The EF 50mm f/1.8 STM is an upgrade of the old standard (and much-loved) EF 50mm 1:1.8 II. The optics are said to be very similar, but with the new lens there is a clear improvement in centre sharpness at f/1.8. Focusing is also faster, smoother and quieter thanks to the Stepper Motor (STM). Incidentally, the STM provides much-improved video focus tracking. The STM lens is heavier and more solid in feel than the old one and has a metal lens mount as opposed to plastic. The leaf photograph attached shows the fantastic DOF (Depth of Field) control possible with f/1.8. The second shows the amazing resolving power at f/8. This is a lens no Canon shooter's camera bag should be without! Also, it is worth noting that on APS-C bodies, the angle of view will be equal to that of an 80mm lens, making it just perfect for beautifully controlled portraits and studio work. An added plus, of course, is the cost - I don't know of any other lens that delivers such great performance at such a low price. Highly recommended for enthusiasts and professionals alike. Just get one - you'll never regret it. (Professionals who find it hard to accept this statement should test the MTF of the f/1.4 50mm @ 1.8 with this one's MTF wide open (1.8). The results are surprising !