My question actually ranges a bit farther than that. Scenario: I use IrfanView for most photo viewing and manipulation, especially the simple stuff (cropping, reducing size for mailing, etc.). Photoshop is great for publication work, but overkill for E-mail. Most of my photos are taken with a Nikon D600 giving .jpg images 6016 x 4016 pixels. Today I migrated several thousand image files from my Win 8.0 laptop to my Win 7 Pro desktop. I shoot, depending on subject matter, a mix of vertical format and horizontal format images. When viewing those photos on the Win 8 laptop, IrfanView automatically displays them correctly (vertical format vertically, horizontal format horizontally). When I move the same files over to the Win 7 Pro desktop, the vertical format ones are displayed horizontally. File size typically 13.3 MB. If I rotate such a picture so it appears in the correct vertical format, the file size changes to 12.4 MB - regardless of whether I do that in Windows Picture Viewer or in IrfanView. So the same photo is displayed correctly in vertical format on the laptop, file size 13.3 MB, but on the desktop it appears horizontally and if I rotate it into the vertical position the file size shrinks to 12.4 MB.
So what is happening here, am I really losing information (given that JPEG is a "lossy" compression algorithm) and why does IrfanView autocorrect the display format on the Win 8.0 laptop but not on the Win 7 Pro desktop?
I started off as a Leica photographer 68 years ago (at the age of 15) so I am a bit of a paranoid fanatic about image quality. Digital is of course a different game, and especially it lets you collect thousands of images in a short span of time, Shooting film in a 35mm SLR you are very conscious of 36 exposures and then having to reload, so your approach is quite different . . . and if you are shooting for instance wildlife or hunting action there's a lot to be said for the digital capabilities - though I have some amazing aerial falcon action shots taken long ago on 35mm Kodachrome, no less. But even then fast shooting was relevant, I had 4 motor driven Nikons slung around me, each with a different length lens (changing lenses takes WAY too long, especially when you are bucketing across the desert at 80 mph), and at least 500 rolls of Kodachrome in the freezer at all times <G>. And fairly often I had to send the Nikons in to the factory to have the dings and dents taken out . . . which they did with amazing efficiency. Very quick, and what you got back looked like a brand new camera - but with the same serial number as before <G>.
The images on the laptop were downloaded directly from the camera's memory card. If I were really after the ultimate niceties of color tone, etc., I would be shooting RAW instead of JPEG, but the trouble with that is that RAW image files are much bigger and so fill up the camera's buffer much sooner. That limits the burst length of rapid-fire shooting, and burst length is of the essence when you are shooting high speed action. Can't have everything <G>.
Right now I am concerned with getting the images over to the desktop with no loss of information, and with finding out if the rotated ones have in fact lost information - and if so, how to avoid that loss while transferring the images and viewing them on the desktop.
As always, thanks for enlightenment.