Page updated: 10 July 2026
How Lightroom and Photoshop Store Edits: A Clear, Practical Explanation
Why Lightroom and Photoshop Handle Edits So Differently
Lightroom and Photoshop can produce identical-looking results, but the way they store those edits could not be more different. Lightroom records your adjustments as lightweight text instructions, while Photoshop saves every change as pixel data inside the file itself. This fundamental difference affects everything from file size, to performance, to exporting a resized file, and how easily you can revisit or undo your work.
Lightroom behaves differently depending on the type of file you edit. With proprietary RAW files from any manufacturer — Canon (CR2/CR3), Nikon (NEF/NRW), Sony (ARW), Fujifilm (RAF), Olympus (ORF), Panasonic (RW2), Pentax (PEF), Leica (RWL) and others — Lightroom cannot write metadata inside the RAW file. Instead, it creates a "sidecar" file to store your edits. Traditionally this is an .xmp file, but newer AI-based masks may also generate an .acr sidecar file to hold the mask data. The RAW file itself is never modified.
DNG files (digital negative) behave differently. Because DNG is an Adobe-designed container format, Lightroom can embed all edits directly inside the DNG itself. For basic adjustments, no sidecar file is created at all. Only when a DNG contains complex AI masks or large segmentation data, Lightroom may generate an accompanying .acr file. This is normal behaviour and not an error.
In this article, I break down exactly how each program stores edits — including XMP and ACR sidecar files, embedded DNG metadata, layers, masks, and AI segmentation maps — so you can understand why Lightroom edits are usually tiny additions to file size, while Photoshop files can grow dramatically, and when each approach makes the most sense for your workflow.
How Lightroom Stores Edits
Lightroom uses a non-destructive, instruction-based editing model. Instead of modifying the pixels in your RAW file, Lightroom saves your adjustments as lightweight text instructions. These instructions tell Lightroom how to render the image, but the underlying RAW data is never changed.
This is why Lightroom edits add so little to your overall storage — the program is storing instructions, not pixels.
XMP and ACR Sidecar Files
Lightroom Classic stores your edits in one of two places:
- .xmp — a small text file containing slider values, metadata, and mask definitions
- .acr — a binary file created when Lightroom needs to store AI segmentation data
The XMP file is human-readable and contains simple instructions such as exposure, contrast, sharpening, and white balance. The ACR file is only created when you use AI masking tools like Select Sky, Select Subject, or Select Background.
Even with multiple masks, these files usually total less than 1 MB — which is why Lightroom adds so little to your storage footprint.
AI Masks and Segmentation Maps
Lightroom’s AI masking system works by generating a segmentation map — a lightweight data file that identifies areas such as sky, subject, background, people, and clothing. Lightroom reuses this segmentation map for multiple masks, which keeps the file size small even when you duplicate or invert masks.
This is why a heavily masked image in Lightroom might only add 500 KB to 1 MB of extra data, regardless of how many masks you create.
How Photoshop Stores Edits
Photoshop uses a pixel-based editing model. Every adjustment layer, mask, smart object, and pixel edit becomes part of the PSD or TIFF file. Unlike Lightroom, Photoshop cannot rely on simple text instructions — it must store the actual pixel data required to rebuild the image.
This is powerful, but it comes at the cost of dramatically larger files.
Layers, Masks, and Pixel Data
Each layer in Photoshop contains pixel information. Even if a layer appears empty, Photoshop still allocates space for it. Masks also store pixel data — a full-resolution grayscale image for every mask you create.
- More layers = larger file size
- More masks = larger file size
- Smart Objects = dramatically larger file size
- Embedded RAW files = extremely large file size
This is why a simple edit in Photoshop can produce a 100–300 MB PSD, while the same edit in Lightroom might add only 0.5–1 MB.
Lightroom vs Photoshop: Why File Sizes Differ So Much
Lightroom and Photoshop both produce professional results, but the way they store edits is fundamentally different. Lightroom saves instructions, while Photoshop saves pixels. This single difference explains almost every file‑size discrepancy between the two applications.
Here’s a simple breakdown:
- Lightroom stores text-based instructions (XMP) and lightweight AI data (ACR).
- Photoshop stores full-resolution pixel layers, masks, and smart objects.
- Lightroom adds kilobytes; Photoshop adds megabytes — sometimes hundreds.
- Lightroom edits are non-destructive; Photoshop edits are pixel-destructive unless layered.
Understanding this difference helps you choose the right tool for the job — and helps explain why a simple Photoshop edit can balloon into a 200 MB PSD while Lightroom barely adds 1 MB.
