2 Digital Images

Capturing images

A digital camera lens focuses light onto an electronic sensor. The sensor samples the light at many small locations, and each sample contributes to a pixel in the resulting image. A scanner works similarly, sampling light reflected from or transmitted through a physical object.

Focus, lighting, sensor or scanner quality, and capture settings affect the detail and color information recorded. Whenever possible, capture at a size and quality suited to the image’s intended use. Enlarging a small image later can create additional pixels, but those pixels are estimated rather than newly captured detail.

How images store information

Most photographs are raster images: rectangular grids of pixels, with each pixel storing a color value. An image with dimensions of $3{,}000 \times 2{,}000$ pixels contains $6$ million pixels.

Color images commonly use the RGB color model, storing red, green, and blue values for each pixel. Bit depth describes how many possible values each channel can store. At $8$ bits per channel, each channel allows $256$ values; an RGB image with $8$ bits per channel can represent over $16$ million color combinations. More bit depth can preserve smoother tonal changes and give more flexibility during editing, but it also increases data requirements.

Pixel dimensions, PPI (pixels per inch), and Physical size describe related but different properties. Pixel dimensions are the image’s width and height in pixels and determine how much image detail is stored. PPI describes pixel density for a printed size, while Physical size is the image’s printed width and height.

For example, an image of $3{,}000 \times 2{,}000$ pixels printed at $300$ PPI is about $10 \times 6.7$ inches. Printing the same pixel data larger results in fewer pixels per inch. On a screen, pixel dimensions and the display determine displayed size; changing a PPI value alone does not change the pixel data. Resampling adds or removes pixels and can affect image quality.

Editing while protecting image quality

Basic edits can include cropping, straightening, resizing, adjusting brightness and contrast, correcting color, removing unwanted marks, and sharpening. Edits should support the image’s purpose without introducing distracting artifacts or misleading changes.

Keep an untouched copy of the original and edit a separate working copy. When available, use layers or adjustment layers so changes can be revised without permanently altering the original pixels. Avoid repeatedly saving an image with lossy compression because each lossy save may discard additional image information.

Choosing formats and compression

A file format determines how image data and related information are stored. Common formats differ in compression and in the kinds of work they suit.

• JPEG/JPG: Widely used for photographs. It typically uses lossy compression to reduce file size; stronger compression can make visible artifacts more likely.

• PNG: Uses lossless compression, retaining pixel data. It is useful for graphics, sharp-edged artwork, and transparency.

• TIFF: Commonly used when image quality and flexible storage are important, including archival and print workflows. TIFF files may be uncompressed or compressed.

Lossless compression reduces file size without discarding image data. Lossy compression achieves smaller files by discarding some information, which can reduce quality. TIFF, PNG, JPEG 2000, and JPEG are listed among acceptable or preferred formats for digital photographs. High-quality source images should be retained with suitable resolution, bit depth, and color information.

Exporting for the destination

Choose export settings to match the destination. A suitably sized JPEG may be practical for a photograph sent to a website or by message, while PNG can suit a graphic that needs transparency or crisp edges. For continued editing or high-quality preservation, retain a master file and export a separate delivery copy in an appropriate format.

Before exporting, check the following:

• Dimensions: Is the pixel width and height suitable for the screen or print size?

• Format and compression: Does the destination require a particular format, transparency, or file-size limit?

• Color: Are the color mode and embedded color profile appropriate for the workflow?

• Metadata: Should descriptive information, such as the title or creator, be retained?

• Appearance: Open the exported file and check that it looks as intended.