How to use Lightroom Smart Previews

Lightroom 5 brought a whole bunch of exciting enhancements, but they weren’t all headline-grabbing editing tools – Adobe also introduced Smart Previews, which could be the most important advance of all for those of use with image collections too large to store on an internal drive.

I do my day-to-day work on a laptop with a 256Gb SDD. It’s fast and power-efficient, but my image library has approximately 50,000 images and takes up 500Gb. I’d need a laptop with a 1Tb drive to cope with that and all my system files, apps and documents, and how long would it be before this too was full up?

So I store my Lightroom catalog on my laptop’s internal drive and my image library on an external 1Tb portable drive. I can carry out browsing, sorting, organising and tagging tasks without the external drive connected, but I need it plugged in to do any image-editing.

Until now, that is. Lightroom Smart Previews are a fraction of the size of the original files, but you can edit them even when the original isn’t present. Adobe says 500 RAW images from a high-end D-SLR might take up 14Gb of hard disk space, but their Smart Previews will take up just 400Mb. This means you could store an entire, ‘editable’ catalog on a low-capacity internal drive.

There are some limitations, as we’ll see, but on the whole this feature is really useful and really easy to use. I’m going to show how it works on this sample image (above), which has got some serious colour and exposure issues. Can I really fix it without the original high-res file?

01 Building Smart Previews

You can get Lightroom to create smart previews automatically when you import images, but most of us will have libraries that predate Lightroom 5, so it’s more likely you’ll be building smart previews for images already in your library.

It’s simply to do. First, select an image or images, then use the Library > Previews > Build Smart Previews command.

02 One or all?

Even if you’ve just selected a single image, Lightroom will ask you if you want to build smart previews for all of them. I’m going to stick to one image because I can then show the differences between smart previews and the ordinary sort.

03 Spot the difference

At first it looks like nothing much has happened. But if you select an image thumbnail and check the information under the histogram in the right sidebar, you’ll see there’s a difference.

Lightroom uses this area to display information about image previews stored in the library. For the image that has the smart preview (left), it displays ‘Original photo + Smart Preview’. With a regular image (right), Lightroom displays ‘Original photo’.

This makes no practical difference for as long as you have your external drive connected, but if you disconnect it, things change…

04 Disconnected drive

Now if you check the thumbnails in grid view, you’ll see that the image with the smart preview (left) has a preview icon in the top right corner, so it’s still editable. The regular image (right), on the other hand, simply displays a question mark, showing the original image is disconnected and you can’t edit the picture.

You’ll see a difference under the histogram, too. For the image with the smart preview (left), Lightroom displays ‘Smart Preview’. For the regular image (right), it simply says ‘Photo is missing’.

05 Basic adjustments

I can still edit the image with the smart preview, even though the original file is not currently available. I’m using the Basic tab to fix this picture’s colour and exposure problems.

06 Zooming in

But if I switch to the spot healing brush tool to smooth over a wrinkle, there’s a difference. My smart preview is only 2560 pixels across, so when you zoom in to 100% view, you don’t get the same degree of magnification. (It’s worse here because I’m using a Retina display on a Mac, with twice the resolution of a regular monitor.)

07 Small adjustments

I’m using the adjustment brush here to enhance my subject’s eyes, but it’s made more difficult by the reduced size of the smart preview. Global adjustments are fine, then, but pinpoint editing like this can be trickier.

08 Reconnecting the drive

If I plug the external drive back in and wait a few moments for Lightroom to work out that it’s reconnected, you’ll find your smart preview edits transfer automatically to the full-res image. You can also zoom properly now, which is important for judging sharpening and noise reduction properly.

09 The finished image

The result’s no different to editing the image in the normal way, but the point is that thanks to smart previews I could do it without having the full-res file available. In fact, you can even export a finished image without the full-res version, from the smart preview alone. The resolution is lower – mine came out at 2560 pixels wide – but more than adequate for web or screen use and even printing at small to medium sizes.