schachtb wrote:
I can see that the JPG file timestamp changes after a LR edit. However, the edits appear only visible from within LR. Viewing the file from Finder doesn't reveal any of the LR edits. Can anyone shed light on how that could be?
The editing parameters are written (though, they do not have to be) as a list of metadata fields - a recipe. This recipe is executed when the JPG is opened into a program which both (a) understands them - since they are proprietary to Adobe - and (b) has been told that it should pay attention to them. For example. Photoshop can be set either to ignore such information when it opens a JPG - and just display the JPG normally - or else, to take the JPG as it would normally appear, and carry out the recipe onto that, on-the-fly. Lightroom always does this, if such a thing is present.
FInder is an example of a program which simply does not see these edits - they are meaningless to it.
Each image file acts as a container, with the actual image data as the contents. Around this is a wrapper, mostly consisting of a file header, plus some stuff at the end to "close" the file neatly. The file header contains routine file stuff such as the modification date etc, standard image related stuff such as the copyright / camera model / aperture / pixel dimensions / colourspace etc, plus (if this has been written also) proprietary metadata which in this case, is meaningful only to Lightroom, to ACR, or to other programs which are emulating those.
Anything that is not understood is just ignored. A lot of the "standard image related stuff" mentioned above, will be accessible as extended properties of the JPG, using Finder or other viewers, or presented automatically if you upload the image to e.g Flickr, or some such.