Over the 2+ decades that I have been fooling around, in my spare time, with scanning the huge collection of negatives and what I think of as ‘slides’ (a most inaccurate word with which to name what you call ‘diapositive’), I have had just a few (say 0.5%) results from scanning (on either Nikon Coolscan or Epson V.700) which have been most pleasing to me. This suggests that the basic hardware - especially the Nikon device - is capable of producing a good scan. The really overwhelming factors which prevent this hardware configuration from consistently producing acceptable results are:
1). the skill of the original photographer in getting a properly exposed ‘slide’, That was almost always me, meaning that only the minority of the slides were good to start with - 60 years ago I knew even less than I know now.
2). the degree of care exercised by the developing lab - which has been unknown, obviously variable, and out of my control
3). the choice of scanner settings - which we have agreed is pretty important and the subject of an equally important lack of guidance.
4). the settings of the final (raw) image processing software (now darktable).
None of these will be dramatically improved, in my opinion, by my making a further investment in getting a ‘DSLR’ scanning set-up, such as you describe. I find it difficult to believe that anything I put together will be superior to my Nikon Coolscan V ED. By the way, does it have to be a ‘DSLR’? Any reason why my Fuji mirrorless camera wouldn’t work?
Specifically what information do you have from your ‘DSLR-scanning’ equipment which you didn’t have when using a ‘professional service’ and how does that help you to adjust your settings in darktable? For example, although it is available (I think), I have never been aware of the need to know what the colour temperatures of the scan LEDs in my scanners are. Even if I did know it, where would I use it ?
I asked about the advice on the ColorPefect web-site not because I wish to use PS or a plug-in to PS, but because of the suggestions the web-site owner makes on getting good scans from Vuescan before they are processed by ColorPerfect. I was wondering if those Vuescan recommendations would assist in darktable.
As a rule I prefer to work in Linux too; the only barrier to my working totally that way is that Nikon Scan (withdrawn by Nikon at least 10 years ago) is, obviously, a Windows-only application. It is, however, functionally equivalent, better documented and so easier to use than Vuescan, under either Linux or Windows.