Saving Images In JPEG?
 

I read that resaving a photograph after working with it in say Photoshop (JPEG)- can cause you to loose some "quality" each time you open and resave it? Can any one suggest a safe way to work with one's digital images. Would this quality loss be happening each time an image is opened and then saved? Say for instance you made changes to the size for printing or cropped it? Any good books on this subject?

I have never personally noticed that if I just open an image and close it, without doing anything else, that it loses quality. However, I've read that this is the case.

Of course any image will lose some clarity if taken from a larger size down to a smaller size. And, it's impossible to make a .jpg file larger simply by resizing and still maintain good quality.

One of the things you need to be concerned with in .jpgs is the compression level. Often people compress too much resulting in too much image degration. Once you've lost that quality you can never recapture it without rescanning or saving again from the original image which is not "lossy" (such as a .tif file).

.jpg is a "lossy" type of file compression. Pixels are actually thrown out during the compression. If the quality level of the compression chosen is high, this will never be seen with the naked eye.

Below are 4 versions of the same image. Top left is the original image. On the right is the same image with a little compression, which still renders high quality. You can see some slight differences, but the quality is still very good.

Bottom left is a medium level of compression. Notice how the quality is starting to degrade. On the right is a high level of compression. The quality is now severely degraded.

The level of compression is an option at the time you save a file as .jpg. Look for it when you do the save.

Be aware that .jpg and .gif files are all that's recognized consistently on web pages. .png is an up and coming file format, but is not recognized yet by all browsers.

Here's some information you can read, which explains better than I have. If you focus on .tiff (or .tif), .jpg (or .jpeg) and .gif, you'll minimize any confusion.

http://www.scantips.com/basics09.html

The Number One Rule: don't play around with your only file copy of an image. Relabel your best digital image as a "Master" file/folder and don't do anything to it except duplicate it. Then do your work on the duplicate. If things go awry, all you've lost is your time.

My understanding is that most of the degradation in quality occurs with multiple resizings of an image. When you make an image smaller, the software has to decide which pixels to "throw away" to accommodate the downsizing. When you enlarge an image, the software has to fill in the gaps by interpolating information from the surrounding pixels in the original file. The processes are not precise reversals of each other, so if you repeatedly resize a file, you begin to accumulate a lot of information that is based on the computer's educated guesses, rather than on your original work.

And so, if I want to print a notecard, a bookmark and a business card, and send an e-mail attachment, all using the same image, I start each of those projects with a fresh duplicate copy of the original, rather than serially tweaking one image.

That's just the tip of it, but someone on better terms with computers than I am will have to step in at this point.

[Such as Cynthia, who arrived while I was typing my note.]

Good luck
Steven