Levels vs Curves
Photoshop Basics Levels vs. Curves
The tonality of an image may be adjusted in Photoshop (all versions) by one of two basic methods Levels or Curves or a combination of both, but which is the better method? In presenting Photoshop tutorials on a fairly regular basis to Clubs and Societies in the MCPF area I often pose the question How many of you use Levels and how many use Curves? to which the majority invariably answer Levels.
In Photoshop there are generally several different ways of processing an image to achieve a specific end result, but the quality of the final image depends on the method used. In terms of adjusting the tonality or tonal range of an image the choice lies between Level or Curves, so which of these options provides the greater degree of control? NOTE All adjustments should be applied non-destructively, i.e. as an Adjustment Layer and NOT directly on the image
Looking first at the Levels dialogue box (CS5 shown, but essentially the same for earlier versions of Photoshop) there are a limited number of adjustments available. Using the sliders below the histogram the tonal range of the image may be compressed.
In the example shown above the black slider is set at 21 which forces tones in the range 0 21 to be regarded as black and the white slider is set at 184 which forces tones in the range 184 255 to be regarded as white. Movement of the central grey slider (default value 1) lightens or darkens the tones between white and black. In other words there are but 3 basic adjustments available in Levels and individual tones cannot be adjusted independently.
The sliders on the grey scale below the histogram may be used to force white or black to a shade of grey. This was useful when printing monochrome images with a single black ink in which the tones were determined by the amount of ink put on the paper. Adjusting the white slider ensured that pure white areas which would not normally receive any ink would have a few black dots applied which generally enhanced the image. With the advent of printers using multiple black inks this facility is now of limited use in monochrome printing at least.
The Curves dialogue box (CS5 shown, but essentially the same for earlier versions of Photoshop) may at first sight appear a little daunting, but it contains the same basic adjustments as the Levels dialogue box as shown below although the output grey scale is absent. For comparison purposes the same adjustments have been applied in Curves as were applied in Levels.
The advantage of Curves is that any one of the 256 tones can be adjusted individually by selecting the tone either in the image or on the curve.
In versions of Photoshop up to CS3 Ctrl clicking on a tone in the image places a marker on the curve; dragging the marker up or down lightens or darkens that tone. Note that the tone is adjusted wherever it appears in the image, i.e. the adjustment is global. For the adjustment to be restrained to a specific area the layer mask must be used. The curves dialogue for CS3 is shown below in which 2 tones have been selected. To remove a marker click on it and drag it out of the curves box.
When a tone is lightened or darkened the resultant curve provides a smooth transition throughout the image rather than an abrupt change and the deviation from the linear curve, actually a straight line, can be clearly seen as shown opposite in CS5. For very fine control over the tones multiple points on the curve are required as shown opposite.
As Photoshop has evolved so have the options in the Curves dialogue box, but the basics have remained unchanged. A major advance introduced in CS4 was the ability to adjust the curve by dragging in the image which simplifies adjustments no end. Just click on the hand icon (see opposite) and click and drag in the image to adjust a specific tone there is no limit to the number of tones which may be adjusted. To place a marker on the curve click on the hand icon and Ctrl click on a tone in the image.
|Location of Click & Drag Curves Adjustment - CS4 or higher|
In both Levels and Curves individual channels may be adjusted if so desired by selecting the channel from the Channel drop down list, but if simply adjusting in RGB it is a good idea to put the Levels or Curves Adjustment layer into Luminosity mode to avoid any colour shift. Both Levels and Curves have a number of presets accessed through the drop down list at the head of the dialogue box.
The use of Curves increases your ability to process the image with much greater finesse than is possible in Levels. In Levels 5 basic sliders are available, but in Curves there are effectively up to 256 sliders and changes can be made which are just not possible using Levels or Brightness and Contrast.
I thoroughly recommend the use of Curves - give them a try, they dont bite!
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