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To understand why, first we need to understand that Lightroom Classic uses a different type of preview in the Develop module than it does in the Library module.
Here’s how it works.
Previews in the Lightroom Classic Library module
When you import your images into Lightroom Classic you’re given the choice of building either Minimal, Embedded & Sidecar, Standard or 1:1 previews. These are used by the Library module, but not the Develop module, to display previews of the photos in your Catalog.
Note: I recommend you select Standard if you don’t want to zoom into your photos in the Library module, or 1:1 if you do. 1:1 previews take longer to build as they are larger and contain more data.
Above: Lightroom Classic uses Minimal, Embedded & Sidecar, Standard or 1:1 previews to display photos in the Library module.
Previews in the Lightroom Classic Develop module
Lightroom Classic builds a different preview in the Develop module based on the current state of your Raw file, taking into account the developing you have carried out on it. The preview is updated every time you do something new to the photo.
Above: Lightroom Classic builds previews in the Develop module as they are needed. Using DNG means Lightroom Classic can build previews faster, saving you time.
This is where converting your Raw files to DNG comes in. DNG (short for digital negative) is an open source Raw file format created by Adobe.
When you convert a file to DNG in Lightroom Classic it adds something called Fast Load Data to the file. This is a partially processed preview that lets Lightroom Classic render previews faster in the Develop module.
The speed savings are impressive. Adobe says a DNG file with fast load data will load up to eight times faster than one without it. The more cores your computer’s processor has, the greater the gains, as each core can build a different part of the preview at the same time. This isn’t possible with other Raw files.
Another benefit is that DNG files are smaller than other Raw formats, and take up to 20% less hard drive space. High volume shooters will appreciate this.
Converting files to DNG in Lightroom Classic
The easiest way to convert your Raw files to DNG is to do it when you import them into Lightroom Classic.
Make sure you go to the File Handling tab in Preferences and tick the Embed Fast Load Data box (below), otherwise you will miss out on the speed increase that DNG gives you.
You should also import your files from the camera’s memory card rather than copy them to an external hard drive before the import.
These are the key steps to follow during the import.
Select Copy as DNG in the Import window. This tells Lightroom to copy the Raw files from the memory card to the specified folder on your hard drive and convert them to DNG.
Set Build Previews to 1:1 or Standard in the File Handling panel.
After the import, go to the Catalog panel, select Previous Import, select all the photos in the Collection and go to Library > Validate DNG files. Lightroom checks the new DNG files to make sure they aren’t corrupted, and puts any corrupted files in a Temporary Collection in the Catalog panel. I’ve never seen a corrupt DNG file, so I don’t think it happens often.
This ability to check DNG files to see if they are corrupted is another benefit of the format.
What if my camera uses the DNG format?
Some Leica and Hasselblad cameras already save files in the DNG format. There’s also at least one Pentax camera that gives you the option of using either DNG or Pentax’s own Raw format (.PEF). Some iPhones and Android smartphones also use DNG.
This seems convenient, but bear in mind that camera generated DNG files don’t contain Fast Load Data. Only DNG files created by Adobe software do.
The case against using DNG in Lightroom Classic
The case for DNG is impressive but there are some situations when DNG might not be the right solution for you.
It extends the time required to import your photos into Lightroom Classic. If you’re in a hurry, you may choose to avoid using DNG to save time. Otherwise, get into the habit of importing photos when you have something else to do while your computer gets on with it.
Not all applications open DNG. If you ever use, or think you may use, software that doesn’t recognize the DNG format you should probably stick with your camera’s native Raw format. Examples include Canon’s DPP and Capture One Pro.
Smart Previews and DNG
If you really want Lightroom Classic’s Develop module to run as fast as possible then you should use Smart Previews. They are even faster than DNG (with one exception, which we’ll come to shortly). You may be tempted to use Smart Previews instead of DNG, especially as it saves time during the import.
The only thing with Smart Previews is that they are limited if you like to zoom in to 100% in the Develop module. For example, portrait retouching is a good example of where you might want to do this. Smart Previews are smaller (2540 pixels along the longest edge) and that restricts how far you can zoom into the Smart Preview itself. If you want to zoom into 1:1 Lightroom Classic needs to build a new preview. You need DNG to maximize Lightroom Classic’s speed at this magnification.
You can learn more about Smart Previews in our tutorial The Ultimate Guide to Using Smart Previews in Lightroom
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