When I get my film processed, I order CDs as well as prints. The files on the CD are jpegs which I then edit. I understand from various sources that the jpeg format loses data.
That is correct. The extent of loss / how much compression is applied, can be set in many programs. I've found juggling the compression level more or less can find an acceptable compromise of file size and image output quality. This can often be a long way from the common default of 85% compression.
Should I save the jpegs as tiffs or some other file format? If so, at what point should I save in another format, immediately, before editing? After editing?
Once the image has been loaded into a program, then there are no "accidental" losses of data. It's when the image is _saved_ as a JPG that the losses occur, in the saved file. If you keep editing, the data is still complete (as originally loaded) in the program.
I find it useful to load the image, and immediately do a "Save As..." with a different name specified, so I don't lose the original image. A useful format for me is PNG, which is not "lossy", so the one file can be repeatedly opened, edited and saved without deterioration. Thus, if I start from a JPG, I'll immediately open it, save as PNG, and then work from there. (Of course, opening the _original_ JPG, which has not been saved over-the-top will still have all the information it ever contained.)
The Pentax Photo Laboratory will only save JPG (five levels of compression) or TIFF (8- or 16-bit), so I save my PEF (Pentax RAW) as TIFF, then open the TIFF in The GIMP, and save as PNG. (Then delete the TIFF, since they are _much_ larger than PNG, with no extra information.) Alternatively, I'll open the PEF in IrfanView Thumbnails and save directly as PNG.
&i (:
