Today’s Question: I know you don’t convert to DNG on import in your workflow, but for those of us that do I wonder if there are any advantages or disadvantages of also converting JPEG files (as from my phone) as well as RAW files?
Tim’s Quick Answer: I would say there aren’t any real advantages to converting a JPEG capture to the Adobe DNG file format, and there is certainly a disadvantage in the way of a file size that would be at least slightly larger for DNG as compared to the original JPEG.
More Detail: It would be reasonable to assume that converting a JPEG image to the DNG format would open up the possibility of applying “better” adjustments via Lightroom or Adobe Camera Raw, but that would not be an accurate assumption. You can already apply adjustments to JPEG images in Lightroom or Adobe Camera Raw, and there is no real benefit to converting a JPEG to DNG first.
In theory you might gain a slight advantage in terms of enabling a high-bit workflow and avoiding additional loss of quality and detail due to file compression. But in reality you are already starting with low-bit data, and with an appropriate workflow you don’t need to be worried with quality loss. Within Lightroom, for example, applying multiple adjustments in multiple stages does not have a cumulative negative impact on image quality.
While the DNG file format does employ lossless compression when converting RAW captures, yielding a DNG file that is generally around 20% smaller than the original RAW capture file, that does not hold true for JPEG images. Instead you will generally find that the DNG file is about the same size or possibly slightly larger depending on the settings for the original JPEG image.
So, on balance I would say there is no reason to convert JPEG images to the DNG file format. There are certainly reasons to consider DNG as a “replacement” file format for your original RAW captures (though as you note I am not a proponent of this approach), but not for JPEG images.