File size needs depend on intended uses. As a rule, stock image distributors require the largest file sizes, since final usage is unknown. Magazines can be more specific, although they may request Double Page (11 x 17 inches, A3 or 420mm x 297mm) for maximum flexibility in placement and cropping. Graphic designers and advertising art directors should be very specific about file sizes, since it’s best for the photographer to deliver files within 10 percent of final size — unless the designer will handle resizing and output sharpening. If designers do resize images, it is best to start with a larger size and reduce it, rather than attempting to increase the resolution of smaller files. Web use and digital projection require much smaller file sizes, measured in pixels, and usually saved at screen resolution of 72 ppi. Widescreen HDTV requires a minimum of 1920 x 1080 pixels for uncropped images, though it’s common for producers to ask for more.
| RGB TIFF at 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) |
The current uncompressed file-size standard for commercial stock images is 48 to 50 MB. A 50-MB file works for larger than A3 reproduction, because posters, banners, point-of-purchase displays and billboards are printed at lower linescreens (lpi). As the final size for such uses goes up, the viewing distance goes down and thus the screen ruling can go down, allowing a 50-MB file to serve all these uses. Software RIPs used by wide-format printing devices have built-in algorithms that interpolate image files on the fly. This is usually a better way to resize large images than using Photoshop, and then having to handle and deliver very large files.
Sensor size and file dimensions offer only a rough guide to final printed quality. Digital image files may be made larger or smaller 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 pixelation for a smoother look. For more information on “up-rez” techniques, see this excellent article by Jeff Schewe.
| Sensor size (megapixels) |
Pixel Dimensions |
Approximate Doc. Size at 300 ppi |
8-bit File Size |
|---|---|---|---|
| 17.5 MP | 3413 x 5120 | 11 x 17 inches | 50 MB |
| 11.2 MP | 2731 x 4096 | 9 x 13 inches | 32 MB |
| 6.3 MP | 2048 x 3072 | 7 x 10 inches | 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% 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 1.3–2.0 times the line-screen resolution, so as a practical matter, smaller files can successfully work for a given final size.