Understanding Photoshop is a biweekly column that takes an in-depth look at how digital photographs are built and manipulated. It is a college-level course in plain English for free at Photofocus. To learn more see this article.

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Some of the core tasks of processing digital images with Photoshop involve sizing, manipulation, and processing. Even though their contents may vary, all digital images are essentially the same: They are composed of pixels that contain color and luminance information. Photoshops powerful features allow you to adjust those pixels to better match your needs and desires.

Fig 03_01 Pixel Detail

Pixels in detail: When you zoom into an image at 1600% magnification, the pixels are very easy to see. You can open the photo Owl.tif and use the Zoom tool (Z) to magnify the image. In fact, you can zoom up to 3200%, which makes pixel viewing quite easy. To toggle the visible grid at high magnifications, choose View > Show > Pixel Grid.

Digital cameras

Shooting a photo digitally produces a less accurate image than scanning a photo shot on film with a flatbed scanner using a high samples per inch setting. This is because digital cameras capture data using photosensitive electronic sensors. These sensors record brightness levels on a per-pixel basis. However, the sensors are usually covered with a patterned color filter that has red, green, and blue areas. Although the filter attempts to capture all detail that the lens sees, it is unable to completely do so due to its design.

Fig 03_03 BayerA

Digital camera technology

A CMOS sensor (above), such as this one from Nikon, is the standard imaging device on a digital camera. The filter used is typically the Bayer filter arrangement, which contains a repeating pattern of two green pixels, one red pixel, and one blue pixel. The Bayer filter uses more green because the human eye has an increased sensitivity to green

Fig 03_03 BayerB

The Bayer filter arrangement (left) uses red, green, and blue pixels and is very common in digital cameras. This filter allows the image to record the brightness of a single primary color (red, green, or blue) because digital cameras work in the RGB color space.

The RGB values combine using the additive color theory (which was briefly discussed in this article) and form an image when viewed from a suitable distance.

Not all the properties of film can be fully imitated by the computer sensors in a digital camera, so the camera must interpolate the color information of neighboring pixels. This averaging produces an anti-aliased image, which can show visible softening. When anti-aliasing is present, hard edges are blended into one another. Sometimes this can be desirable (with low-resolution Internet graphics where you reduce file size by limiting color). Other times, anti-aliasing can produce an undesirable softness when you print an image. Depending on the colors in the original image, a digital camera might only capture as little as one-fourth of the color detail. For example, if you had a desert scene with lots of red detail and little green or blue, the sensor would rely on the red areas of the filter (which only cover a fourth of the sensor face).

Does this mean you should shoot film only? Of course not; its getting awfully difficult to even buy film these days. Ultimately, film captures a high-quality image that can be optically enlarged using the negative. However, digital capture can be more convenient and affordable because you eliminate the time-consuming processes and costs associated with developing the film. Huge strides have made in the improvement of image quality in digital cameras, and the ability to experiment and shoot multiple exposures with real-time feedback makes them a much better learning tool.

menusmallrawIt is important to shoot at a high pixel count (which can be accomplished by setting the camera to shoot in a high- or best-quality mode or choosing to shoot raw). You can always crop or shrink the image for output or display, but you should avoid enlarging the image if you don’t have to. When a digital image is enlarged, it can create unwanted image softness or pixelization (a visible blockiness). Capture as much pixel data as possible to minimize digital upsampling (increasing the resolution of the image).

Workaround for unsupported cameras

If Photoshop does not support a particular raw format used by your camera, use the software that shipped with the camera. The image can be converted into a 16-bit TIFF image (a high-quality file with no compression), which Photoshop can open.