As for magnification of the subject, 50mm is generally considered 1:1, or same size about like the eye sees.  With a 1.5 crop, remember 50mm crops the image as a 75 mm field of view but the size is the same.

Zoom factors are not the same as magnification.  300mm  magnification is approximately 300/50= 6x magnification(like with 6x binoculars).  Too often lenses are sold(mostly in the non-dslr cams with one lens) with a "zoom factor", like somethng of 10x zoom factor.       If the lens goes from 18mm to 180mm that is a 10x zoom but is not a 10x magnification.  The magnification of the 180 would be about 3.6x, as with binoculars specs.

If I missed it here, someone please correct my blunder.       TomDart.