I own both the 1.7mm and 1.4mm versions of this lens. I bought the 1.7mm first and then saw a brand new 1.4mm in John Lewis at a price less than the then current secondhand value. They are both really good lenses, the 1.4mm is slightly sharper but focuses slower and the non reversible lens hood is a pain. The 1.7mm focuses really quickly and is great for shots of my dog and for my travels. Overall, I'd say that the extra cost of the 1.4mm isn't really justified for the relatively small increase in image quality and at £99, the 1.7mm is a steal.