300mm is 11.811 inches.
300mm is 9.72 x 10-18 parsecs.
450mm x 300mm 45cm x30cm 1350cm
A 300mm lens is a telephoto lens, which will magnify the subject. It used to be that a 300mm lens was actually 300mm long, but with the use of compound lenses, they can be shorter, but have a "virtual" length of 300mm. On a 35mm camera, about 50mm is the "natural" lens size -- most cameras come with a 50mm lens. You can divide 300mm by 50mm to find out how much it magnifies from the "natural" position. 300/50 = 6, so it will magnify 6 times.
30cm = 300mm
About 300mm a year