Digital Photography - Two Tips for Submitting Digital Images to Stock Photo Libraries

Two Tips for Submitting Digital Images to Stock Photo Libraries

1. File Size

When preparing digital image files for stock libraries, it is important to be aware that File Size needs will depend on their intended uses. As a rule, stock image distributors require the largest file sizes, since final usage is unknown. Magazines tend to 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 are often very specific about their file size requirements. This means 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.

Uncompressed file size requirements for print
	RGB TIFF at 300 ppi
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 image file is perfectly adequate for projects requiring images larger than A3 reproduction. This is 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, which means the screen ruling can also go down, allowing a 50 MB file to serve all these uses. It is also worth remembering that Software RIPs (Raster Image Processors) 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.

2. Digital Capture Quality

Sensor size and file dimensions offer only a rough guide to the final printed quality of a digital image. 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. However, you need to be aware 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 ‘The Art of the Up-Res’ 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, it is important to be aware that pixel quality generally trumps pixel numbers. This means that a high-quality 6MP sensor will often be sufficient for 95% of intended uses, while pictures from a 10MP sensor in a point-and-shoot camera may look poor when enlarged beyond 8×10 inches at 300 ppi. This is especially so if the image was captured 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, in practice, smaller files can successfully work for a given final size.

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