How Lightroom Works, Where to Store Files, and What to Back Up?

Lightroom is the main software for many photographers and hobbyists. Even if other software such as Photoshop are used, Lightroom is designed to be the main place for organising files, keywording, and basic editing, while other editors work as external editors from there.

While Lightroom is quite intuitive and easy to use, it works in very different way than other image editing software. Therefore, it can be a bit intimidating to start with and many give up before or because of not understanding how it actually works.

Lightroom (or LR here, referring to Adobe Lightroom CC Classic, not the new cloud version) does not work like other software where you just open a file, edit, and save. Instead, Lightroom has a catalog database where you import all photos so they are kind of always open. Edits are data stored in the catalog in real-time. When you need an actual JPEG photo, you need to export it from the catalog.

Common Lightroom questions I will try to give some answers to are:

How does Lightroom work? What is Catalog?
Where are my photos?
Where do I store my Lightroom files?
How to make it faster?
What should I backup?

When you understand how Lightroom works, you can answer the rest of the questions.

I do not consider myself as a Lightroom guru but I have been using it since version 2 and do know quite well how it works. I am not a professional so the amount of photos I have is not huge; however, these principles should be easy to scale up as well.

Also remember, Lightroom is not a replacement for Photoshop! They are often compared but have been designed to work together, that is why Adobe CC Photographer subscription has Lightroom and Photoshop. Especially for hobbyists Lightroom may be all that is required but if you want to do detailed editing and use layers, Photoshop is the way to go. Lightroom is still meant to be the main software where you jump to Photoshop from and come back when you are finished.

I also have Photoshop (although cannot use it well), Aurora HDR 2018, and Nik Collection, and all of those are used as external editors and plugins from Lightroom.

How Adobe Lightroom Works?

Image data in LR consists of following parts:

Originals (RAW, JPEG, TIFF, DNG)
Lightroom Catalog (database)
Previews
Smart Previews (optional)
Adobe Camera RAW (ACR) cache

Very briefly

Originals are only used as source data in the background and they are never modified
- RAW file is not even an image per se but data required to create an image
Catalog is a database containing all photo information: file location, all edits, keywords, etc.)
Previews are actual photos shown to the user, created from Originals using Catalog information
Smart Previews are smaller lossless versions of Originals
ACR Cache contains some form of partly processed Originals used for faster access

Next I try to open these a bit more.

LR’s main idea is to keep the Originals intact despite all editing while still remaining fast enough. Non-destructive editing that does not modify original files have been there since the beginning, while for example Smart Previews were added later to make LR faster and more flexible.

Edits are actually just instructions stored in Catalog along with Originals location, keywords, rating, and other metadata. Catalog is a database. When an image is shown, resized copy of the Original is created and all edits rendered on it as per Catalog instructions. On Export all edits are applied on a high-quality copy of the original image. Therefore, original files are not modified in the workflow.

Reading the original files and creating copies of them every time an image is edited or shown is slow. To ease the processing load, various forms of cache and previews are used. Previews are processed images of various sizes (e.g. screen-size and 1:1 full-size previews) which are loaded instead of reading and processing the large Originals every time. When edits are performed, new Previews are generated and shown to user.

Think of Previews the photos you actually see on screen and Originals as underlying data that is never shown.

Camera RAW cache is something between original photo and preview, some form of partly processed RAW file. Smart Previews also sit between Originals and Previews; when enabled they are used instead of original photos for generating Previews.

Smart Previews

Smart previews were introduced in Lightroom 5 and are not automatically generated in the background unless user chooses to do so. When adjustments are made and Smart Previews in use, LR renders new Preview using the Smart Preview instead of the original photo. This leads to one major difference: Originals are not needed when editing photos using Smart Previews. Only on Export original photos are required to export highest-quality files.

This means that Originals can be on external USB drive or NAS but photos are still available for editing even if the drive is not connected – very useful for laptop users. Only on Export the drive needs to be connected.

But even if Originals are available on LR (the drive connected all the time), there is a setting to use Smart Previews instead of Originals on editing. This can be used to boost performance if Originals are on slow drive.

Once More

This simple functional description is probably not 100% accurate but gives an idea how LR works. When a photo is opened in Develop module for editing and Smart Previews are enabled:

LR loads the Smart Preview of the photo as a starting point
Reads the edit instructions from Catalog
Applies the edits on (a copy of) the Smart Preview
Shows the photo and saves the Preview

If no further edits are made, next time the same photo is shown LR loads the already-generated Preview. This likely also happens in Library module when just viewing photos. Note that Originals are not accessed at all.

If Smart Previews are not used, large and heavy Originals need to be read and processed in the first step although Camera RAW Cache helps this in some way.

Enabling and Generating Smart Previews

I recommend using Smart Previews even if your original files are available all the time. For example, my Originals are on NAS which is connected to my desktop computer always but I use Smart Previews stored on SSD for better performance.

Choose to use Smart Previews as source files even if Originals are available

While normal previews are generated automatically as they are part of normal Lightroom funtionality, Smart Previews are not. When you take them in your workflow, it is best to create them on Import. Just tick “Build Smart Previews” box on Import, or save it in your import preset (I recommend using one). I also generate full-sized 1:1 on Import. It takes time to generate all these for larger batch of photos but I rather do it once and then enjoy smoother browsing of photos.

Select Smart Previews on Import

If you did not create Smart Previews on Import, it can be done later on Library module.

Generate Smart Previews and Previews on Library

This is all that is needed for using Smart Previews – have them generated and ensure Lightroom uses them on editing.

Where to Store Lightroom Files

In terms of performance, we can decide how to store all files after understanding how they are used.

In the example above, Smart Previews, Previews, and Catalog are accessed during normal workflow. As long as these files are stored on fast SSD, there should be no serious performance bottlenecks. Therefore, when using Smart Previews, large RAW files can be stored on slow but cheap HDDs, USB drive, or NAS, without sacrificing performance. And that is why we like Smart Previews even on desktop workstations!

If Smart Previews are not used, LR needs to access the original file in the first step, which becomes bottleneck if these are on slow drive. Camera RAW cache helps this in some way but I cannot find much information how it exactly works. Anyway, Camera RAW Cache should be stored on SSD as well although I am not sure if it is even used when using Smart Previews.

One file type not mentioned yet is Presets. Presets can be used for everything: import, copyright information, develop settings, export, etc. There is a box in Preferences to store presets with Catalog. Then the Catalog, Presets, Previews, Smart Previews are all stored in the same folder, making backing up easier.

Stores Presets in the same folder with Catalog

What Files to Back Up?

It should be clear at this point that in order not to lose any work, you must backup Catalog and Originals. You probably want to backup Presets as well but the rest – Previews, Smart Previews, and ACR Cache – are optional as these can be re-generated.

Lightroom Tips

Some things that came to my mind when writing this.

Use only one Catalog – unless you have A LOT of photos. One Catalog should work fine at least with tens of thousands of photos, even more. Obviously huge number of photos make Catalog and Previews folders big as well, in which case you may want to consider using multiple Catalogs at least if you need to move them around.
Do not move files outside Lightroom. Among other information Catalog stores location of Originals. If you move them outside LR, it loses track. You can move files but just do it on LR.
Do not copy Originals if you want to give someone a copy of your image (unless you deliberately want to give the Original without any modifications). Export is the way to do it as it combines Originals and Catalog data. You can export not only JPEGs but also TIFFs, DNGs, and even Catalogs.
Because of this Catalog-based operation you can go back in editing history. You can also create Snapshots of edits or create Virtual Copies to have multiple different edits of the same photo. Snapshots and Virtual Copies do not copy the Original, just the Catalog entry. The same goes for Collections – they are nothing but new entries in the Catalog.
The fact that Originals remain unmodified is extremely useful for backup. Incremental backup does not need to copy your large files even if you edit them as the edits are stored in the Catalog. This is very useful if using cloud backup.
- In Lightroom Catalog settings there is a tickbox “Automatically write changes into XMP” or on Library module you can choose “Metadata -> Save Metadata to File”. This saves Catalog data to the Original file itself. I never use this as I do not want to modify the Originals. You just need to take good care of your Catalog and its backups – see next tip.
Backup your Catalog often! Set Lightroom to ask for Catalog backup every time it closes (but tidy the backup folder occasionally as it becomes large). Remember, all your work is in the Catalog – which may get corrupted if LR crashes! Make sure you do not lose too much work if LR cannot recover the corrupted Catalog.

Conclusion

Not sure how good job I did on answering the original questions but here is a recap:

How does Lightroom work?
- Impossible to put in one sentence but it is vital to understand this in order to answer the rest of the questions.
Where are my photos?
- All over the place! Originals are where you put them and remain unmodified. All editing, keywords, and anything done on Lightroom are stored in Catalog as metadata instead. Therefore, these should not be separated. Final images (e.g. JPEG) are created on Export using Originals and Catalog data.
Where do I store my Lightroom files?
- See below.
How to make it faster?
- Use Smart Previews, then store your Catalog, Previews, and Smart Previews on fast SSD drive. Large Originals can be on HDD, USB drive, or NAS.
- Generate Smart Previews and 1:1 Previews for all photos before editing them to make browsing smoother (especially if you want to check photos 1:1 if they are sharp).
- Obviously having decent hardware makes it faster. I have written another post regarding this.
What should I backup?
- Catalog and Originals. You probably want to backup Presets as well.

Hope this helps! Feel free to drop a line if you have something to add or ask.

References and Further Reading