Archived posting to the Leica Users Group, 2006/02/26
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]If you shoot jpg instea of RAW - a major mistake unless you have no choice - immediately upon downloading the image to your computer, save it in a lossless TIFF format, and preserve that file as your "negative." ___ Sent with SnapperMail www.snappermail.com ...... Original Message ....... On Sun, 26 Feb 2006 07:48:09 -0600 "Eric" <ericm@pobox.com> wrote: "Larry: " ">If I open a JPEG file in Graphic Converter (for the Mac), make a minor al= ">teration, and save in Best quality, the file size more than doubles. As a= "> test I down loaded a JPEG file from my wife's P&S camera (a Leica Digilu= ">x Zoom). The file was 197.3 KB. I then saved the file without alteration = ">at the programs's Best quality. The file size was now 412.8 KB.=20 "> ">What's going on? What was added to increase the file size? " "I'm giong to open a 1000x1500 picture. That's 1.5 million pixels. If it's "24-bit color, that's 3 bytes per pixel, or 4.5M bytes. Save it in medium "compression, and you'll probably end up with about a 100k file. " "4,500,000 --> 100,000 " "That's pretty good compression. The jpeg algorthm is tossing out some bits "that are so similar to the surrounding bits that we won't really notice the "loss. " "Then, when you load that 100k image, it becames a 4.5M picture because the "program reconstructs all the tossed out bits. " "Let's say you now save at a better quality/lower compression jpeg file. You "end up with a 150k jpeg. Where did the extra stuff come from? There is no "extra size. It's always a 4.5M picture. That hasn't changed. What has "changed is that instead of getting a 45:1 compression ratio, we're now "getting a 30:1 ratio. The lower the ratio, the better the jpeg quality. "There is little value in re-saving with lower compression, because the "made "up" pixels have already been made up from the first round of "compressing/uncompressing. " "Does this make sense? " " " "-- "Eric "http://canid.com/ " "_______________________________________________ "Leica Users Group. "See http://leica-users.org/mailman/listinfo/lug for more information