How to apply minor PP to SOOC Jpegs and understanding its effects on image degradation ?

Started Jan 28, 2013 | Discussions thread
Tom Axford Veteran Member • Posts: 4,659
JPEG loss varies according to the jpeg quality setting

What you are referring to is the size of the jpeg image file, not the image size (which is measured is pixels not in bytes). The size of the jpeg file varies hugely according to the "jpeg quality" setting when the jpeg was created. Most cameras offer two or three different jpeg quality setting which you can select in one of the menus.

If you files are normally over 6MB then you probably have chosen the highest quality setting (often called 'Fine' in the menus). The image loss at this setting is very slight indeed and you should be able to PP the image without any noticeable loss of quality.

The size of the new jpeg file created after completing your processing of the image will depend on the jpeg quality setting used by the software you used for PP. Most software allows you to change this jpeg quality setting if you wish. Unfortunately the jpeg algorithms used vary a great deal: some are better than others (i.e. the size of the resulting jpeg file is not always the same even at the same jpeg quality setting). To complicate things further, the jpeg quality is sometime specified qualitatively (e.g. 'good', 'very good', 'best') and sometimes on a numerical scale from 1 to 10 or 1 to 100.

I generally use 85 (on a scale of 1 to 100) and my files come out around 3MB (very approximately) for 16 Mpixel images.

The most important thing to remember is to not keep reprocessing from one jpeg file to another as there is a very slight loss of quality each time you save a jpeg. Suppose you want to do part of the processing, then save the image so that you can return to it to do further processing. Rather than saving this intermediate image as a jpeg, it is better to save it as a tiff file (which is a lot larger, but is 'lossless' - there is no degradation of the image whatsoever).

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