How are Levels and Curves related in photo editing?
Asked 10/31/2013
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I’ve started using both the Levels and Curves adjustment tools and I’m trying to understand how they relate mathematically. My current assumption is that Levels is essentially a simplified form of Curves: the black and white point sliders define the endpoints, and the adjustment between them is constrained compared with a full curve.
Is that basically correct? Can every Levels adjustment be reproduced with Curves, or are there things Levels can do that Curves cannot? I’m also interested in a more precise description of what the Levels sliders are doing.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Levels are a special case of curves, where the curve is pinned at both ends and has one control point in between. The two pairs of "input levels" and "output levels" specify the coordinates of the end points of the curve.
The "middle value" specificies the degree of curvature. Levels can only have one radius of curvature, so the curve you posted is not possible using levels. A value greater than 1 means the line is bent outward, a value less than one means it's bent inward. Here's what a levels adjustment would look like in the curves tool:
Curves may have many more control points and can "double back" having negative slope, meaning two or more different input values can reach the same output value (levels is strictly "one to one" from a mathematical point of view). This is not possible using levels:
As such curves are slightly more powerful, but often levels allows you to get to the desired result more quickly as there only five numerical parameters to adjust. Getting exactly the same result in curves is possible but probably requires more clicks.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—Levels is generally a special case of Curves.
In Levels, the black and white input sliders set where shadows map to black and highlights map to white, with clipping beyond those points. The output sliders set the darkest and brightest possible output values. The middle slider adjusts the midtones (a gamma-style bend), but with far less freedom than a full Curves adjustment.
So mathematically, Levels is a constrained tone-mapping function, while Curves is a much more flexible one. A Curves tool can reproduce the same endpoint remapping and midtone adjustment that Levels does, and also do much more by adding multiple control points.
Curves can create shapes that Levels cannot, including multiple bends or even sections with negative slope. Levels remains one-to-one and much simpler.
In practical use, Levels is often quicker for basic black point, white point, and midtone corrections, while Curves is better when you want finer control over tonal ranges.
So: if your question is whether Curves can replace Levels, the answer is essentially yes. Levels is mainly a simpler interface for common tonal adjustments.
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