To follow along with this tutorial you will need to download this image: levels.jpg (right-click and choose: Save As)

Tone
The first and foremost item you should be correcting when you bring a photo into Photoshop is the tone, or the variance of dark and light, of the image. Most people do this with the Brightness/Contrast feature. Brightness increases the overall white in a photo, while contrast increases the difference between black and white. Although they may seem effective, using such simple controls can do more harm than good. To become a better Photoshop user you must forget the Brightness/Contrast feature and begin using Levels instead.

What is Levels?
Open up your image and go to the menu item: image>adjustments>levels (or press Ctrl L). This will bring up a graph call a histogram. It plots the tone of an image from black to white (left to right) across the X-axis, with the y-axis being the amount of image information within that range of gray. The higher the peak of more of your image information falls within that range of gray. If you're histogram leans to the left, your image has lots of shadows or black and is possibly underexposed. If your histogram leans to the right, your image has lots of highlights or whites and is possibly overexposed. Histograms that are balanced and symmetrical often stem from images that are more pleasing to the eye and are generally considered to be exposed properly. These images are often referred to having a balanced tone. It's not wrong to have a histogram that leans to the left or to the right, but generally creating a vivid image requires a more balanced tone.

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The above histogram shows an image with lots of shadow information. The very left end of the histogram is clipped off the chart, meaning a portion of this image is underexposed and there is no data available for that range of tone.	This histogram demonstrates a well exposed image with plentiful information across the whole range of tones. None of the ends are clipped, so it is exposed well.	The above histogram shows an image with lots of highlight information. The very right end of the histogram is clipped off the chart, meaning a portion of this image is overexposed and there is no data available for that range of tone.

Exposure
What does it mean when an image is over- or under-exposed? When image is so dark that many areas contain pure black and no detail, that portion of the image is considered to be under-exposed. Which means not enough light was getting through the lens to provide enough detail to the film or digital image sensor. If too much light was getting through, portions of the image may become so bright that they lose all detail and become pure white. This called over-exposure.

A histogram shows brightness from pure black to pure white. Everything in between are shades of gray. If a histogram shows the image data is bunching up on the pure white or pure black ends of the graph, and the graph looks as if the head and tail were clipped off, then the image may have under-or over-exposure problems.

Using levels
Open levels.jpg.
Press Ctrl. L to bring up the Levels window.
The levels window should look similar to the screen below. You can ignore everything I marked in the blue box.

Take note of the following controls and how they work:

• The Black Point slider. By moving the slider you're telling Photoshop that you want the range of tone within the image to start as black here. Any total information to the left of the slider will be lost.
• The White Point slider. By moving the slider you're telling Photoshop that you want the range of tone within the image to end as white here. Adjusting the Black Point in the White Point sliders are similar to adjusting the contrast of an image, only you have far more control.
• The Gray Midpoint slider. By moving the slider you're telling Photoshop that you want the range of all tones between black and white to center around this point. Moving this control is similar to adjusting the brightness of an image.
• Below where it says “Output Levels” is where you can manually clip the highlights and shadows of your image. By moving the black and white sliders along the Clipping Scale, you can force the image to exclude either highlight or shadow information. By shifting the black slider to the right, you are purposely removing some shadow detail in the image. By shifting the white slider to the left you are purposely removing some highlight detail. Essentially this function allows you to remove detail from the tale ends of the histogram if needed.

What does this histogram tell me?
The first thing you need to do when you bring up levels, is to study the histogram. Find out what the issues are, then adjust your sliders to their optimal positions. In this example the far left side of the histogram is lacking information. This means that there are no deep shadows in this photo. You could almost tell by looking at it: The darkest points only reach a dark gray. On the far right a thin line in the graph shows that there are some highlights, but not many. Luckily it appears as though almost no histogram information is being clipped off the far ends of the graph. If too much information was clipped off the far ends, it would be very difficult to fix.

Most of the information in this histogram leans slightly to the left of the gray midpoint. This image was taken in the shadows of a deep gully, so it’s OK for it to be slightly dark, but the range of tone certainly needs fixing.

Remapping tone.
Essentially what you need to do is tell Photoshop that you want the image’s darkest shade to become pure black, and the image’s lighter shade to become pure white. This will re-map all the shades in the image, giving it better contrast, detail, and dynamic range.

• The Black Point slider. You want to position this slider just under where the left side of the histogram data appears or bulges. You'll notice in the preview, the shadows darken.
• The White Point slider. You want to position this slider just under where the right side of the histogram data appears or bulges. You'll notice in the preview, the highlights brighten. Since there were small bits of information on the highlights end, moving this slider passed them has clipped these highlights from the image. Sometimes clipping highlights or shadow detail is a sacrifice that must be made to gain a more pleasing tonal range. It's OK in this case since the highlight data that is present is very small (not a large bulge on the graph).
• The Gray Midpoint slider. You can fine tune the brightness of the image by moving this slider, but in most cases this can be left as is.

Before	After Levels adjustment.
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