How much better is the 1.4 than the 1.8 at low light focusing? I just sold my 1.8 as i`d bought a 17-50 2.8 but if the 1.4 is considerably better im tempted to buy one for that price.
It's not a night and day improvement, but it is better. However, I was expecting more. The fact is that the lens is a very old design and does not have the ring USM motor of more sophisticated lenses. It has micro USM, which is inferior. See this post -
http://forums.dpreview.com/forums/read.asp?forum=1029&message=15260787.
When performing AF calibration checks I found that the lens would not focus at the same point repeatedly, just the same as the 50/1.8. At f/1.4 that's something you notice. Still, it is quieter and you do stand a fighting chance of being able to focus manually with better precision than the (almost impossible) 50/1.8.
It is also a little bit "dreamy" when used wide open. Not unpleasant, and maybe even an advantage for portraiture, but definitley not bitingly sharp.
There is also a known mechanical fault (or is it a feature?) in the design of the 50/1.4. If pressure is exerted on the front of the lens when it is focused at other than infinity it is possible to wreck the AF drive mechanics. Such pressure might come from a knock or getting a bit squashed in a camera bag. The solution is either to always remember to store/secure the lens at the infinity end of the scale and/or to fit a hard hood of proper design (not one that screws to the filter thread) and leave it permanently in the ready to use orientation. If you do mash the AF drive there might be a DIY fix, requiring a rather robust twist of the lens to try to realign the gearing, but it might also mean a trip to a service centre.
I do prefer the 50/1.4 to the 1/8, but at more regular prices it's hard to see a leap in quality that justifies the price hike, but then that's probably true of the 50/1.2 over the 50/1.4 as well, with bells on. But at these Jessops prices, why not?
