David Lawson

The first law of optics is: 1/OD + 1/ID = 1/FL . Therefore, if you take a lens at infinity focus, the Object Distance (OD), must be at infinity (generally, that's assumed to be 1000*FL of the optic), and the Image Distance (ID) is going to be 1*FL away from the center of the optic. As you get closer to the Object, the Image Distance increases, and thus, for a 50mm lens, if we focus at 5000 mm (5 meters), the ID is 50.505050505050... You can play with this formula at https://mathsolver.microsoft.com/ It'll open your eyes to a lot of reasons why that lens won't focus closer than 1/1000 *FL.