USM vs. STM Canon Lens

[ahbnyc]

I am thinking of buying the Canon 55-250 EF-S lens, but it comes in both an STM and a USM model.  What is the practical difference between the two?  Also, for a camera with an APS-C sensor is the alternative of the 70-300 USM IS EF lens worth the extra $400 cost (and added weight)?  Thanks.

[MoreOrLess]

If I remember correctly the old 55-250mm isnt a USM lens.

[AlterEgo]

I am thinking of buying the Canon 55-250 EF-S lens, but it comes in both an STM and a USM model.  What is the practical difference between the two?  Also, for a camera with an APS-C sensor is the alternative of the 70-300 USM IS EF lens worth the extra $400 cost (and added weight)?  Thanks.

stm is a linear stepper motor model that works fine with CDAF (=dSLM, they are optimized to execute a series of fast small/fine moves, hunting for a contrast maximum), USM motorized lens needs PDAF or PDAF on sensor for a good work (they are optimized to execute a fast move to a certain focusing position on a single command from camera and may be one more move if PDAF is setup up to double check... they are not very good for CDAF contrast hunting)

[Chris_Brown]

From Canon's STM info:

. . . a stepping motor (STM) drive, designed to deliver smooth and quiet continuous AF during video shooting when paired with the Movie Servo AF feature on select EOS cameras.

[ahbnyc]

Thanks for the responses -- I must have misread the Canon website the first time when I thought the non-STM model was USM.