The Curves command remains one of the most potent tools in Photoshop’s arsenal. Versatile, powerful and intuitive, it’s the weapon of choice for most tonal tweaks. Unlike Levels, which only allows you to adjust the whites, blacks and midtone points, Curves lets you place up to 14 points (although you’ll rarely need more than four or five) to make precise adjustments to different parts of the tonal range.
The fluid curve line ensures a smooth transition between these different tonal shifts.
How Curves work
Click on the diagonal curve line to add a point then drag this up or down to lighten or darken the image. lt’s important where you place points along the curve line as this affects different parts of the tonal range from blacks (on the left) to whites (on the right). Add more points along the line then use them to target and adjust the brightness of different tones. Anywhere the curve goes above the initial diagonal starting line will mean the tones will be made brighter, and in any parts where the line goes below it, they will be made darker.
Curves values explained
Curves work by giving you intuitive control over brightness, which is mapped out on a scale of 0 (black) to 255 (white). The horizontal axis represents the original tonal values and the vertical axis represents the values after the curve settings are applied.
In a sense, the Curves box shows a ‘before and after’ of the tonal tweak, with the horizontal axis the ‘before’ and the vertical axis the ‘after’. The curve line is initially at a 45 degree angle because the input and output tonal values start off the same, so each X-axis value has an identical Y-axis value. resulting in a 45 degree line.
If you click right in the centre of the line then drag the point to three quarters of the way up the box then you’re targeting the midtone pixels that were originally at a brightness value of 127 along the tonal range and giving them a new brightness value of around 191, or 75% white. lf you make a second point a quarter of the way in from the left and drag this down to the starting line, you’re effectively resetting the pixels in that part of the tonal range to their initial value.
How to add Tones and Contrast
Images with punch will jump out of the screen compared with those with lifeless flat tones. The amazing S-curve will give your images this vitality with almost no effort. Create a new Curves Adjustment Layer, then drag a point up in the top half of the line and a second point down in the bottom half (a third point at the centre anchors the midtones). This curve brightens the highlights and darkens the shadows, resulting in greater contrast. Wherever a curve is steeper there’ll be greater contrast, so the more pronounced the S-curve, the stronger the effect. Take a look at the Curves adjustments below to see how different curves affect the highlights and shadows.
Use Selective Adjustments
One of the great things about applying Curves as an Adjustment Layer is the opportunity for selective control with masks. Adjustment Layers automatically come with an active Layer Mask attached. It’s usually easiest if you make this mask completely black, then paint with white to reveal the effect. Once you’ve added a Curves Adjustment Layer (Layer>New Adjustment Layer>Curves) and tweaked the curve. press Cmd/Ctrl+l to invert the mask to black, thereby hiding the change in tone. Then simply grab the Brush tool and paint with white to reveal the change in the areas of your choosing. This selective control is very effective on portraits because you can use masks to change different parts of the face, such as the eyes and lips. and even introduce colour shifts to enhance make-up.
Dragging up on the green channel introduces green. A Layer Mask restricts the effect to above the eyes.
A pronounced S-curve boosts contrast, while a mask restricts the effect to the irises
A slight downwards drag on the curve darkens the image. then a mask restricts the effect to the lashes.
Dragging the reds up and the greens down adds red and magenta. while a mask restricts the effect to the cheeks.
An increase in reds combined with a boost in overall contrast gives the lips extra impact.
Curves and Colour channels
Both Levels and Curves enable you to tweak an RGB image’s Red, Green and Blue colour channels. This is useful if you need to correct a colour cast or remedy a white balance mistake, and also for creative or dramatic colour shifts to change the atmosphere of a scene. While Levels is fine for basic colour correction, the Curves command’s more intuitive interface makes it the best choice for creative colour shifts.
By tweaking the colour channels you can make a huge range of effects. just as the main RGB curve gives you precise brightness control, you can similarly restrict colour changes to parts of the tonal range by manipulating the individual channel curves. For example, dragging the bottom part of the Blue curve up then pinning the top half back into place will add a blue cast to the shadow tones only. You can also get dramatic colour shifts by dragging the top right or bottom left points on each colour channel to shift the brightest or darkest tones towards different colours.
Dragging the Red curve upwards adds red, and dragging It down adds cyan, so increasing the bottom half of the line adds red to the shadows, while lowering the top half introduces cyan into the highlights.
An upwards shift of the bottom left point on the blue curve makes the blacks in the image look blue, thereby changing the colour of the background to a dark blue.
Dragging one point downwards on the Green curve here adds a Magenta cast, which is particularly noticeable on the stem of the flower.