Levels adjustments are are pretty basic and straightforward way of maximising the tonal range of your pictures. You move the white and black point sliders to line up with the brightest and darkest parts of the picture, and that’s it – you’ve got maximum tonal range and contrast.
Curves are more complex because they enable you to adjust the brightness and contrast within different tonal ranges in the picture, for example in the shadows, midtones or highlights.Curve adjustments are both more sophisticated and more difficult to get right, which has led to the general assumption that they are more ‘expert’ than levels adjustments. That’s not quite right.
Levels vs curves adjustments each have pros and cons and there is in fact some crossover, as some curves tools also incorporate levels adjustments. Sometimes levels are better than curves, sometimes it’s the other way round, but neither is always ‘right’.
This overhead shot of a succulent in a glasshouse is a good photo to work with. It has an interesting pattern but very low contrast, and it’s a good candidate for levels and/or curves adjustments. Let’s take a look in a program that offers both Levels and Curves panels – Capture One (the same principles will apply in any other program).
(the same principles will apply in any other program).
1. Levels adjustments and their effect
First let’s take a look at this picture with the Levels panel. We can see that the histogram occupies only about a quarter of the full tonal range, with a large ‘dead’ space either side in the shadows and highlights. You can click/tap on any of these screenshots to see a larger version
So the quick fix here is to drag the black point and white point sliders to the points where the histogram starts and ends. Immediately this has increased the contrast. If the picture looks too dark or light at this point, you can drag the midpoint slider left or right to adjust the brightness without affecting the black and white points you’ve already set.
There are two key points here. One is that just because a job is simple, that doesn’t mean it’s not effective. The other is that this adjustment has maintained a straight-line linear relationship between the tones in the picture, whereas curves adjustments produce non-linear relationships and contrast changes which may not be what you want and which may be difficult to manage.
2. Curves adjustments are a little different
The point of these adjustments on this picture is to increase the contrast, and the classic way of doing this with curves is with an S-shape adjustment. You add a control point in the shadows and drag it downwards, you add another in the highlights and drag it upwards, and this has the effect of steepening the curve – and hence the contrast – in the midtones.
Straight away, it’s much more difficult to achieve the same balance of contrast and brightness offered so quickly by the levels adjustment. With a curves adjustment we don’t have to ‘clip’ the upper and lower parts of the histogram, which may contain very subtle shadow and highlight that might be worth keeping, but it’s very difficult to get enough contrast without ‘flatlining’ the upper and lower parts of the curve anyway.
Something else is happening. Curves adjustments do no produce a linear response. They increase or reduce contrast in different parts of the picture when it’s not really necessary and actually quite difficult to control. The curve we’ve made here to try to replicate our levels adjustment is flattening the shadows and highlights in a way the levels adjustment did not.
3. Levels adjustments with curves
In fact, though, most curves tools offer levels adjustments too in an indirect way. Instead of trying to make a curve that matches our first levels adjustment we can simple replicate it by dragging the curve black and white points to line up with the start and end of the histogram, just as we did with the levels adjustment.
The initial result is the same. And we can achieve the same lightening/darkening effect as the levels midpoint adjustment by dragging up and down on the middle of the curve.
It’s very easy to look down on levels adjustments as a rather blunt and simplistic tool, but even if you’re an expert with curves, an initial levels adjustment can make your curves adjustments either much simpler or even not necessary at all.
4. Luma curves and how they’re different
Regular levels and curves adjustments are applied equally to the pictures RGB (red, green, blue) color channels. They don’t just increase the brightness (luminance) contrast, they increase the colour contrast, i.e. saturation. Levels and curves adjustments are great if you welcome a saturation increase at the same time, but sometimes you just want more contrast without exaggerated color.
That’s where luma curve adjustments come in. These are not available in all photo-editing applications. Here, the image is not edited as a regular RGB images, but is instead treated as a Lab image – a lightness channel and two color channels. With luma curve adjustments, only the lightness channel is adjusted. The result is more contrast but without the saturation increase.
You can see the effect in this picture. It’s a more accurate rendition of what this subject might have looked like in very high contrast lighting, and without any artificial saturation increase. Capture One offers luma curve adjustments but there is no luma mode in its Levels panel. At the same time, though, you can see how applying a curve adjustment to darken the picture has produced a non-linear response, with ‘flattened’ shadows and increased contrast further up the tonal scale.
Levels vs curves: the conclusion
It’s often difficult to decide between levels and curves adjustments. It’s easy to assume that curves are more sophisticated because they appear to offer more control and are certainly more difficult. In fact, though, curves adjustments generally become a lot easier after you’ve adjusted the levels, and if you check the levels first you may not need a curves adjustment at all.