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Digital Photography Workflow

Evaluating Your Images—Basics

 
 
 
Click for a movie on adjusting tonal ranges.
 
 
Cropping (top) is one way to emphasize the key parts or make it fit a format, perhaps in a magazine layout.
 
 
One way to evaluate the tonal range is with a histogram that charts the various levels of brightness in the image.
 
 
Hues can be arranged on a color circle or wheel.
 
 
Click for a movie on adjusting hue, saturation and brightness.
 
 
As you point Lightroom's white balance selector tool at a pixel in an image, the pixel's color mix is displayed. If you click that pixel, it and all like it will become neutral.
 
 
Click for a movie on sharpening an image.
 
 
Noise can significantly degrade smooth tones.

When you open an image on the computer, you really get to see it for the first time. The display on the camera's monitor is so small, captured images are hard to evaluate. So what do you look for when deciding if the image you are looking at can be improved? In this section we'll try to get you started. As you learn to identify characteristics that can be improved, you'll also discover there is frequently more than one way to adjust them. Many people start with the automatic adjustments because they are so easy. However, it won't be long before you find yourself migrating to more powerful tools that take more practice, but which give results that make the extra effort worthwhile.

Without you even being aware of it, your camera is making changes to your JPEG images that cannot be undone. These include such things as sharpness, white balance, and contrast. If you want to adjust these yourself, use the camera's RAW format if it has one.
 
To properly evaluate images your system should be color managed, as we discuss later. At times you should also enlarge images to 100% (sometimes called 1:1 or Actual Size) or use a digital loupe to zoom in to examine details.

As you examine your images, here are some things to look for.

Evaluating Image Size and Orientation

The initial size and orientation of an image is determined by what the camera captured. There are situations in which you may want to change these characteristics.
  • Resizing can be done in two ways, by changing the number of pixels in the image, called its pixel dimensions, through a procedure called resampling. This process adds or removes pixels to make the image larger or smaller. You might want to do this to reduce the size of images you will be sending by e-mail or posting on the Web. You might also want to increase the size of the image when making large prints. However, increasing the number of pixels in an image doesn't always make the image better. In fact it usually has the opposite effect. You can also change the size of the image without changing the number of pixels it contains, called its document size by specifying the pixels per inch. You normally do this when making a print or exporting an image to another application.
  • Cropping removes distracting or unimportant parts of an image. You might also want to crop if the image has to fit into a specific design such as a newsletter or greeting card.
  • Rotating an image may be necessary if you turned the camera vertically to capture a picture, or if the horizon line is tilted.

Evaluating the Tonal Range

Dynamic range in music is the range between the faintest and loudest sounds that can be reproduced without distortion. In photography the dynamic range, called the tonal range or contrast, indicates the range of brightness in an image between pure white and pure black. There are two ways to evaluate the tonal range of an image—visually and using a histogram (discussed in the next section). You should use both approaches because they are not mutually exclusive. For example, you can analyze an image visually and then learn why it's the way it is by checking its histogram.

Visually, images that use the full tonal range look rich and crisp, with vibrant colors and smooth transitions in tones. Those that don't use the full range lack contrast, often looking flat and dull. Details may be missing in highlight and shadow areas or the image may be too dark or light. In these cases you may want to adjust or expand the image's tonal range.

Evaluating Colors

The human eye perceives color in terms of three characteristics—hue, saturation, and brightness and there is even a color model based on these characteristics, called HSB. The color monitor uses a different color model called RGB because images are displayed using varying amounts of red, green, and blue light. When evaluating images, you can think in terms of these two models—one to evaluate colors and the other to look for color casts.

To evaluate colors in an image, think of them in terms of hue, saturation, and brightness because these three aspects can be adjusted.

  • Hue is unique in one respect, it is the actual color, as measured by its wavelength, while the other two characteristics (saturation and lightness) modify the hue in some way. The hue can be red, orange, yellow, green, blue, purple, or any intermediate color between those pairs.
  • Saturation, sometimes called chroma, is the strength or purity of the color. If you adjust saturation through its entire range, colors go from rich and vibrant to dark gray.
  • Lightness, also called luminance or brightness, is the relative lightness or darkness of the color. Brightness is reduced by adding black to the color mix and increased by adding white. If you adjust brightness through its entire range, colors go from white to black. This is the only one of the three color attributes that gray scale images have.

As you decrease saturation (top), colors become muddier and finally gray. As you decrease lightness (bottom), colors become darker and eventually black.

A color cast is usually caused when one or more of the three color components (red, green, and blue) are too high or low over the entire image. This can be caused by not setting white balance correctly, by photographing a scene illuminated by more than one type of light source, or even a subject picking up reflections from a colored surface. Color casts are very noticeable when shooting during sunrises and sunsets—but there we usually like the effects. It's easiest to identify a color cast by looking at areas that should be neutral white or gray. If these areas have any colors mixed in, the image has a color cast that you should remove. Pure white areas should have R, G, and B settings of 255. Gray areas should have R, G, and B settings that are equal, for example, 128, 128, and 128 for middle gray. Pure black areas should have R, G, and B settings of 0. Regardless of which neutral tone you are examining, if one or more of the RGB values is higher or lower than the others, these tones won't be neutral and will have a color cast.

A neutral color has equal amounts of red, green, and blue and appears as a shade of gray.

Evaluating Details

When examining an image, look for small imperfections that can be retouched. The sensor may have dust that shows up on the image as dark spots. A portrait subject might have a small blemish that will be very noticeable when you enlarge the image. There may be reflections, or even telephone wires you want to remove. Small areas may benefit by being made a little lighter or darker than their surroundings. Portrait subjects may have red-eye caused by flash in a dark room.

Evaluating Sharpness

The apparent sharpness of an image depends a great deal on how much contrast there is along edges and lines. If an image looks soft, it can often be improved by sharpening, a process (technically called unsharp masking) that adds contrast along lines and edges. Many photographers sharpen almost every image, ignoring this aspect only for images such as fog scenes that are deliberately soft.
 

Evaluating Noise

If you used a long shutter speed or high ISO setting to take a photo, it may contain noise. Look in dark areas for randomly colored pixels that look like grain.

Global vs Local Editing

You may have noticed that some of the adjustments discussed in this section affect the entire image and others affect just specific areas. These are referred to as global and local adjustments and are discussed in sections that follow.


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