If you shoot in JPG and import your photos into Lightroom, you might wonder whether any changes you make in Lightroom degrade image quality.
Under normal circumstances, whenever you open and then save a JPG image it degrades the quality of the image. JPG uses what's known as lossy compression. To keep the file size small it discards information while using sophisticated algorithms to try to make sure that discarding information has minimal visible impact on quality.
But whenever you open and then save a JPG, the problem is compounded. Because what you're doing in that case is further compressing an already-compressed file. Each time you save a JPG you're discarding more information. So if you're using something like Photoshop or Gimp or some other image editor to edit your photos, it's a good idea to work on copies, not the original image, and to minimize the number of times you open, edit, and save the file.1
That said, all is not necessarily lost of it do resave JPGs. Sometimes the result can be quite subtle. Here's an example where I've opened and then saved the same JPG file, repeating the process 10 times. The resulting file is 10 generations removed from the original. For each, I've used a JPG compression setting of 60 and haven't made any other changes at all. The second image definitely is degraded--you can see some banding around the helicopter and Washington Monument, and there are extra lens flare spots in the sky on the second as the gradations get flattened. But for much of the image, the difference is barely noticeable.
So you might be wondering... doesn't that mean that editing in Lightroom degrades the image? And the answer is no.
Lightroom does things differently. It uses what is known as non-destructive editing. When you use the develop module in Lightroom to edit a photo you're not actually saving over the original file. Instead, Lightroom stores the instructions for the changes you've made and then uses those instructions when it displays the image in Lightroom or when you export the file.
That workflow is specifically designed so that it doesn't touch the original file no matter what changes you make within Lightroom itself. The analogy is if you're using film, you don't cut up or edit the original negatives--you always leave the original negative as the master. Lightroom treats your original images, whether they're RAW, JPG, or TIFF, the same way.
So a normal workflow for editing JPGs in Lightroom might look something like this:
- Import the photos.
- Add metadata in the Library module (caption, title, keywords, location, etc)
- Process the photos in the Develop module (exposure, color balance, contrast, etc).
- Export the photo or use the Print module to print the file.
Throughout all of those steps, your original images aren't being edited and remain completely intact. If there's any image degradation being introduced, it's only when you get to step 4 when you export the file out of Lightroom. If you export to a JPG file, you'll have a round of compression applied then. But no matter how many times you fiddle with steps 2 and 3, the resulting file will still only be a second generation JPG.
But there are some qualifications.
The first is that when you export a JPG from Lightroom to JPG format, there's inevitably some degradation. But it's only one generation of saving, and you can control how much compression to apply to the exported file.
The second is that if you edit a file outside of Lightroom and save it back into Lightroom as a JPG, you've broken the non-destructive workflow. That applies also to round-trip editing when you might use Lightroom's Edit in Photoshop external editor. If you're concerned about it and need to edit the file externally, a good option is to have the external editor use a 16-bit TIFF file. The TIFF will be much larger in file size than the JPG, but it's the best way to preserve the quality if you're going to edit the file outside of Lightroom.
- There are exceptions, but they're limited. There is some software that can edit only the metadata block of a JPG file and can resave the file without affecting the section of the file with the visual image data. Other software, like IrfanView, can do certain lossless transformations to JPG like rotation. In general, though, the rule stands in nearly all cases that resaving a JPG degrades image quality. ↩
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