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Mac Forum / Applications / Mac Applications / February 2005Photo file sizes
First post here...hi, guys...ummm... When I load a photo with preview and rotate it 45 degrees then save it, then load, rotate back and save the file size goes down sometimes dramatically but the dimentions are still the same. What's going on with that and is there an easier way to take several photos at once and reduce the file sizes? I'd like to make them smaller for my web page to save space. Thanks in advance. -Chris > When I load a photo with preview and rotate it 45 degrees > then save it, then load, rotate back and save the file size > goes down sometimes dramatically but the dimentions are still > the same. What's going on with that My guess is that you're LOSING something. Exactly WHAT, I don't know. > is there an easier way to take several photos at once and reduce > the file sizes? I'd like to make them smaller for my web page > to save space. iPhoto will do that, and much more. It's what I use to downsize my images for my website. :) JR > > When I load a photo with preview and rotate it 45 degrees > > then save it, then load, rotate back and save the file size > > goes down sometimes dramatically but the dimentions are still > > the same. What's going on with that > > My guess is that you're LOSING something. Exactly WHAT, I don't know. Losing information through compression. Which, btw, is why you don't want to edit jpeg files if you can help it. Every time you edit and save the file, you lose some more. > > is there an easier way to take several photos at once and reduce > > the file sizes? I'd like to make them smaller for my web page > > to save space. > > iPhoto will do that, and much more. It's what I use to downsize my images > for my website. >> When I load a photo with preview and rotate it 45 degrees >> then save it, then load, rotate back and save the file size >> goes down sometimes dramatically but the dimentions are still >> the same. What's going on with that > >My guess is that you're LOSING something. Exactly WHAT, I don't know. That is correct. The quality of the picture is being severly degraded. Do it enough times and the picture will be rendered a completely indecipherable blur. It's unfortunate that people work with JPEG files without understanding what a JPEG is or how it works. A JPEG picture is a picture that has been compressed to save disk space. JPEG compression is "lossy." In English, that means the quality of the picture is deliberately, intentionally degraded in order to make the file smaller on disk. This degredation is permanent and irreversible; nothing on earth can repair the degredation, and a degraded image is degraded forever. When you open a JPEG and then do something to it and save it again, it is compressed again, and that means it is degraded still more. If you open the image and save it again, it is compressed again, and degraded more. Every re-saving of the image degrades its quality again, and like all degredation, this cumulative loss is permanent and irreversible; nothing can repair it. In general: - Never save a JPEG multiple times. Ever! If you edit an image, save it as a TIFF or PICT, not a JPEG. - JPEG is good only for situations where file size is critical but image quality is unimportant--for the Web, for example. if image quality is important, do not use JPEG. - If you think you will be editing a picture multiple times, save a master copy in a non-degraded format like PICT or TIFF, then save copies in JPEG. When you edit the picture again, go back to the undegraded TIFF or PICT, not to the degraded JPEG.  SignatureArt, literature, shareware, polyamory, kink, and more: http://www.xeromag.com/franklin.html > >> When I load a photo with preview and rotate it 45 degrees > >> then save it, then load, rotate back and save the file size [quoted text clipped - 8 lines] > It's unfortunate that people work with JPEG files without understanding what a > JPEG is or how it works. [snip] I don't wish to be argumentative, but having read a number of explanations and advice similar to the one I snipped above, I conducted a little experiment. Starting with a 4 megapixel JPEG photo (2240 x 1680, or about 31 x 23 inches at 72 dpi), I opened and saved it 10 times, changing exactly one pixel each time to ensure that the pic was truly different and being subjected to JPEGging. I then cropped about 1/4 x 3/8 inch out of the picture that I saved and re-saved, and an equal size crop of the same location from the original picture. I enlarged both those small crops and printed them as 4 x 6 inch prints. Yes, they were quite blurry (because of the enlargement), but they were equally so. In other words, neither I nor others I've shown them to could tell which one had been JPEGged 10 times and which one hadn't. So I'm not sure what to make of the "offical" explanation about JPEG being lossy. Has anyone any insights? SignatureRemove *NOSPAM* from edress In article <earl.misanchuk-0802051621150001@s0106000393c61ace.ss.shawcable.net>, > > >> When I load a photo with preview and rotate it 45 degrees > > >> then save it, then load, rotate back and save the file size [quoted text clipped - 28 lines] > So I'm not sure what to make of the "offical" explanation about JPEG being > lossy. Has anyone any insights? JPEG is "lossy", but the degree of lossiness is under the control of the software doing the compression and saving. Most apps let you set the degree of fidelity on a scale of 1 to 12 (don't ask me why 12 instead of 10 or 100 or whatever). If you set the fidelity to 12, you'll be able to edit, recompress, and save a great many times with no discernible loss and very little compression compared to other settings. If you set the compression to, for example, 6, you'll save a lot of space, but repeated open/edit/save cycles will show the quality degradation relatively soon, especially in areas of fine detail or subtle color shifts. Go to a lower number, like 3, and it will happen even faster. If you have Photoshop or Photoshop Elements, you can experiment in the Save for Web dialog and examine the detail loss at different compression settings (use the 2-up or 4-up views). SignatureSpenser > First post here...hi, guys...ummm... > When I load a photo with preview and rotate it 45 degrees [quoted text clipped - 4 lines] > I'd like to make them smaller for my web page to save space. > Thanks in advance. What's happening in Preview is that when you save the image, it gets re-compressed. So every time you do that, there's less data to work with. If you keep doing it over and over on the same file, you'll eventually end up with a really crappy looking image, but a tiny file size. There are a few ways to shrink image file sizes without recompressing, including getting rid of thumbnails and icons, and lossless progressive recompression. Search on Versiontracker.com for 'jpeg' and you'll find utilities to do all this stuff. |
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