Originally posted by acarlay Adam you can edit a 645z jpg with Pentax's desktop software and have all the features available to you.. it doesn't only apply to raw files.
This is quite true, almost any processing software (I use Lightroom for example) will handle RAW or jpeg without any issues, often without you even noticing what format you are using. But as already noted the jpeg has already been derived from the RAW in camera and a lot of info was tossed out. So first you can develop the jpeg and you have all the tools, but you no longer have all the data so your range of latitude for developing is reduced. And second if you develop a jpeg and re-save you toss out even more info.
In practical use, this is not a big issue. If you develop a jpeg and save you lose some info but not a lot. Where it gets bad is if you edit and save multiple times, then it gets ugly. But if you are making your edits, saving and done then sure it's no big deal.
Working from the RAW will always be a better workflow though, IMHO. I never touch a jpeg. Basically I consider a jpeg exactly the same as a print, you don't edit prints, you don't edit jpegs.