Email attachment can make quick file transfers possible without the complexities of FTP software, but as a method of conveying high-resolution images from one user to another, email's file size and bandwidth limitations can make it less than ideal. When you can't reduce image dimensions or resolution to make your files more portable, you may need to resort to other alternatives to enable big bitmapped images to piggyback onto email transmissions without your messages bouncing back to you as undeliverable.
Use Archival Compression
Unlike JPEG and other compressed file formats, archival compression exists solely to reduce the size of a digital asset for storage or transfer purposes. To use the file, you first must extract it from its compressed receptacle. Some compression formats, including the SIT files produced by StuffIt, dominated on the Mac in the days before OS X. The ZIP file format provides cross-platform support for compression needs under OS X and Windows. Not all high-resolution images compress to the same extent. If you've saved your file in a compressed format, for example, it may respond less to archival compression than if your image consists of an uncompressed file like TIFF or PSD.
Skip High-Bit Versions
Unless your recipient absolutely requires a 16-bit image file, you can reduce the size of your document substantially by creating an 8-bit version for transmission via email attachment. At 300 pixels per inch, a 5-inch square 8-bit image requires 2.15MB of disk space in grayscale and 6.44MB in RGB color. At 16 bits, its sizes balloon to 4.29MB and 12.9MB. High bit depths support extended tonal ranges that can create smoother gradient blends and allow for color correction methods that otherwise would degrade the image. These higher bit depths bulk up file sizes unnecessarily if the person on the other end only needs an image file for a page layout.
Remove Extra File Resources
Along with high bit depths, optional file resources also enlarge document sizes. Alpha channels, invisible layers, paths -- all these extras may be unnecessary to the person who requested your high-resolution image. If you've saved your document as a layered TIFF, creating a flattened alternative may help you economize further without compromising image quality. When time crunches preclude the use of flash drives and optical media through overnight shipping, you can save each channel of a large RGB or CMYK file as a separate document and allow your recipient to reassemble the file.
Switch to Website File Transfer
Email providers limit the sizes of the attachments their customers can send. Some accounts can create 25MB messages whereas others are limited to 10MB or smaller per message. Both the sender and the recipient must be able to accommodate the attachment size. To avoid the potential problems that can plague this method of sending files, look for a Web-based file transfer service -- such as Copy, WeTransfer and Dropbox -- to which you can upload your image so your client or colleague can download it from the same site.
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