|
Basic Enhancements: Curves
This is probably the most powerful tool you have for getting the brightness, contrast and colour in your image just right. Most images that come off your camera could probably be improved with the use of curves and although it may look complicated at first, it is worth learning to use.
Let's start by looking at the curves dialog box ...
The main part of this dialog box is taken up by a square box showing the histogram for the current image, and a diagonal line through it. This diagonal line is the coolest and most powerful diagonal line you will ever come across. This diagonal line represents how the pixels in your original image are changed to create the pixels in your processed image.
The horizontal axis of the graph represents the colours in your original image, the vertical axis, the colours in your processed image. Both axes range from pure black (lets call this 0%) to pure white (100%). The diagonal line shows that nothing is current changed in the image (i.e. 0% on input is 0% on output, 50% on input becomes 50% on output etc. therefore no change).
What you do is take this perfectly straight diagonal line, and you add a curve or two into it ("Ahh ... so that's where the name comes from!"). To do this you simply click somewhere on the line and bend the line by holding the mouse button down while you move it. Easy!!! Try loading up an image into your photo software, make sure the preview is turned on so you can see the changes as the happen and have a play.
Brightness and Contrast with Curves
The most common use for curves is precise control over brightness and contrast of the image (although it is often used to control colour balance as well). Let's look at a few different graphs and see whats going on.
 |
To increase the brightness of an image you need to increase the brightness of the pixels and therefore the output brightness needs to be greater than the input brightness. You may think therefore that sliding the line upwards as shown will achieve this.
This will indeed make the image brighter and is in fact how the brightness function in most software works. However there is a problem.
If you do this you end up with any 0% blacks being mapped to dark grey (about 16% in the image), as well as burning out your highlights (everything above about 84% in your image is output as pure white.
|
 |
A much better way to achieve a brighter image is to just increase the mid tones. Do this by clicking on the diagonal line in the middle and dragging it upwards. This small movement will have a significant effect on the brightness of the image, although your extreme shadows and highlights are hardly effected.
What you are actually doing here is increasing the contrast in the darker colours (the left side of the graph) and reducing it slightly in the lighter colours (the right hald of the graph). |
 |
Image contrast is also very easily adjusted using Curves.
Whereas brightness was governed by the height of the line, contrast is controlled by the steepness of it. The default contrast is the diagonal line of 45 degrees. If you increase the steepness you are increasing the contrast.
The contrast function in the "Brightness/Contrast" control works like the graph shown on the left. Not good. You are increasing the contrast of the midtones, but losing all detail in the shadows and highlights. In this illustration, anything less than 17% or greater than 83% is being output as pure black and pure white respectively.
|
 |
Here's a better way to increase the contrast. By making the dark tones (the left side of the graph) a little darker and the light tones (the right side) a little lighter you can increase the contrast without losing shadow or highlight detail.
The adjustment shown here may look quite small but this would give a very noticable increase in constrast.
The graph is a little steeper between about 25% and 75% which is where the contrast has been increased.
This curve is sometimes called an 'S' curve. |
 |
You can end up with some very weird effects with the curves technique. The one shown on the left would produce a very bizarre effect as some of the very dark colours are made very bright and then some of the colours that were originally brighter are made darker. |
 |
This graph will produce an image that most film photographers will be very familiar with. It makes darks light and lights dark. The image will end up looking the same as an old film negative. |
An example
 |
The original image - a particularly flat image which is too bright and lacks contrast.
The histogram for this image shows that it has virtually no dark areas.
|
 |
We've slid the input slider to the right to create some shadow detail and increase contrast
|
 |
Further darkening the image and increasing contrast by making the curve even steeper. I've also reduced the 'green' channel slightly.
|
 |
Something a little more wild!
|
Other Features of Curves
It is sometimes worth trying the 'Auto' button. The software will analyse the image and suggest some curves for you that will improve the image. You can always discard them or use them as a starting point for a bit of further tweaking.
Presets can be very useful. Photoshop gives you some to start with such as:
- Lighter
- Darker
- Increase Contrast
- Medium Contrast
- Strong Contrast
- Colour Negative (probably not the most useful one!)
- Cross Process (definately not that useful!)
You can also save your own presets by clicking on the little icon to the left of the OK button.
The pencil tool allows you to draw your curve freehand, and then click the smooth button a few times to smooth it out.
|
Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"Classic Quote"
- Quote by
|
