From Wikipedia:
EF 50mm f/1.8
The EF 50mm f/1.8 is a discontinued economy level lens, which has been replaced by the EF 50mm f/1.8 II. It is constructed with a plastic body and a metal mount, and features a distance window with infrared index. A 5 blade maximum aperture of f/1.8 gives this lens the ability to create depth of field effects.
The optical construction of this lens contains 6 lens elements, with no special elements. This lens uses a front extension focusing system, powered by an AFD motor, Front of the lens does not rotate when focusing, making it compatible with circular polarisers.
Auto focus speed, despite the AFD motor, is moderately fast although audible.
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EF 50mm f/1.8 II
The EF 50mm f/1.8 II replaced the EF 50mm f/1.8. Because of its low price but relatively high optical quality, this lens has excellent value. It is constructed with a plastic body and mount and has a very simple design with no features.
Changes compared to the mk I model are the plastic mount, the absence of a distance window, a micro motor instead of an arc-form drive and a rotating lens front. Optically, this lens is almost identical to the first version.
The focus ring on this lens is even smaller than the one on the mk I model and placed on the front of the outer barrel. Also unlike the earlier model, it isn't decoupled in AF-mode, so accidently turning it can cause damage to the autofocus mechanism.
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