when setting white point and black point. If you hold down the Alt key (Option key on the Mac) while moving the black-point slider, the entire display will turn white with color only showing up where you start to clip the shadows. Similarly, holding down Alt/Option while adjusting the white point will turn your image black with color only appearing where your highlights are being clipped. Since we readily accept some dark shadows in nature, you can feel free to clip the shadows to some degree if it improves the appearance of your image, but blown out highlights are rarely a good thing. Regardless of whether you want either though, this technique with Levels can let you tell exactly what you are doing. Correctly setting black point and white point allow you to maximize the total contrast of your image by using the full tonal range from black to white.
In terms of what you can do between those two points though, Levels is rather crude, providing only a single slider to shift the mid-tone point. As you move it towards the left, more of the tones in your image will fall to the right of middle gray, and the image will get brighter. As you move it to the right, the reverse happens. Your image will progressively appear darker since more of it will fall below the point where you have the mid-tone slider set.
Curves can be used to adjust white point and black point too. As you slide the far left endpoint on the Curve towards the right along the bottom of the Curves grid box you will be mapping progressively brighter values to pitch black. So too on the upper right-hand end. As you slide the control point there towards the left along the top of the grid you will be mapping ever darker tones to pure white. But there's no easy way to tell when you are clipping either highlights or shadows so you have only what the image looks like as a guide to just how much to adjust either. Alt/Option doesn't work here.
But Curves provides excellent control over how contrast and brightness between those two ends of the tonal spectrum are distributed. You can place as many control points along the Curve line as needed and move them up or down to get the effect you desire. In Curves, slope equals contrast so you can locally increase contrast in one section of the curve by making that section steeper. You have to stay within the grid box of course, so doing so will obviously make some other section of the Curve line less steep so you still end up at the endpoints on both ends. As such, Curves is best though of not so much as increasing contrast but as rearranging contrast. If your image lacks important information in a portion of the tonal spectrum, you can steal contrast from there and move it to where you need it more.
Given all this, I tend to use both Levels and Curves, each for what they do best. After creating a Levels Adjustment Layer to place the black and white points where I want them, I then create a Curves Adjustment Layer to adjust the brightness and contrast of everything in between. Adjustment Layers allow you to revisit your choices later if you decide you overdid something or perhaps want further modifications, all with no additional loss or degradation.
With a bit of practice, using both Levels and Curves together can give you an amazing degree of control the tonal relationships within your image.