Almost any image-editor worthy of the name offers curves adjustments, but they can be tricky things to get right. It’s easy to overcook the results or fix a problem in one area only to cause a problem in another. So here are ten top curves tips to show how they work, what they can and cannot do and how to get them right.
By the way, all of the screenshots below are full-screen so that you can see both the before (left) and after (right) images and the curves settings. You can simply click on the screenshot to see a full size version. They’ve been taken in Lightroom, but the same principles apply in any other image-editor with curves adjustments.
1. Levels vs curves
Levels adjustments are useful for maximising the brightness range of a photo. By dragging the black point slider to the start of the histogram and the white point slider to the end, you make sure your photo has a full range of tones from solid black to maximum white. This is a good starting point before you make any curves adjustments. Some programs offer a separate levels adjustment, others combine levels with curves, so that you set the white and black points in the curves dialog. Both methods have the same result. In this instance I’ve dragged the top of the curve – the white point – to line up with the brightest highlights in the histogram.
2. Overall contrast is finite
When you increase the slope of the curve, you increase the contrast in that particular tonal area, but you’ll see that this immediately flattens the curve, reducing the contrast, in others. So typically you might make the curve steeper in the middle to add midtone contrast. This inevitably flattens the curve in the shadow and highlight areas, and you just have to decide if the overall effect is worth it. Basically, you have to decide where in the tonal range you want more contrast, and where you’re prepared to sacrifice it. In this example, I’ve steepened the curve slightly in the midtones and highlights, but this has slightly flattened out the curve in the shadows.
3. Parametric adjustments
It’s a fancy term for a simple set of tools. Instead of leaving you to add and adjust curve points manually, parametric adjustments offer a series of simple sliders for adjusting, for example, ‘shadows’, ‘darks’, ‘lights’ and ‘highlights’ (Lightroom). When you move a slider, it adjusts the brightness of that particular zone in the tonal range, producing a smooth curve shape in the process. This can often provide better, subtler outcomes for both novices and curves experts.
4. Manual curve points and ‘nudging’
Sometimes you can see exactly where you need to give your photo a contrast boost, often in a small area of similar tones. You can do this by adding a manual curve point for the brightest and darkest points in your key area and then dragging them up and down – in some programs, e.g. Photoshop, you can ‘nudge’ them more accurately with the arrow keys. It’s a good way of adding a strong contrast boost to a small and specific range of tones. In Photoshop and Lightroom you can use the targeted adjustment tool in the curves panel to drag up and down directly on areas of the photo.
5. Don’t go flat
If you let parts of the curve flatten out too much you risk getting some extremely ugly image effects. A flattened-out curve section produces dull, muddy tones that will almost certainly spoil the overall look of the picture – even the shallowest part of a the curve needs to have a bit of an upwards gradient to keep a natural look. This is one of the trickiest balancing acts for making effective curves adjustments. So here I’ve boosted the shadows and kept the midtones and highlights more or less unchanged – but that flat-spot has ruined the picture.
6. Regular vs luminance curves
Most curves adjustments work in the RGB colour space, which means any contrast adjustments apply to all three colour channels. As a result, where the contrast is increased the saturation tends to increase too. This is often exactly what you want, but sometimes the extra saturation is too much. Some programs, however, offer an alternative ‘luminance’ curves adjustment – Capture One Pro does, for example – and here the image is swapped to HSL or Lab mode and only the luminance (brightness) channel is adjusted. This changes the contrast without affecting saturation.
7. Simple brightening and darkening
Curves adjustments don’t have to be complicated. If you have a photo that’s just too dark or too light, and you’ve already adjusted the levels (black points and white points) to get the maximum tonal range, then the solution is simple. Just drag the centre of the curve upwards to make the image lighter overall, and downwards to darken it. This lifts the shadows and compresses the highlights (brightening) or tones down the highlights while compressing the shadows (darkening).
8. The classic s-curve
This is a classic technique for adding contrast to flat-looking photos without clipping any shadow or highlight detail. The steepness of the curve controls the contrast in that region, so the s-shaped curve is steepest in the middle, where the midtones are, and flatter in the shadows and highlights. In most cases, it’s the midtones where you really want the extra contrast, and the flattening out in the dark and light areas is either unnoticed or acceptable.
9. The tricky m-curve
It is possible to boost both shadows and highlights with a single curves adjustment, but it’s difficult to get right when you’re moving curve points manually – this is where parametric adjustments are really useful because (a) they work only within certain brightness zones and (b) they keep any curve flattening under control. The way to think of m-curves is as two s-curves joined together, so you have one s-curve to boost the darker tones in the image, another to boost the lighter tones further up the scale, and you just have to hope that the flatter part of the curve where they meet in the middle (the midtones) corresponds to an area of the image that’s not too important. I’ve had to swap images here because there aren’t that many where an m-curve can really make a difference – you’ll see that both the shadows and the highlights look a little better than the did in the original.
10. Curve reversal
Never, ever, allow the curve to change direction and head back downwards again – unless you do it on purpose. Done by accident, it produces awful and obvious tonal artefact, done on purpose it can create a striking solarisation effect, both in colour and black and white. Solarisation is an old darkroom technique that re-exposed a print to light part-way through development. This produces a part-positive, part-negative effect and you can achieve the same with a curves adjustment. The simplest way is to ‘peak’ the curve in the centre and then drag the right hand ‘white point’ back down to zero, but you can get a more unusual effect with an up-down-up curve shape. This can make the shadows too light, so just drag the black point to the right to clip the shadows to compensate.