Today’s Question: I built Smart Previews for some images in Lightroom Classic so I could work with them without having my external hard drive connected. I am able to edit the images in the Develop module, but the option to use any external editor (including Photoshop) is greyed out. I was under the impression that smart previews allow for all editing, but is it just limited to editing in Lightroom Classic only?
Tim’s Quick Answer: Smart Previews in Lightroom Classic enable you to edit photos in the Develop module (or export a lower resolution copy of the image) even if the source image is not available, such as when an external hard drive is disconnected. However, you are not able to send a smart preview from Lightroom Classic to an external editor, as the original image is required for that purpose.
More Detail: When you built a Smart Preview for a photo an Adobe DNG (Digital Negative) file is created at a reduced resolution of about 2500 pixels on the long side. The Smart Previews are stored alongside the catalog, so that they can be accessed even if the source image files are not available, such as if you have disconnected the external hard drive containing the source images.
Smart Previews can be used in place of the original image in the Develop module within Lightroom Classic, so you can apply adjustments even without the source image. When the source image is available again the adjustment updates will be synchronized to the original automatically.
You can also export copies of images based on Smart Previews, though in general I recommend exporting based on the original file instead. Among other things, the Smart Preview will generally be of a lower resolution than the original.
However, you are not able to send a Smart Preview to an external editor from Lightroom Classic using the Photo > Edit In option on the menu. That is because these options require a derivative image (such as a TIFF or PSD file in the case of Photoshop) based on the original image file. You therefore need to make sure the source image file is available when you want to use an external editor.