Let me say categorically here that both Tools do similar things, but Curves gives you more options in Photoshop.
Both Levels and Curves enable you to set a shot’s tonal endpoints – that is, define the brightness values of the lightest and darkest tones.
In the Levels Dialog, drag the Black Point input slider in to darken shadows, the White Point Slider to lighten highlights, or both; moving either slider will increase contrast.
You can set the endpoints in the same way using Curves. In CS3 and newer, drag the sliders beneath the histogram inwards; as in Levels, you can hold down ALT in Window and OPTION in Mac as you do this to see if and where shadows and highlights are being clipped. In older versions, click on the bottom-left (shadows) and top-right (highlights) points and drag them inwards.
NOTE: For contrast and Clipping, when you’re adjusting the endpoints with Levels and Curves, you don’t necessarily have to move the Black and White Point sliders right to the edges of the histogram. If a scene didn’t contain deep shadows or bright highlights, you don’t need to create them. Conversely, if a shot contains small areas of clipped highlights and/or shadows, it’s fine to move the sliders a little way past the ends of the histogram to improve overall contrast.
The main different between Levels and Curves is midtones adjustments. As we’ve seen, the further you push the Levels midtones slider, the more you weaken contrast, so you may need to move the Black or White endpoint slider in to compensate. If contrast is good to start with, however, and you just want to tweak an image’s midtone brightness, Levels is the simplest option.
With Curves, when you drag a point up (to lighten) or down (to darken), the effect tails off towards the shadows and highlights, so overall contrast is maintained. The big advantage of Curves is that you can target particular areas of the tonal range without affecting others – a good example is using an ‘S-curve’ to boost contract without affecting the darkest and lightest tones.
Also NOTE: For Weakening tones you can also lighten shadow tones and darken highlights with Levels and Curves. In Levels, move the black Output slider (the lower-left slider) inwards to weaken blacks, and the white Output slider inwards to weaken White. If you are using Curves, drag the black points (the bottom-left point) upwards to weaken Blacks, and the White point down to weaken white. Weakening tones isn’t the same as recovering clipped highlights, and if you go too far you will create unsightly patches of grey, but it can be useful as an emergency fix to make clipped areas less obvious.
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