Estimated Cost: 80€ and up
Copy, or reproduction lenses are and were made for this specific purpose: Projecting a flat surface such as a drawing, a map or, indeed, a piece of film. These lenses are also used in the darkroom on enlargers, to make darkroom prints in an enlarger, therefore the most available ones are often called enlarger lenses. Because they are designed for this specific purpose they have a set of features that are ideal for accurately digitising film, in terms of colour, focus, sharpness and distortion. Note that there are good enlarger lenses, which produce spectacularly good results, but there are also lesser lenses that produce decent but not spectacular results - though they can be an economical direction to go in under the right circumstances. There is a list of lenses and categories of lenses with their expected performance.
Fitting your lens to a camera
The main challenge with enlarger lenses is that they never come with a focusing mechanism (the helicoid that your turn when focusing a lens), and therefore require extra hardware to make them work for our purposes. There are two options for focusing: a screw-on focusing helicoid or (macro) bellows. In addition, you need the relevant adapters to adapt the lens to your camera system, and probably various adapters to adapt your lens to and from the helicoid or bellows. Most enlarger lenses have an M39 screw thread, but check the specific lens you get - it will be out there on the internet if you search for your lens and add ‘thread size’ behind. To find the right adapters for you make sure you consider all the mounts and threads on each component. As an example, here is how you would use an M39 enlarger lens on Sony E mount:
Sony a6300 with E-mount + Sony E-mount to M42 adapter + M42 (on both ends) Helicoid + M42 to M39 Adapter + Rodagon 50mm f/2.8 with M39 screw mount.
Potentially very high quality
A good enlarger lens will have a very flat field, be very sharp and be neutral in color rendering - they are made to not be in the way, or impose their own character on the image.
Good cost to quality ratio
Can be affordable with the right combination of adapters and helicoids and lens - the setup mentioned above will cost 100-120€. At the same, good enlarger lenses will rival or surpass the best macro lenses.
Bad enlarger lenses
While there are some really good enlarger lenses out there, there are also some that aren't good. They will be better than a normal lens adapted, but they won't be as good as a decent macro lens. These can, however, be very cheap, sometimes costing 10 or 20€.
Hard to adapt
It is no secret that these are quite hard to figure out - you need so many adapters and a focusing system! It's hard to figure out and you can get things wrong. Be very sure what you are buying and do your research properly.
Note that it might be easier to use an 80mm lens, than a 50mm lens particularly to a DSLR because the focal-flange distance of a DSLR will be in the way of focusing far enough away (even on a 35mm negative).
In order of priortiy
Rodenstock Rodagon Apo (all of them, 50mm, 75mm, 80mm) (expensive)
Schnieder Componar S 50mm f/2.8 (affordable)
Rodenstock Rodagon 50mm f/2.8 (affordable)
Nikon El-Nikkor 50mm f/2.8 (affordable)
80mm equivalents of the 3 above
These are in quite different price ranges. The non-APO lenses will be similar in performance to a Nikkor 55mm f/2.8 (see Vintage Macro).
Make sure you get the S version of the Schneider Componar, not the C version - it is a much inferior lens.