What moves and what doesn’t?
I wasn’t being pedantic on this but it seems like it’s six of one and half a dozen of the other.
My experience was with a Nikon 9000ED but I’m happy to accept that it may be different in other scanners.
I sold the 9000ED last year (for more than I paid for it) because with the advent of high resolution digital cameras, there are clear advantages in speed and convenience with no downside as regards quality.
I am well aware of the closeuphotography.com website and use it as a valuable resource. Another is coinimaging.com with lots of tests of all kinds of macro lenses.
Your view of the Sigma Macro 70mm F2.8 EX is incorrect in the wider scheme of things. Yes, it’s a good macro lens but not a patch on the Sigma Art 70mm f/2.8 DG Macro which in turn, is not a patch on almost any scanner lens. These, like the Nikkor Scanner 100mm ED and Printing Nikkor 105mm lens and of course the minuscule Dimage 5400 are simply in a different league. General purpose macro lenses are not designed to do what scanner lenses do and so don’t do it as well. That said, scanner lenses have a very narrow field of application and are pretty much useless at doing anything other than high resolution, flat field photography - which is why they excel at photographing film. Whether it matters or whether “you can notice a difference at regular viewing” is a moot point. It all depends on what standard you work to. Some are happy with the results from a reversed zoom lens while for others nothing but a Printing Nikkor will do.
DSLR scanning doesn’t have to be limited to a diffused light source and it baffles me why it’s so popular given the increased risk of vibration and unwanted colour casts. For me, flash with diffusion dialled in to taste solves all those difficulties. Diffused light sources by the way, don’t make softer images. A sharp original is sharp whatever the light source. Diffused light sources lower the contrast or perceived sharpness (acutance), not the resolution. Perceived sharpness is readily tweaked in post process.
Film flatness is always a dilemma. Glass carriers solve that problem but introduce added difficulties with dust and reduced image quality. Slide mounts are almost universally unsatisfactory as they fail to hold the film flat especially if using a warm light source. Most complaints about unsharpness at the edges of an image are due to this more than any other cause. The conventional advice about stopping down the lens to gain more depth of field is misguided because what is out of focus at F4 is still out of focus at F16 - just less obviously. With film there is only one plane of sharp focus.
There is only one slide mount which fulfils the brief for film flatness - the now rather rare WESS AHX500K and the MF equivalent. These have little pegs top and bottom which accommodate the film sprocket holes. When closed together the film is put under tension achieving almost perfect flatness. When gently tapped with a finger, the film ‘sings’ like a drum skin and will not ‘pop’ or distort even in a hot projector.
Retouching is always a pain whether it’s dust on the film or a dirty sensor. For dust on film digital ICE is one answer but also has an effect on sharpness. You can’t beat getting the film spotless in the first instance.