This is quite frankly the most remarkable piece of glass I own! The lens its self is made of an alloy-so its reassuringly heavy!I presume this would be billet Aluminium,this reassured me of the build quality,especially after my last purchase which was the 100mm F2.8L Macro-which was disappointingly plastic(apparently some engineering grade plastic HOWEVER i must point out that this lens is Optically superb!!),no matter how you think about it,you feel cheated after buying one of Canons "L"s .Back to the 24mm! The build quality is fantastic,and this L is weather and dust sealed with no External moving parts,or extending barrels.I screwed a 77mm Hoya HD UV filter straight on the front of mine.This lens is capable to taking breathtaking images-with almost a 3D effect.For such a big aperture lens,this is very very sharp.Some say in reviews the edges are softer at lower apertures,at 1,4 shooting resolution charts in a studio this may be the case,BUT in real life this is not so true!And if you like taking images that look like they are trying to crawl up out the Paper,this wont bother you at all! Even reviews that have picked this up agree that by 2.8 its Razor sharp.If I ran you a A3 print out,I guarantee you would not be able to tell the difference between 1.4 & 2.8.The other massive advantage to this lens especially to the likes of the 16-35 F2.8 etc is the amount of natural light it lets in! So for internal events like wedding receptions etc,this lens is GREAT.The manual focus is great,and is actually manual through helecoids,not fly by wire like the 85mm 1.2.The only downside is on a full frame there is visually detectable vignetting in the corners,In reality however dont forget your 5DMKIII will correct this for you in camera-It may effect others who dont use that Camera,its easy to remove in photoshop!This is a remarkable lens,and with out doubt Canons sharpest F1.4 prime! If you dont believe me rent one for the weekend-I guarantee you will be buying one Monday morning!