Photoshop
Colour and Tonal Adjustments
Levels
After you've generated a Histogram to see how the tonal values are distributed in an image, the next step is to perform tonal adjustments, if necessary. This should be done before applying any colour corrections. There are a number of different options for this purpose. The Levels adjustment will be discussed in this lecture.
Keep in mind that when working with these adjustments they can be applied to your whole image or a selection that you have defined. You can apply immediate, permanent changes to the active layer by using the commands in the Image > Adjustments submenu. For the options available as adjustment layers, we'll use those instead. Levels falls into this category.
Levels is used for setting the white and black points in an image. After those have been set, you can also adjust the midpoint of the grey values, if necessary. There are several ways that the Levels dialog can be used to adjust the tonal range. We'll look at some of them in a practice exercise.
Practice Exercise: Levels
- Download the waterfall image above by right-clicking (Ctrl-clicking) on the image and save it to your hard drive.
- Open the image and use the Save As command to save it as levels.psd.
Add an adjustment layer by clicking on the button in the Layers panel and selecting Levels from the pop-up menu or go to Layer > New Adjustment Layer > Levels. The Levels dialog will open.
Using the Input Levels Sliders
Let's look more closely at the Input Levels area of the dialog.
We skipped the Histogram step because the Levels dialog displays one. Above the Histogram are Input Levels for the black, grey and white points. They will always display 0, 100, and 255 when you first open the Levels dialog. These values are tied to the sliders that appear below the Histogram. Dragging a slider will change the Input Levels values. Likewise, if you enter in new values, the sliders will be repositioned. The sliders can be used to set new black and white points.
- Drag the white Input Level slider to the left until it is positioned at the point where the graph begins to rise. The white level value should be around 181. What this does is remap pixels with a tonal value of 181 to 255 and the intermediate values will become lighter. If you have the Preview check box enabled, you'll be able to see the effect in your image.
If you were to merge the layers now and generate a Histogram, it would look like the one shown here. It illustrates that the tonal values have expanded to fit the wider range. In this case, we wouldn't adjust the black point because the image already contains pixels with a 0 value. Normally, adjusting the white and/or black points is sufficient to correct the tonal values. In this case, we'll need to do more by adjusting the grey point slider.
- Move the grey point slider to the left to a point where the image lightens sufficiently without washing out the colours. The "correct" value is subjective and may be affected by the operating system and monitor settings you're using. The grey point we set was 1.20.
- Click OK to close the Levels dialog.
Here is our result. Usually adjusting Levels using the sliders works quite well. Although this image is greatly improved, the result isn't great. It has a reddish cast. Let's try another method available in the Levels dialog. In order to compare the results of your different settings, let's create another adjustment layer.
Using Auto Levels
- Right-click (Ctrl-click) on the adjustment layer, select Layer Properties from the context menu, name this layer Levels Sliders and click OK. Hide the layer.
- Click on the Background layer and create a new Levels adjustment layer.
- When the Levels dialog opens, click on the Auto button and then click OK.
The Auto button automatically sets the white and black points to the lightest and darkest areas of the image. This has given a much better result. The red colour cast has been removed by using the Auto option. A word of caution, though. Auto is usually not the best choice, although it worked quite well on this particular image. Auto Levels may result in colour shifts. It worked to our benefit in this case but if an image's colour balance is already good, Auto Levels may cause an unwanted colour shift. Adjusting the Input sliders will never affect the colour balance.
The Auto Levels command is also available in the Image > Adjust menu.
Using the Levels Eyedroppers
Black and white points can be set by sampling neutral-coloured areas of the image with the eyedroppers. The key word here is neutral because if the pixel sampled isn't neutral, a colour shift will result.
- Right-click (Ctrl-click) on the second adjustment layer, select Layer Properties from the context menu, name this layer Auto Levels and click OK. Hide the layer.
- Click on the Background layer. Hold down the Alt (Option) key while clicking on the adjustment layer button in the Layers panel. In the New Layer dialog, name this layer Levels Eyedroppers and click OK.
- In the Levels dialog, click on the eyedropper for the white point.
- Open the Info panel to help you locate the lightest point in the image. Watch the K value in the CMYK section of the Info panel as you move the pointer over the white waterfall. Find an area with the lowest K (black) value and click in the image to set the white point. Here's where we selected a white point with the eyedropper:
You'll notice that the Histogram has changed to show the redistributed tonal values. This is one advantage of using the droppers. Note that the Input Levels values readout doesn't change when the Eyedroppers are used.
This has yielded the best results of the three methods. In a side-by-side comparison, Auto Levels looks washed out.
- Save your file.
Other Levels Controls
- Channels
- You can adjust individual colour channels or the composite channel, as we've done above. Manipulating a colour channel will have a greater effect on the colour than adjusting the composite channel. It would be useful in correcting colour shifts after the tonal values have been corrected.
- Output Levels
- The Output values can be manipulated like the Input Levels. The result is a decrease in contrast and is only recommended in images with high contrast.
- Save and Load
- Levels settings can be saved in files on your hard drive with the .alv extension. The Save button will save the current settings. To load a previously saved Levels file into the dialog, use the Load button. This would be useful if you were processing a number of photographs taken with the same camera that require similar adjustments.
Conclusion
Using the Levels dialog can be much more involved than what you've been shown here but this is a good starting point. It's worth investigating the potential of Levels by doing further reading, either in the Photoshop Help file, user manual or other resources.
At the beginning of this lecture, we said that tonal adjustments should be made before applying colour corrections. Now that you've seen how the colour balance can shift during tonal adjustments, you understand why.
Levels Summary
- Tonal adjustments should be performed before applying colour corrections.
- The Levels dialog is used for setting the white and black points in an image. After those have been adjusted, you can adjust the grey point, if necessary.
- The colour balance of an image won't change when using the Input sliders. It can change if Auto Levels or the Eyedroppers are used.
- A new Histogram is generated within the Levels dialog when the Eyedroppers are used.
- The way to avoid colour shifts when using the Eyedroppers is to sample neutral colours in the image.
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