Originally posted by Oggy Pleas Sir, Please Sir ---can I hypothesise?
Not in Public Oggy!
I reckon there have been some SDM failures.
People who have paid quite a lot are justifiably miffed when this happens, and start ranting.
OTOH, there are a large number of SDM lenses out there which work very well indeed and produce exceptional images day in and day out, but no-one rants that lenses are working properly. Thus the problem gets blown out of all proportion.
I am so tired of that arguement.
I expect there are far more screw drive lenses in use then there are USM lenses. How often do you see a post about someones FA50 1.4 falling apart? I can think of one. Now consider how many people have and use that lens. I expect it is one of the most common lenses right after the DA 18-55 kit lens. BUT there are numerous users on this forum that have had issues with their DA* USM lens.
Also consider how many of the people with dud USM's are long time posters on this forum. Many are photographers that know their gear, and know how to use it. When I see a post from these users that their USM crapped out, I believe it.
If it is just "internet chatter" of failed USM, then sure I could see your point. But it is not. These are people that I know through this forum, and their lens has crapped out.
Imagine if we were colectivly part of a camera club, that actually met in person weekly. Say there are 100 people in the club with USM lenses, and say, 5 of the lenses were duds.
A 5% mortality rate on a top product is outragious. But hey, 95% are still working, so why worry right?
But, based on the data collected on this forum (
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/69791-superson...-safe-buy.html), the rate of USM failure might be as high as 25%. Now I understand with a pole such as this, the rate of reporting for the failed USM is higher then the rate of reporting for the good USM. So lets just say for arguements sake that the good lenses get reported 1/4 of the time. So then your failure rate becomes 8%. This is still way to high.
If we really wanted to, some one could look through all of the gear people have listed in their profiles, count all the USM lenses, then take the USM failures from that post, and come up with a new failure rate.
So if you have an 8% chance of failure in the first 4 years (USM was introduced in 2006?), then you have 92% chance that your lens will be fine.
If you have 3 lenses, then you chance of all three working after 4 years becomes about 78%. (0.92 x 0.92 x 0.92) wow, thats a 22% chance that at least one of your lenses will fail over 4 years.
If the data is correct and all of the forums USM users posted, then with a 25% failure rate, and if you own three lenses, then you have a 42% chance of all three lenses will be working after four years.
Maybe I've forgotten my statistics, and I'm sure someone will correct me, but these numbers don't make me want to buy one of these lenses.