rmalarz Loc: Tempe, Arizona

This is not going to be a "this format is better than...". It's simply going to clear up a few misconceptions bandied about on this forum regarding the jpg file format. In fact, it's only going to clear up one misconception about this file format. The jpg format is a lossy file format. This means it is going to lose data. An understanding of discrete cosine transforms would be helpful in understanding the process. However, suffice it to say, here's what this post is going to clear up.

If one opens a jpg file, looks at it, and closes it, nothing happens to the file.

If one opens a jpg file, looks at it, and saves it, it degrades.

This is just the nature of the jpg file format and the save algorithm. It's that simple. Open, Look, Close, nothing. Open, Look, Save, file changes.

Yes, one can edit a jpg. However, in addition to the edits one makes, the save process works just the same making additional changes to the file over which the photographer has no control. The only control the photographer has is how much more gets changed based on the amount of compression selected at the time of saving the file.

If you don't believe this, it's simple to prove. Using the diff function in UNIX, or the free Windows program windiff will show you the changes. Can you visually see them, perhaps yes, perhaps no. The changes are there none the less. Again, it requires nothing more than a Save to instigate a change to the file.
--Bob

lamiaceae Loc: San Luis Obispo County, CA

Good post!

bsprague Loc: Lacey, WA, USA

"It's that simple. Open, Look, Close, nothing. Open, Look, Save, file changes."

No argument with that at all. It is the technical, factual reality.

But artistically or creatively the picture gets less clear. I have stunning (to me!) JPEGs from small sensor cameras and stunning (to me!) RAWs, TIFFs and PSDs.

via the lens Loc: Northern California, near Yosemite NP

Does anyone have a technical reference that I could read to find out about this? So far, I have not been able to find anything on this "open and close" issue in my Manual of Photography, which is a sort of bible about technical issues in photography.

Longshadow Loc: Audubon, PA, United States

Don't need one?
You can view a jpeg a million times without any degradation in the file.
The degradation comes from opening the image file in an editor and saving it instead of just closing. Every time you "save" the file in an editor it runs the file through a compression algorithm and then saves the file. Kinda like (but not identical to) copying a copy of a copy of a copy on a Xerox machine.....

Bill_de Loc: US

What programs that you use offer the 'save' option if you just open it? Usually they offer the 'save as' command, and if you don't at least change the name it tells you the file already exists.

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via the lens Loc: Northern California, near Yosemite NP

Yes, I read that, but I like to read the technical background on photography, thus seeking a technical manual on it. This is not to say I don't believe what I am reading, I just like to do the research.

rmalarz Loc: Tempe, Arizona

Here's a book I'd recommend. https://www.amazon.com/Encyclopedia-Graphics-File-Formats-Reference/dp/1565921615
It came in handy in a few of the programming classes I took.
--Bob

bsprague Loc: Lacey, WA, USA

Technical background.... Try this to start, but check the references at the end. No, I didn't read it. It is too complex!

https://en.wikipedia.org/wiki/JPEG

Hyperhad Loc: Thunder Bay, Canada

Well said and simply explained rmalarz. This was one of the first things I taught my students when we began looking at editing images. The difference between lossy and lossless was a concept I had to get across to them.

It is easy to remember by the term used. If something in your image is lost, that is bad. If nothing in your image is lost, that is good. Of course, it depends upon how you will be using your images. Lossy files are smaller in size, and faster to send electronically. But you lose a bit of your image data as the computer "deletes" some data that can be replicated when the file is opened next. Lossless files are larger in size, and take longer to send electronically. But, no data is lost because the computer takes all the data in the image and copies it in toto.

The same applies to lossy and lossless compression. The more times you copy a lossy file, the greater loss in quality. Photocopy a page of text on paper. Now copy the copy. Repeat several more time. After you are 5 or 6 copies away from the original, the text will be degraded quite badly. The same applies to lossy files. Remember when you were in school, and your handout looked really bad, blurry, and hard to read? Same thing. The teacher made a copy of a copy of a copy *they likely had no electronic file of the handout, so they photocopied an old hard copy". Lots of teachers had to be dragged, kicking and screaming, into the digital age!

A lossless file will remain, for all intents and purposes, identical to the original. I know what format I want to use for my images!

Longshadow Loc: Audubon, PA, United States

Photo Impact Pro will allow saving a modified or unmodified file without asking if you want to over-write it.

Elements 14 (modified or unmodified) asks for a file name and asks if you want to over-write existing if you do not enter a new file name.

Not understanding the program process, someone could think "Yes, I want to save it. Use the same name since I didn't change it..."

LoneRangeFinder Loc: Left field

Most of my programs only ask if you have made a change AND are closing or exiting.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

You don't need one. When you open a jpeg you need to de-compress it. When you simply close the file you are not saving anything so there is no need to re-compress it. Hitting the "save" option triggers a re-compression - even if you have not made any changes. Hitting close does not.

rehess Loc: South Bend, Indiana, USA

This is more an issue of computer literacy than anything else. Try various operations with a write-protected device, such as a CD or a memory stick with the write-protection 'flipped'. If you can do it, then the operation is not trying to change the original file. Read will work, because 'read' doesn't imply 'change', and the the only way to diminish the file is to write to it.

Bill_de Loc: US

Thank You!

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