Above the Trees

Digital Pictures 101: Part 2 - Compression

Awhile back, we explored “resolution” in digital imaging, and discussed tips on how to choose optimal scanning resolution. We also identified the problem that optimal scanning presents, especially in the case of creating a digital archive: image files can be huge – and can easily fill up your hard drive. And although hard drives are getting larger and cheaper every day, the limiting factor might be your backup media – which in this day and age tends to be CD’s DVD’s or online internet backups. Image compression can significantly reduce this burden. As the name suggests, “compression” technology results in smaller file sizes. There are two major types of compression: Lossless and Lossy:

LOSSLESS compression (PNG, TIFF, BMP file formats)

• Reduces file size with no loss in image quality.
• Does not compress to as small a file size as lossy. (See Table)
• Use when archiving and editing images.

LOSSY compression (JPG or JPEG file formats)

• Reduces files size with some loss of image quality.
• Allows for variable levels of quality (compression) to be
• selected by the user.
• Use when sharing images.

Popular Digital Image File Formats

• JPG or JPEG – Joint Photographic Experts Group. Most digital cameras use this by default. Lossy compression.
• TIFF - Tagged Image File Format. This flexible image format allows for many color depths, and can use Lossless or Lossy compression
• PNG - Portable Network Graphics. Handles 24-bit (true) color, Lossless compression.
• BMP - Windows bitmap. Not compressed.

The following MB Comparison Shart is based on a sample image that is 5400x3600 pixels:
The main point to take away from all this is that JPG (pronounced jay-peg) is simply amazing at compressing file sizes with very little loss in image quality. This is especially true when scanning images at very high resolution (300 dpi or higher) and saving files with high quality settings (about 90% of the maximum setting). JPG compression allows you to store and share hundreds of high quality images on a CD instead of dozens.















In these examples, the three cropped images above, cropped from the original, shown are (1) Low-Quality JPG, (2) a High-Quality JPG, and (3) TIFF file (no compression).

Another factor to consider is color depth. Color depth is the number of bits (or bytes) per pixel. More bits per pixel result in more available colors in the final output. Color depth also effects file size, so pay attention to scanner settings.

Generally an uncompressed image will be 1/3 the file size if it is scanned in 8-bit per pixel gray scale instead of True Color— a good thing to remember if you are scanning a lot of documents or black and white photographs and need to save hard drive space.

• Typewritten or handwritten documents should be scanned with 8-bit per pixel gray scale.
• Black and white photographs should be scanned with 8-bit per pixel gray scale unless you want to preserve the subtle sepia or yellowing; then choose True Color.
• All other color photographs or color documents should be scanned using True Color.

Doing the right thing with image compression:

• Archive using lossless compression. (Please!)
• Experiment before picking a compression: Zoom way in to your compressed files to see how the lossy-compression is effecting the quality.
• Choose a compression that allows your project to fit on the media provided.
• Share excellent quality copies using compressed files.
• Use 8-bit gray scale color depth for documents and black and white photographs to save disk space.