I understand that the magnification of the specimen onto the sensor/film is the result of the ratio between the distance from the lens to the sensor divided by the distance between the specimen and the lens.
But as the above are reached, one needs to alter one or both to enable focus and that depends on the focal length of the lens in use.
What is the effect of the lens' focal length in these calculations?
To put it in another way: If I want to have 5:1 magnification ratio, I need to have 1cm distance from the specimen and 5cm distance from the sensor. This doesn't work as simple as that. These figures may not give a focused image. So these need to be adjusted, keeping the 5:1 ratio, until focus is achieved. Perhaps at 1.5cm distance from the specimen and 7.5cm distance to the sensor. I suspect this depends on the focal length of the lens, but haven't found how the lens affects this so I can decide what's the right focal length to achieve the desired magnification when other factors need to be observed (e.g. a given bellows length with the slide copier attachment mounted at the one end of the bellows.