dufflover said:
That's the classic rubbish line about the 3rd party. You can just as easily get a dud Canon or Nikon. You could make a fuss with the exact numbers but unless there is something inherently wrong with the design then "in practice" it is not particularly more likely than the other.
It is not just a question of a good or bad design, although any designer of volume products worth his or her salt will try to minimize the sensitivity of the design to manufacturing variations. The extent to which the manufacturers are able to optimize their process control will play a big part in how likely you are to end up with a dud.
A company which maintains tight control over the materials, assembly equipment, manufacturing processes and externally sourced components will be able to minimize the percentage of out of tolerance products coming off the line. By controlling their test processes they can also ensure that most of the duds get rejected. This is what the science of process control is all about, and big companies like Canon take this very seriously. Not only does it improve the quality of their products, allowing them to charge higher prices, it also saves them money in failures and rework.
Even if two companies share a design, the quality from one may be very different from the other. An example which was quoted in a marketing class I took many years ago featured a gearbox that was built by both Mazda and Ford, who had (and I think still have) significant design sharing agreements. According to the class, Mazda's quality metrics were 8 times better than Ford's for the manufacture of an identical product. (I'm not bashing Ford by the way - this example is several decades out of date, so has little relevance today.) I don't have any hard data to compare Canon's quality with Tamron's, but I would disagree that the quality of the design trumps the manufacturing methods used to build it.