iStock - Stock Photography Training Manual

4.0 Quality Standards

4.7 Compression

Compression, and 'Compression artifacts', refers to visual distortion which occurs in an image when information is lost. The JPEG file format literally compresses an image - makes it smaller - to reduce the file size during saves. The particular compression method it uses is a lossy format, which means that it loses some information to shrink the actual number of bytes used in the file. When too much information is lost it can have a visible impact on the image. This can also happen during the image editing stage. These days, improved camera technology and lots of available storage have helped. Most of the compression that we see now is a result of excessive image editing. Pushing certain filters or processing techniques will cause information to become lost and introduce compression damage into the file.

Compression typically results in a few different kinds of visual distortion, which will typically be called 'artifacting', 'contouring', or 'posterizing':

  • Curved edges in detailed areas take on a jagged stair-case like appearance.
  • Checkerboard style blocks appear.
  • Color gradients become 'banded', meaning that instead of a smooth transition between colors, there are instead abrupt changes from one color to another making visible bands across the image. This often happens in large areas of subtle color change, like a clear blue sky in a landscape.

Unacceptable:

Typical small sensor compression.

Blue skies are prone to detail degradation. In this case, over-saturation and boosted contrast in an 8 bit jpg file have made the problem worse.

Underexposed, low ISO RAW image. Compression was introduced after too much exposure adjustment and sharpening during RAW editing.

RAW conversion with boosted contrast, edited curves and pushed contrast. Visible circular banding appears at 100% in the top corner of the sky.

Heavy editing at the RAW stage to bring out detail in the shadow/highlight areas has introduced ugly halos on the outlines. Image is also over-sharpened.

Underexposed RAW image with overly-aggressive selective exposure editing.

Visible artifacting in the shadow areas.

Tips:

  • Use a DSLR and not a compact digital camera with a tiny sensor.
  • Shoot RAW. Edit your files with care, always observing changes in the detail quality at 100% all around the frame.
  • Watch out when selectively correcting exposure, adjusting curves, or boosting saturation – these can all introduce compression.
  • Do not sharpen too much.
  • If you're using RAW software for conversion only, you might benefit (especially with your nice blue skies) by converting the image to a 16 bit TIFF and downsample the edited TIFF file to an 8 bit jpg at the end.

Please read this article for more examples and information about compression.

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