JPEG compression

I use a Digital Ixus 300 shooting at 1024x768 24 bit. I only take pics for looking at onscreen or putting on the web and apart from cropping don't do much editing.

Generally the file size on download to my PC is about 350-550k. If I save the file with a new name the size drops to about 60-80k, and there is no visible loss in quality, which is all I'm worried about. Is it normal to get such a huge reduction in file size on first resave? Also, since I am happy to settle for the smaller file size despite loss in 'invisible' detail, is there a utility or tip to automate a batch opening and resaving of files? I reckon I could save a couple of gig on my now full hard drive that way! I know it is possible to get batch file renaming, but I think those programmes don't open and save the pics, they just change the names.

First there are several programs which will allow batch renaming and saving at a higher compression ratio, which is what you are doing. Photoshop 7.0 and Thumsplus come quickly to my mind (they being two that I have, I know there are others)

For an original image to jpeg compression with no VISIBLE loss in quality it is quite possible depending on the content of the image to get 10:1 or more in file size reduction. I have even seen visually lossless images at more then 100:1 (usually used to sucker people in to buying product).

The catch is you are losing actual information that is in the image, so it is technically no longer as good as the original. Jpeg (except the newest jpeg 2000 standard) is a lossy format. Meaning you lose data. However, if it you final save and you will not be doing further processing, you obviously can get an acceptable image from jpeg compression.

The catch is you original image might have been able to print a 11x17 and now you can only get an 8x10 without seeing visible artifacts. These jpeg artifcats which tend to look like blocks on the image tend to be in 8pixel x 8pixel blocks.

Just remember jpeg is fine as a final product, but you don't want it as an interim save as you will continue to lose quality with each save.

[Trivia: 4.3:1 is a known ratio for mathematically lossless complex images. Theory says 12:1 is possible with 8:1 being realistic. 6:1 is the highest I have heard that consistently worked, but it took forever to compress and uncompress the image. Visually lossless images typically start at 8:1 with 20:1 being maximum routine compression. Anything more then that is usually a result large areas of the image being contiguous in form and color (the technical joke is I can get you 10,000:1 mathematically lossless compression - Of course it will be a picture of an Artic snow field on a moonless winter night)]

Perhaps what you should consider doing is backing up the photos that are clogging up your hard drive. Put them on some CD's and then keep a thumbnail of the shots, and have them lead to the correct CD. THis way you could just load the photo you want (at full resolution) but your drive won't be bogged down with gigs of photos.

It would be horrid to want to print a large version of a photo and find you can't get anybigger than 8X10 or 5X7 without problems...

If you must keep all the pictures on your hard drive, perhaps it's time to get a bigger drive?

Thanks for your replies. As I mentioned, I'm only interested in on-screen viewing, so don't care if I lose detail that would only show on printing. I travel a lot, with a slim laptop that only has external CD, which I don't take with me, and I like to e-mail photos to friends etc. Smaller file sizes is what I need.

Prompted by Longwatcher's comments I ran a search for freeware batch JPEG compression utilities and ended up with a product called Mihov Image Resizer (http://freeware.mihov.​com)

Seems to do exactly what I need.

Thanks again!!

Pictures are precious memories, and though you may not need a higher quality now, your needs may change in the future. It takes such a little effort to save them, on a CD, as negatives.