Files of lower-than-optimum resolution will not deliver as much image detail and crispness as the output device can reproduce. Excessively high resolution, on the other hand, is wasteful of computer resources; the surplus data can only be discarded, yet the file will take longer to transmit, more memory to process and more disk space to store.
The resolution of digital images intended for computer displays is usually described by absolute pixel dimensions: width and height in pixels. For printing, width and height are often stated in inches, requiring a third value: of pixels per inch. (Outside the U.S., dimensions are stated in centimeters or millimeters, and resolutions expressed in pixels per centimeter [ppc] or per millimeter [ppm].) Beware: It's easy to confuse ppi with dpi (dots per inch), which refers to the resolution of a printing device, or with lpi (lines per inch), which describes a halftone grid or screen used for printing images on a press.
rules of thumb
- Low-resolution images destined for computer displays (monitors) are best defined by pixel dimensions, such as 640 pixels wide by 480 pixels high.
- Inkjet printers normally work best with resolutions of 180 ppi to 360 (some say up to 480) ppi, such as 8x10 inches at 180 ppi.
- Continuous-tone printing (e.g., on film recorders or dye-sub printers) requires resolutions of 240 ppi to 400 ppi.
- The most commonly quoted offset-printing standard is 300 ppi. But resolutions of 1.3 — 2 times the halftone screen for the project are considered safe. For example, if the images will be printed as 150-lpi halftones (common for magazine printing), the appropriate image file resolution range would be 195 ppi to 300 ppi. Newspapers print with coarse screens. A common newsprint resolution is 85 lpi, which works best with an image resolution of 170 ppi.
- Screen-display and electronic-projection image resolutions will vary substantially, depending on the native resolution of the projection device. It's important to obtain information about the native resolution of the screen or projector before delivering "screen resolution" images.
- The resolution of digital images intended for screen display and electronic projection is best described by absolute pixel dimensions: width and height in pixels. Example: 1024x768 pixels.
- Images should be cropped to the correct aspect ratio for intended use. Examples: 1024x768 is a 4:3 (1.33 to 1) aspect ratio, while widescreen 1920x1080 is a 16:9 (1.77 to 1) aspect ratio. Where cropping is undesirable because it will change the composition of the image, you should set the image size along the longest dimension. You can either accept that two borders (either the top and bottom or the left and right) will have bands of the background color, or you can add bands of black or another color to the image to fill out the screen.
- Determine whether the intended use of an image involves zooming or panning. If it does, deliver enough resolution to accommodate this intended use.
- The size of the image on a computer monitor is based on the physical size of the screen and the setting of the screen resolution. When sizing images for use on multiple computers, consider that a 600 x 400-pixel image will almost fill an old 15-inch display set at 640x480 and will be very small on a 30-inch display set to 2560x1600.
- Size images delivered for projection specifically for the intended use whenever possible. Sizing images for the exact native resolution of the projector produces the best results. Undesirable effects, such as stretching, shrinking, pixelation and loss of sharpness can be caused by software and hardware rendering of unsized images. Besides these drawbacks, it's not useful to make the image larger (in terms of pixels) than the screen size, since it will take much longer to redraw. Size it too small, and the image quality will deteriorate.
- VGA (640x480) — Once the standard, now only used on small devices, such as graphics tablets
- SVGA (800x600)
- XGA (1024x768) — By far the most common resolution for PowerPoint and projection video
- SXGA (1280x1024)
- SXGA+ (1400x1050) — Newer projectors
- Widescreen (1920x1080) — HDTV 1080i and 1080p
| RGB TIFF @ 300 ppi (default for 150-line screen) | |
| Double page (A3) | 50 MB |
| Full page (A4) | 25 MB |
| Half page (A5) | 12 MB |
| Quarter page | 6 MB |
| Eighth page | 3 MB |
| Billboard | 48 MB at 600 ppi (but 300 ppi is acceptable, i.e. 24 MB) |
digital capture quality parameters: sensor size and file dimensions
Sensor size and file dimensions offer only a rough guide to final printed quality. You can enlarge or reduce digital image files by interpolation in imaging software, which adds or removes pixels. RIP software and hardware can also interpolate during printing. Properly handled, interpolation can increase digital image files in size by as much as 400 percent. However, success depends on the qualities of the original file and the final output required. Note that increasing resolution through interpolation cannot add detail to a file. It may, however, prevent obvious pixilation, offering a smoother look.
| Sensor Size (megapixels) | Dimensions (pixels) | Dimensions (inches @ 300 ppi) | File Size (8-bit) |
| 21.1 MP | 5616x3744 | 12x19 | 60 MB |
| 17.5 MP | 3413x5120 | 11x17 | 50 MB |
| 11.2 MP | 2731x4096 | 9x13 | 32 MB |
| 6.3 MP | 2048x3072 | 7x10 | 18 MB |
Stock-image distributors, magazine publishers or others requesting files may specify a certain minimum size for camera sensors used to generate image files. However, pixel quality trumps pixel numbers, so that a high-quality 6-MP sensor will often be sufficient for 95 percent of intended uses, while pictures from a 10-MP sensor in a point-and-shoot camera may not look good beyond 8x10 inches at 300 ppi, especially if used at an ISO exposure setting above 200. Although many publishers have standardized on 300 ppi for 150-line screens, the actual requirement is 1.3 to 2.0 times the line-screen resolution, so as a practical matter, smaller files can successfully work for a given final size.