You need to close the file and re-open it otherwise you're saving repeatedly from the in-memory data which is fine. When you close the file the in-memory image is gone and it reads the file on disk. You might want to add a 1, 2, 3 in a corner each time as you save the file out to keep track of where you are.

Hmmmm. Do I have to close Photoshop after each save too?
I closed,opened and resaved ten times, and I still don't see any significant difference.

If you save as "12" in photoshop it actually is pretty good. If you did the same experiment saving as "6", I bet you'd see a lot more - also perhaps something needs to change to really see a difference (like if you resized up and down - which would cause its own problems), or maybe have something over white and shift it a few pixels before each save.

Merely opening and saving something as a jpeg a few times is not going to ruin it. Still, if you are planning lots of re-edits, save it as a .png or tiff.

I don't think it's a real issue, then, for my purposes.
I'm lost at thinking of several significant, believable edits I would do to that pic (that wouldn't elicit, 'why on earth would you want to do that anyway?'); and my class will not be shooting isolations on white.

It's just for 'normal users' - nothing to do with stock.

Thanks, all.

Hmmmm. Do I have to close Photoshop after each save too?
I closed,opened and resaved ten times, and I still don't see any significant difference.

You can use "Save for web" and show them four different images immediately.

Unless I misunderstand what you are saying, you are opening file 1 every time and then save it as file 2, open 1 again save as 3, etc.?

If this is the case I doubt if any difference will show, as the "master file" is the same every time. You should open file 1, save as 2, close 1 and open 2 and save as file 3, close 2 and open file 3, etc.. If there is any deterioration, only then will I think it be visible.

Unless I misunderstand what you are saying, you are opening file 1 every time and then save it as file 2, open 1 again save as 3, etc.?

That's what I did the first time

If this is the case I doubt if any difference will show, as the "master file" is the same every time. You should open file 1, save as 2, close 1 and open 2 and save as file 3, close 2 and open file 3, etc.. If there is any deterioration, only then will I think it be visible.

That's what I did the second time.

I've decided that it's not that much of an issue to bring up in the first total beginners' course.

I'll throw another wrinkle into the mix.  Does the resolution of the original image enter into the equation?  This whole issue of re-saving JPEG images started way back with the earliest digital cameras.  Not uncommon for them to be 2, 3 or 4 mp.  Now we've got much higher resolution cameras with vastly more pixels.  If it's true that JPEG processes in blocks of 8 pixels, that would have a much greater effect on low resolution images as opposed to today's high resolution ones.

Thoughts?

I'll throw another wrinkle into the mix.  Does the resolution of the original image enter into the equation?  This whole issue of re-saving JPEG images started way back with the earliest digital cameras.  Not uncommon for them to be 2, 3 or 4 mp.  Now we've got much higher resolution cameras with vastly more pixels.  If it's true that JPEG processes in blocks of 8 pixels, that would have a much greater effect on low resolution images as opposed to today's high resolution ones.

Thoughts?

The example above is a full size crop from a portrait, and is a small image. But I was saving it at 12.

I guess I'll wait to see the composition of the class. The last class I did, on Windows Live Photo Gallery, had some people who were neither camera nor computer literate, and I spent a lot of time explaining what click, double-click etc mean.  This class Ill definitely have one lady like that, but another is a former colleague who dragged me kicking and screaming onto a BBC Master in 1996! I'm just preparing one class until I find out what they're hoping to get out of the course.

Thanks all; I'll know on Monday for sure, but I doubt if some artefacts visible at 300%+ will be of that much concern to my 'clientele', though of course this issue is important for stock.

Just getting a "beginner class" to understand the difference between raw, tif,  jpeg or eps will be tricky (all options they will run across if they will be using any photo editing software). They have to get to this point before it will make a difference. Don't even start with photo editing and when you do, stick to the basics.

Are these beginners taking the class to simply learn how to take a "good" photo? I guess you have to make your teaching decisions based on the students who take the class. Is it important for them to learn camera basics (which could be anything from how to turn their cameras on and when to use or not use the zoom, what their particular camera settings mean, info about aperture, dof, iso, rule of thirds and when to break that rule, etc)? Do they need to learn the dif between photographing objects and people?

One trick I've used is to get them all to take the same photo of the same object or outdoor scene  from 4 dif angles or in 4 dif types of light and then compare them to point out photo principles. But, these days no one likes to be criticized. Teaching is tricky, good luck! You will learn as much about human nature as you teach about photography.