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Curves: Increasing contrast

Curves (and levels) isn't only a Photoshop thing, but exists in most photo (and video) editing software. It can be used for a wide range of adjustments and effects, but this tutorial will focus on increasing contrast.

This is what I'll show how to do:

It might be tempting to use the adjustment named "brightness/contrast" for this, but i don't recommend it as it does not give you the same degree of control over the result.
A simple levels and/or curves adjustment will fix the colours and contrast in 90% of your photos in a matter of seconds!

This tutorial will show you how to adjust curves and levels solely using the curves dialouge in CS3 and newer.
It is the same for older versions but they do not have a histogram in the curves dialouge. To get a histogram in CS2 or older use the levels adjustment layer for adjusting the black and white point.

Adjusting your image:

Start off with our image. As you can see there's only the background layer:

Now, add the 'curves' adjustment layer:

The curves window pop up. As you can see it has a faded histogram in the background (in cs3).
The adjustment layer will by default have an empty white mask on it. I have turned that off on mine.

The equivalent of adjusting the white and black point in the levels dialogue is moving the top and bottom point inwards. Just like with levels. Move them inwards until they "hit" the edges of the histogram (or further if you want to clip the colours). There's a checkbox at the bottom to preview where clipping will occur.

If you want do increase the contrast furhter without getting more clipping, give the curve a soft s-shape like this:

Levels & s-curve

That's it. You can now adjust contrast like a pro! The next step will be to learn how to adjust curves for individual channels, effectively changing the white balance or color cast! This we will do in the next Curves tutorial.

Presets:

If the levels are already adjusted (or are naturally good) then you might want to adjust the contrast without changing the white/black point. You would do this by making an S-shaped curve as shown. In CS3 you can choose some presets from the drop-down at the top to get you started; This shows the preset for a medium contrast increase.

Masks:

Adjustment layers (like curves) can be used with masks to only affect parts of an image. Masks are a very powerful tool, and would allow you to brighten only the face and darken the sky, while leaving the rest intact.
A 3-part tutorial on masking can be found on this site.