Advanced Editing Techniques

Once you’ve mastered the basics of image editing you can move on to explore the tools that serious photographers use to correct minor flaws in images before printing them or posting them on websites. The most useful tools are those for selective adjustment of brightness, contrast and colours.

Levels
One of the best tools for adjusting image brightness is the Levels control, which is based on a histogram that shows the tonal range within the image. Most applications have an Auto Levels tool, which seamlessly analyses the image and adjusts brightness, contrast and colour to provide an optimal distribution of tones. This setting can bring out the brightness, contrast and colour saturation (richness) in pictures that have been taken under difficult conditions, such as through the window of a plane.

Manual Levels adjustment lets you finetune settings selectively. Selecting the Levels adjustment displays a histogram that maps the tones in the image on a scale ranging from 100% black to pure white. The black triangle on the left end of the scale represents pure black while the white triangle at the far right is pure white. In between these extremes is another triangle representing mid-tone grey.

A shot from the window of a plane, showing typical loss of contrast and colour saturation.

The same picture after Auto Levels adjustment.
Your objective is to move these triangles so that all image tones shown in the histogram are evenly distributed.

The Levels histogram superimposed on this screen grab shows the image to be under-exposed.
Clicking on the white triangle and dragging it to the left moves all tones towards the right end of the histogram. This brightens the image.

Moving the white triangle (circled) to the point where the histogram begins to rise will brighten the image.
However, because you are effectively stretching a relatively narrow range of tones, some intermediate tones are ‘dropped out’ in the process and the histogram will take on a comb-like appearance. As long as the ‘teeth’ of the comb are closely spaced, this is unlikely to affect image quality.

When you stretch the brightest tones up to their maximum level, intermediate tones may be lost and the histogram will look like the teeth of a comb.

Moving the grey triangle to the left brightens the mid tones without changing either highlights or shadows.
You can fine-tune the intermediate tones by moving the grey triangle to bring out details in highlights or shadows.

Shadow and Highlight Adjustment
The Shadow/Highlights adjustment lets you selectively adjust the brightness of the darkest and lightest areas in an image to bring out details that would otherwise be lost. Three sliders are usually provided: Lighten Shadows, Darken Highlights and Midtone Contrast. By adjusting each of them, you should be able to achieve your objectives.

The Shadows/Highlights adjustment in Adobe’s Photoshop Elements.

Over-adjustment with the Shadows/Highlights sliders can produce unnaturallooking results.
A light touch is required to use this control effectively. If you’re heavyhanded you may end up with an unnaturallooking result caused by a loss of too many intermediate tones. Be guided by the appearance of the preview image.
Colour Adjustments
Many digital photos can benefit from colour adjustments, although most will require minimal changes. Most image editors let you adjust three parameters: Hue, Saturation and Lightness.

Hue, Saturation and Lightness adjustments in Adobe Photoshop Elements.

The Hue setting controls the colours in the image and ranges from positive to negative colours.

The Saturation slider controls the intensity of the colours and ranges from lurid colours at the high end . . .

. . . to monochrome at the low end.
Working with Layers
As your understanding of the editing process deepens, you may wish to try some of the advanced controls in the more powerful image editors like The GIMP and Photoshop Elements. These provide both a wider range of adjustments and greater selectivity in where the adjustments are made.

Always make adjustments on a copy of the image, preserving the original as your archival copy which is never edited. Creating an adjustment layer as the first stage of the adjustment process will allow you to make adjustments non-destructively. It’s easy to delete the layer if your adjustments produce poor results. This will return you to your original image.

Layers are like stacked, transparent sheets of glass. You can see through transparent areas to the layers below and work on each layer independently. Each layer remains independent until you combine (merge) the layers.

The bottom layer in the Layers palette is the Background layer, created when you load an image into an editor. It remains locked to allow you to add components to an image and work on them one at a time, without permanently changing the original image.

Creating a new adjustment layer in Adobe Photoshop Elements.

The new layer is shown circled in red in the palettes section of the editing workspace.

You can individually adjust the colour and brightness of each layer, reposition layers, specify opacity and blending values and apply special effects. You can also rearrange the stacking order or link layers to work on them simultaneously.

Selective Brightness Adjustment
One of the most practical applications for layers is selective adjustment of brightness levels in digital images. It’s useful when you wish to change only a small part of the image without altering the brightness or contrast in other areas. The illustrations on these pages show you how to go about it.

Using the Magic Wand selection tool, choose the area in the image you want to adjust. You can enlarge the selected area by holding down the Shift key and clicking on areas you wish to add.

The white dots show the area that has been selected.
Feather the selection to suppress any hard edges. Depending on the size of the image, set the feathering to between 5 pixels (for images between 2M and 4M size) and 25 pixels (for large images above 15M). You will notice the edges of the selected area becoming smoother as feathering is applied.

From the Select drop-down menu, click on Feather and set the degree of feathering.

Now select the Layer drop-down menu, click on New and choose Layer via Copy. This creates a new layer containing only the area you have selected.

Now select the blending mode from the layer using the drop-down menu in the Layers palette (lower circle). Photoshop Elements provides plenty of options; the most useful ones are Screen, which lightens the selected area and Multiply, which darkens it. We will choose Screen.

The immediate effect of 100% Screen adjustment is to lighten the selected area. The degree of lightening can be adjusted with the slider on the right side of the Layers palette (circled in red), which we have pulled back to 82%.

When you are happy with the degree of adjustment you have made, selecting Layer>Flatten image merges the two layers and locks in the adjustments you have made.
USEFUL URLs
The following websites provide additional information on the topics covered in this chapter.
www.photoreview.com.au/tips/editing/ for some useful tips on image editing.
http://en.wikipedia.org/wiki/Image_editing contains an excellent overview of the processes used in image editing plus some handy links to additional articles on the topic.
www.adobe.com/products/dng/ has information on the ‘universal’ Digital Negative raw file format and its advantages to photographers.
www.luminous-landscape.com/tutorials/bit-depth.shtml has an easy-to-understand tutorial on bit depth.
This is an excerpt from Mastering Digital Photography Pocket Guide 2nd Edition.
Click here for more details on this and other titles in the Pocket Guide series.

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