JPEG compression, defined by an ISO norm in 1990, was developed for the purpose of efficient storage of photographic data. Lossy compression (such as JPEG) takes advantage of the human eye’s imperfection and stores data with a certain loss of information, thanks to which it achieves much higher compression than lossless compression does. The mentioned imperfections include for example a greater sensitivity towards large changes than fine details, and towards brightness changes than color changes. JPEG takes advantage of this to reduce the size of color information by recalculating color channels to a lower resolution (subsampling). The amount of detail preserved by JPEG is configurable, and is measured by a “quality level” on various scales; Zoner Photo Studio uses a scale from 1 to 100. Higher values mean less distortion and a larger output file. Smaller values mean more distortion and a smaller output file. JPEG compression settings affect only the picture’s quality and file size. Its size in pixels remains unchanged.
Choose compression levels based on how you plan for the picture to be used. Use values from 30 to 60 to maximize size saving. Use values from 70 to 80 for usual purposes, e.g. for pictures published on the Internet. Use values from 80 to 100 for desktop publishing. Although higher values give more detail, this relationship is not linear. Values above 90 give nearly imperceptible improvements while markedly increasing file sizes.
Because of rounding errors in calculations, and in some cases due to color subsampling, JPEG compression causes quality less even when you use a quality value of 100. Therefore, JPEG compression is inappropriate for pictures that require absolute precision (pen drawings, line drawings, etc.). However, for photographs, it is invaluable, since with proper settings, the changes to a picture are imperceptible. The JPEG Group also created an ISO standard for lossless JPEG compression, but this standard is unused. It is outdated today, since the more modern PNG standard generally gives better results than lossless JPEG.
To eliminate unneeded quality losses when resaving in the Editor, use high JPEG compression quality values such as 90 to 95. To set what values are used automatically, use Settings | Preferences | General To set the value manually while saving, use “Save As” and then the control offered during the saving process. To avoid quality loss altogether, use a format with lossless compression, like PNG or TIFF. If you need support for EXIF picture information, do not use PNG: it does not support it.