Today’s Question: Regarding the advice [from the May 7th Ask Tim Grey eNewsletter] to maintain the JPEG format instead of converting to DNG when importing an image from a smartphone [into Lightroom], is there any other disadvantage other than increased file size? I’m concerned that with a JPEG photo that’s optimized (in Lightroom) and saved through several cycles may degrade due to the lossy nature of the JPEG format.
Tim’s Quick Answer: There is not a cumulative loss of quality caused by working with a JPEG image in Lightroom’s Develop module. The only cumulative degradation in image quality would occur if you exported the adjusted photo as a JPEG, since that exported JPEG copy would have JPEG compression applied to it as well.
More Detail: When you are adjusting an image in the Develop module in Lightroom, that work is non-destructive to the original image. What that means is that you are not altering the original pixel values in the source image file.
Cumulative degradation of image quality in a JPEG image only occurs when you make changes to the actual image data, saving the result as a JPEG image (even if you are simply saving to update the existing file. So, for example, if you apply adjustments to a JPEG image in Photoshop, close the image, then re-open the image, apply adjustments, and save again, the overall image quality would suffer. That is because the compression applied to the pixel data would be performed more than once, reducing overall image quality.
Even that cumulative loss of image quality would not be significant provided you were using a moderately high Quality setting for the JPEG image, and didn’t adjust and re-save the image a very large number of times. In other words, this isn’t an issue you really need to worry about too much in a typical workflow.
This is the reason I don’t really recommend converting an original JPEG capture to a different file format that would result in a larger file size. You aren’t getting a significant quality benefit for the JPEG image, even if you convert it to a higher bit-depth and save it in a file format that avoids lossy compression. There would be very tiny differences, but in most cases not enough benefit to justify the larger file size and slower workflow.
There is certainly an advantage to capturing more data in the first place, such as by opting for raw capture over JPEG capture. But once you have a JPEG capture, the benefit of converting to a different file format in general is rather modest. That is especially true in Lightroom, where your ongoing adjustments are not causing a cumulative loss of image quality.