Quote:
Originally Posted by treeve
Excellent article, a question I have wanted to ask ....
Something I note is that when stored in the camera,
a 9500Mp picture takes up 2.2 Mb. If I rotate the photo
it become 1.3mb; and if I alter the contrast, or something,
and save again, it becomes 800Kb. What is being lost,
if anything?
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When you save as a 'JPEG' (jpg) file a certain amount of detail in the image is thrown-out, so to speak, to keep the file size small, but is then 're-invented' when you open up the image again.
Some of the finer detail gets lost and, though it may still look fine as a photo, what you've got is a sort of 'simpler' image so every time you alter anything you have to 'save' when you close the image down and by saving as a JPEG again you throw out detail from the 'simpler' image.
This is why the file size is now smaller - the simpler the image the more the system can throw out because it's easier to re-invent it back again (check the file-size for a jpg image containing a large area of one colour).
Doing it several times will just make the image worse and worse each time as more and more fine detail is lost so always save as an un-compressed file type such as 'TIFF' (tif)