CURVES

This brief tutorial explains the basic concepts behind Curves adjustments in image editing programs such as Photoshop.

This tutorial was prepared by Roy Killen and is copyright. It is part of the supplementary training materials for members of the Belmont 16 Ft Photographic Club and is not intended as a complete, stand-alone instruction package.

Comments and suggestions on this tutorial are welcome - they should be emailed to Roy Killen.

NOTE: There are two ways of applying adjustments (such as bightness/contrast adjustments) in Photoshop. One way is via the Image>Adjustments selection from the menu - this is referred to as a "static" adjustment because it makes permanent changes to the pixels in the image. Static adjustments cannot be reversed or modified. The other approach is to make "dynamic" adjustments by creating adjustment layers. Dynamic adjustments can be reversed, or altered or disabled at any time (unless you flatten the layers). This tutorial describes static Curves adjustments in detail and makes only brief reference to dynamic Curves adjustments. This demonstrates the principles that can be applied in any image editing program (even those that do not allow for the use of layers). The use of adjustment layers is described in detail in another tutorial.

WHY ARE CURVES ADJUSTMENTS NECESSARY?

The Curves adjustment is a very powerful and versatile part of Photoshop. If you master Curves adjustments you will be able to use this approach to make image enhancements that might otherwise require you to use Levels, Brightness/Contrast, and Colour Balance. Not only that, the results you achieve will be better than those achieved with the other "more basic" adjustments because you will have more control over the changes that you make. In particular you will be able to make adjustments (such as contrast) over much more specific tonal ranges.

PHOTOSHOP OR CAMERA RAW?

In an earlier tutorial I explained that it was possible to make tonal adjustments to raw images using either the "parametric" or "point" curves adjustments in Adobe Camera Raw. These adjustments can be very convenient and effective but for some images they do not give you the flexibility that is available through Curves adjustments in Photoshop.

My advice is to try making initial tonal adjustments in Camera Raw (perhaps using a preset such as "medium contrast") and then fine-tune the results in Photoshop.

One of the major advantages of making Curves adjustments in Photoshop is that you can use adjustment layers and masks to limit the effect of the adjustment to selected areas of the image, rather than applying them to the whole image as you must do in Adobe Camera Raw. ( In the CS4 version of Camera Raw you can make some localised adjustments but not localised Curves adjustments.)

Some preliminary settings in Photoshop - the eyedropper

Several of the adjustments that you will be making with Curves involve using the eyedropper tool. The default setting in Photoshop is that the eyedropper samples just one pixel. This does not work very well for colour adjustments so we need to change it to sample 3x3 pixels for medium resolution images or at least 5x5 for very high resolution images. This requires two steps:

1. Select the eyedropper tool from the tools palette.
2. Go to the options bar near the top of the screen, click on the arrow near the words "Sample Size" and select the sample size you want.

Neutral background

Before making any colour/tonal adjustments in Photoshop it is best to display your image on a neutral grey background because coloured backgrounds can alter your perception of the colours in the image.

This is easy to do - just right-click anywhere on the area outside the image displayed in Photoshop and a pop-up menu will allow you to select "gray", "black" or "custom" background. Either select "gray" of seelct "custom" and then choose a grey colour that is the brightness you desire.

If you repeatedly press the letter "F" on the keyboard, you can cycle through different views that show the effect of the different colour backgrounds - one view is a "full screen" view without the clutter of memus and panels.

CURVES ADJUSTMENT LAYER

If you are using adjustment layers, just click on the adjustment layers symbol at the bottom of the layers palette and select "Curves" from the pop-up menu.

A Curves adjustment layer will be created and the Curves dialogue box will open.

When you open the Curves dialogue by creating an adjustment layer, a new "Curves adjustment layer" is added in the layers palette. It is automatically called "Curves 1". If you add other Curves adjustment layers they will be called "Curves 2" etc.

On the Curves adjustment layer a "layer mask" is automatically created. Its icon (the white rectangle) indicates that the mask is white - so for now it is not masking any of the effects of the Curves adjustment. Later, you can use this mask to limit the effects of the Curves adjustment to just those parts of the image where you want it to be effective.

Note: Being able to mask the effects of the Curves adjustment (limit the effects to selected parts of the image) is very useful. However, it is not the main reason for using a Curves adjustment layer rather than selecting Curves from the menu. The main reason is that adjustment layers give you the flexibility of being able to come back later and remove or alter your adjustments.

FINDING YOUR WAY AROUND THE CURVES DIALOGUE

Note: This diagram on t eright is the Curves dialogue box you will see in Photoshop CS3 on a Macintosh - it will look slightly different on other versions of Photoshop and on a Windows computer.

1. At the top of the curves dialogue box there is a "Preset" selection -if you leave it on "None" (as shown) the "curve" will initially be a straight line diagonally across the display area. The other presets that are available simply let you select from a range of pre-defined shapes for the curve.

2. The channel selection allows you to display the curve for the complete image (RGB) or for the individual red, green and blue channels. For now, leave it on RGB.

3. The central part of the curves dialogue box is the GRID. By clickiing on the icons (near the red arrow) you can change this from a 4x4 grid to a 10x10 grid. (Some people prefer to use the finer grid when they are making precise Curves adjustments. For now, leave it as 4x4.)

4. The gradient (black to white bar) that runs across the bottom of the grid represents all the possible tonal values (luminance levels) in the image BEFORE you make any curves adjustments - that's why it is called the "input". The left hand end represents total black and the right hand end represents total white.

5. The vertical axis represents the tonal values in the output. That is, the tonal values in the image displayed on the screen when you alter the tone curve. Total black is at the bottom, total white is at the top.

6. When you first open the curves dialogue there will be a diagonal line running across the grid from bottom left to top right. While ever this line runs straight across the grid (as shown ) it indicates that no "Curves" adjustments have been made to the image. When adjustments are made this line will change to a curve - and the shape of the curve will indicate what adjustments have been made. If the line remains straight but its position changes (so that it no longer runs from corner to corner) this also indicates that adjustments have been made. Actually, in CS3 (and some other programs), when you make curves adjustments the original diagonal line remains and a new "curve" appears in a different colour so that you can easily see how far you have changed the settings.

7. The histogram displays the relative magnitudes of the tonal values in the original image. In this example we can see that there are relatively few light tones but lots of mid tones. This is the same histogram you would see if you opened the "Levels" dialogue. This is a new feature in Photoshop CS3 - in earlier versions the histogram was not displayed in the Curves dialgoue.

8. Make sure you have a tick in the "Preview" box or you will not be able to see the effects of any adjustments that you make.

Don't worry about any of the other things in the curves dialogue box at this stage. We will come back to some of them in a later tutorial.

Now we will look at how the INPUT and OUTPUT are related via the "curve".

When the image is opened . . .

When the "curve" is adjusted . . .

To adjust the curve, simply click on any part of the curve and drag the mouse.

When the curve is moved, the tonal values in the dispalyed image (the "output") change.

In the example shown, the curve has been "dragged upward" simply by clicking on a point on the curve and dragging it upward with the mouse. The two end points (black and white) have not changed, but all tones in between have been lightened. For example, the pixels in the image that were originally at a luminance level of 115 have been lightened to a luminance level of 141.

Because all pixels in the image (except those at luminance levels of 0 and 255 - black and white) have been lightened slightly, the total image looks lighter. The overall effect on this particular image is shown below: [This does not improve the image, I am just demonstrating what happens.]

In the next example the curve has been "dragged downward" so that all tones other than the black and white end points have been darkened. For example, the tones that were originally at a luminqance level of 115 have been darkened to a level of 90.

The overall effect of this adjustment on the example image is shown below:

In the next example, two adjustments have been made. First, the tones below the mid-point of 128 have been darkened and second the tones above the mid-point have been lighntened.

The type of adjustment illustrated here is often referred to as an "S curve".

In this example, the tones that were originally at a luminance level of 192 were lightened to 216, and the tones that were originally at a luminance level of 64 were darkened to 40 (these last figures are not shown on the screen shot).

The small circle in the centre of the grid indicates that an "anchor point"" was placed there by simply clicking at that point before the other adjustments were made. This ensures that the mid-tones (brightness level of 128) do not change.

Darkening the darker tones and lightening the lighter tones increases the contrast in the image.

The overall effect is shown below.

After you make this initial adjustment (which is similar to the type of adjustment you would make with "Levels") you can then make other adjustments to the "curve" - such as the "S-curve" adjustment described previously.

An example of such a curve and the resulting image are shown below.

SUMMARY

Many people get intimidated by Curves when they first start to learn about Photoshop. But really it is quite simple:

(a) You have a starting image (the "input") that has pixels with certain levels of luminance (brightness).

(b) The Curves adjustment dialogue provides you with a way of changing the luminance levels of some or all of the pixels in the image.

(c) The way you make these changes is by altering the shape of the "curve" - from its original straight line to something else.

(d) You can think of the "curve" as being like a translator - it translates the original luminance levels of the individual pixels to new luminance levels. (Some people think of this as "mapping" the orignal input to a new ouput.)

(e) If you make careful Curves adjustments you can improve the tonal range and contrast of the image.

THIS IS JUST THE BEGINNING

What I have shown you above is just a basic introduction to Curves adjustments.

Some of the things that you might like to learn later are:

How to make colour corrections using curves.
How to fine tune your tonal adjustments by working on individual (red,green, blue) channels.
How to isolate areas of an image and make localised adjustments to contrast or colour.

All images on this site and all documents and tutorials linked to this site are copyright.

This page last updated 19th May 2009 (RK)