Lightroom: how to not lose quality by exporting a photo from a previously exported photo collection?

Question

I'm thinking to use Lightroom in order to organize catalogues by year (to avoid having large areas of files that I normally have in the cloud and download locally to work them).

Having catalogs per year precludes me from creating trans-year collections. Therefore the idea could be to work with collections per year and then on the folders of the exported items per year create a super catalog (of the best photos) from which to create trans-year collections.

The question is: is there any way to do not lose quality by exporting a photo from a previously exported photo collection?

Answer 1

TL;DR: Unless you're shooting 25k+ shots per year, skip all this nonsense and just use one monolithic catalog. There used to be a performance benefit to fragmenting catalogs like this, but the advantages are small in modern LR. Otherwise, see below.

In your proposed methodology, you've correctly deduced that you will lose quality due to applying multiple sequential .jpeg compression operations. Importantly, I think you'll still encounter repeat .jpeg compression even if you export using the Original option for file format - how else would Lightroom apply any adjustments to the file? It may be smart enough to export an unmodified copy if there are no adjustments, but I wouldn't count on that. You could check this by comparing the md5 hash of the twice-exported .jpeg vs. the once-exported .jpeg.

You could avoid this by manually copying the .jpeg from its location on disk to your desired export location, but there's a much better way. What you actually want to do instead is to export a catalog of the selects from each year, rather than exporting photos themselves. See "Export a Catalog" here. You'll want to uncheck the option to export the negative files and previews. Lightroom catalogs can embed the negative files themselves so that you can send one monolithic file, for example to a postprocessing service. In your case, assuming the multi-year catalog exists on the same machine as the individual-year catalogs, you won't need to embed the negative files.

Once you've exported a catalog of selects from each year, you'll want to create your "super catalog" as a new empty catalog and open it. Finally, import each catalog of selects that you exported earlier (see "How do I merge two catalogs or import folders from another catalog?" here). You should now have a catalog of just your selects, and it should already be properly linked against the original raws (run Lightroom's "Check for missing files" to verify).

Answer 2

Preamble:

As stated in Lightroom Classic catalog FAQ "A catalog is a database that tracks the location of your photos and information about them. When you edit photos, rate them, add keywords to them, or do anything to photos in Lightroom Classic - all those changes are stored in the catalog(1). The files themselves are not touched."

A) A catalog can embedded information on images in different locations.

B) Lightroom can move files from one location to another one

C) Under Lightroom the preferred organization of images is done by collection, these are collected logical links to images. Operations on collection do not affect the location or the physical files.

D) In Lightroom, exportation can be either:

1) Exportation as new files: This "burns" new files from the image file and the processing made. The new files are no more related to the original ones, except by name. The new files as created where it is stated during the export process.

This type of export can alter the quality of the image depending of the option chosen for the type of file , its size, and of course the processing already made.

This kind of export is very similar to publishing.

2) Exportation as new catalog: This creates a new catalog embedding the files locations and they modifications. The catalogs are no more synchronized (2)

This does not create new image files unless the "Export negative files"(3) option is checked. Beware that this option seems to be set by default in the export as catalog dialog box.

This cannot modify the quality of an image

Answer to the question

The question seems to mix different concepts "exportation" and "files locations", unless it was implicit that the "Export negative files" option was set.

The move/copy from the cloud to local location is not an export, but can be a operation on the files.

When the files will be on location, you can as indicated above have multi catalogs referring to them. These does not affect the quality of the files as they will be not touched by the duplication of the catalogs.

The catalogs will be unrelated so will need to be very conservative on operation affecting the physical files (moves or any other operation that modify the physical the files).

Unless use of sidecars, processing in one catalog will be independent of processing made in the other one.

But beware, having the same file in two (or more) catalogs is risky.

(1): hpchavaz: Or in XMP side-cars files if the option is set

(2): But if the processing information are embedded in XMP side-cars files, two catalogs can sort of share information on the image processing information

(3) "negative files" means "physical files"