I don’t like too much this lens, because has a very vulnerable construction and not too precise autofocus system. 85 mm F1.8 is much much better.
But anyway, there are a lot of this lenses on the market, and after a certain time each of them will have a loose front lens.
Why? because it’s idiot construction.
As you see the focusing barrel is hold by 2 pieces of gliders (I think of PTFE), which are guided in two helicoidal channels. This would be not a problem.
The problem is, hat this channels end at approx 2 mm (!) distance from the barrels end. This makes the lens very sensitive, as if the lens front lens is in the MOST RETRACTED position, even the smallest axial force on the moving element deforms the barrels end – end instead of the precise moving You will get on undefined position.
In my last case I measured this: originally the groove is 3,50 mm wide – deformed to 3,95 mm at the infinite position of the lens.
The barrel is aluminium alloy, so easy to be deformed.
So i took my small wice, and after 4-5 trials the groove was adjusted back to 3,50 mm-s again.
But this was only one of the problems. the lens was working only 10-20 times normally, after that no moving at all. This lens have a special small USM micromotor through a gear transmission. It has a full time manual focus – realized with a small differential gear and a sliding clutch.
the motor has a description on the canon site. http://software.canon-europe.com/files/documents/EF_Lens_Work_Book_9_EN.pdf (page 181)
The rotor of the USM motor was too loose, so i decided to disassemble it. Disassembling was not too easy, as the nut was glued on the end of the shaft with a non-dismountable glue – I suppose it was not pretended to be disassembled.
After disassembling i understood how it works – so here you have the pictures:
The scratches on the gear are NOT original – I didn’t knew the disassembly, so i forced it a little bit. In the middle of the gear is a tiny ball-bearing. I tried to do my best by cleaning, polishing (better to say: matting) the surface of the aluminium rotor – the motor is not working after the operation. Anyway, there is possible to purchase it from the internet for approx 78 USD plus postal fee.
Yesterday i have seen an STM motor. Amazing how it works. Does anybody know, how it works? (i know, it’t a stepper motor. But deeper)