Back Focus Question?

How does a lens develop back/front focus?
This seems to be an infamous problem of Canon.
I have purchase not a lot of lens, but 2 of them suffered from that.

If a lens focus perfectly before (right out of factory), would they develop back/front focus after?

I haven't heard a lot of back/front focus from other brand (Sony/Nikon), but Canon...............

All lenses bend light to hit an optimum point in space. Lenses and cameras are built seperately then expected to mate and perform perfectly. It sometimes doesn't happen becuase there are manufacturing tolerances and not absolutes.

No brand is totally immune from this.

so for the lens I got (they are 2nd hand), they turned bad (back/front focus) after extended period of time?
Or they were as bad when they were out?

mahoro wrote in post #6852513
so for the lens I got (they are 2nd hand), they turned bad (back/front focus) after extended period of time?
Or they were as bad when they were out?

Hard to say since they were second hand. Could have been dropped or jarred in some fashion and knocked out of alignment. Have you had this issue since day one or has it been progressively noticeable?

The cams and rollers that make the internal parts move can wear (although it's probably very slight .. there are people with lenses that are 30+ years old or older and they work just fine.)