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Old Oct 25, 2004, 9:41 AM   #11
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Yes, I meant in one sesson. Thanks to Jurgen Eidt & Amateur for clearing that up.

As far as I know, there is no way to tell how much has been lost to compression. The JPG compression is very smart and advanced. From reading that jpg FAQ NHL posted its much fancier than I thought it was, using our understanding of the human eye when they designed how it works.

My understanding (which I've learned is a bit off, so take this as you will) is that one way you'll see the effect of jpg compression is in a blocky-ness in skys. When it should have had a variety of subtle blues colors those will be changed into blocks (yes, with square edges) of a single color.


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Old Oct 25, 2004, 10:39 AM   #12
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Jpg compression will change from image to image.

So two images taken with the same level of compression could turn out to be of differing file sizes depending on the image. This is down to the level of detail the image posesses; as far as I know .jpgs like to compress areas or blocks of similar colour and leave the detail in a picture alone.

This I think is why images with a lot of detail and small areas of similar colours will be compressed to a smaller degree (and have a larger file size)then the blue sky shot Eric S reffered to.

While I guess this is good to maintain a fairly recogniseable image even into fairly high rates of compression, it mightn't be the best for printing purposes. Anyways, just my pennies worth of knowledge, don't know if it'll help anyone (or if it's entirely correct - mwa-ha-ha-ha! :?)

One way or another, the rule of thumbe is to shoot in the best quality .jpgcamera setting as much as possible. Better yet to shoot in an uncompressed or raw mode, but many compact cameras still don't offer that option or else it is so slow to write to the card that it may as well not exist.
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Old Oct 25, 2004, 3:01 PM   #13
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To get the quality of a picture is not easy. How should this be defined?

For example, you load an image of a portrait with a 'in focus' tree in the backand then you blur the tree. The size shrinks because of less detail but does the quality decrease?

You can measure the quality level of a JPEG and make sure you don't save the edit one not with less. cPicture from http://cpicture.de/encan measure the quality level.
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Old Oct 26, 2004, 10:31 AM   #14
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The Tech corner of this website contains a very good article on this subject, including comparative images at different compression & generation levels...Harvey

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Old Oct 27, 2004, 12:06 AM   #15
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eric s wrote:
As far as I know, there is no way to tell how much has been lost to compression.
Well, if you have the patience, it's easy enough to display crucially detailed bits of the images alongside each other in an image editor, at such a high magnification that you can see the pixels. I usually find I can't tell the difference between a tiff and a good quality jpeg using that method, or by viewing prints.
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Old Oct 27, 2004, 6:31 PM   #16
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My experience is all theoretical since I ration myself on saves. I saytheoretical because I have never actually seen any degradation in my images that wasn't intentional. If I downsize or use heavy compression for email I expect to seedegradation.

I do as much processing in RAW as I can, then convert to highest quality Jpeg. Then I do further post-processing one time in Photoshop. If I want to do a lot of processing (multiple edits) I save the PSD file and create layers to work on. the background layer remains original. And I always have the original JPG and RAW tooif I want to start over.

Happy Editing!
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