What is 50x magnification in mm?

Could you please clarify what exactly you mean by "50x magnification in mm"? Typically, magnification is a ratio that describes how much larger an object appears through a lens or microscope compared to its actual size. It's not usually expressed in millimeters, as millimeters are a unit of measurement for length, not magnification. If you're asking about the resolution or clarity of an image at 50x magnification, or the size of the field of view, perhaps it would be helpful to rephrase your question to better reflect what you're looking for. Alternatively, if you're trying to understand the relationship between magnification and the size of a physical object being viewed, it may be helpful to consider the magnification formula and how it relates to the size of the object and the size of the image.