If I Import a Photo, Edit The Metadata, Then Export Will I Degrade The File?

I've never really understood compression but think I recall talk of opening and closing jpegs can be like taking a photocopy of a photocopy of a photocopy ...

if you do anything, that will require the JPEG file to be compressed again, you may lose a bit of detail when the new ly compressed file is rendered. it will depend on the setting for the JPEG quality, when you export the file.

If you export a JPEG with "JPEG quality set to "Maximum", the details will be preserved. The file size may even increase.

Exporting from Photos with "File > Export > Export unmodified original" will not render the file again - you will export an exact copy of the original, but you cannot embed any changed metadata. You can only add them as a sidecar file.

yes - the unmodified original is just what it says - the exact unmodified original that you imported so no changes you make in photos are made to the unmodified original - to export the changes you have to export the current version

And as a side note yes each JPEG compression loses some information but unless you choose minimum JPEG quality it is not noticeable and there is no need to worry about it - If you use an old fashioned editor that saves a new JPEG with new compression with every edit then the cumulative effect of many edits can be a problem which is why Photos lossless editing is an important feature - no matter how much you edit you only have one recompression when you export -- generally Maximum quality is not worth the increase in size (although that is a personal decision - if you want no compression use the TIFF format for photos so there never is any compression but they have giant file sizes)

LN

rather than"

file / export / export unmodified original

which will not have the edited date / location stuff.

Yes, "export unmodified original" will be exactly the file you originally imported. if you add the checkmark "Export IPTC as XPM" you will get a XPM sidecar file with the metadata in XPM format along with the exported original. As lang as this sidecar is in the same folder as the original image file, Photos will reapply the changed metadata, when you import the photo back to Photos. Other photo editors may be able to read this file as well.

When I save the originals as a backup on an external drive, I export them with such a sidecar file to save the changed dates , locations, keywords, titles.

If you export the edited versions instead, the metadata will be embedded in the exported files.