lens - Do good lenses really last a lifetime?

I’ve heard people say “a good lens will last a lifetime” many times. Is this actually true of modern lenses? More specifically, will a good lens be usable for a lifetime?

With an inexpensive adapter, I can mount up any M42 (Pentax) screw-mount manual focus lens to my Canon EF mount body. The same is not true of older AF lenses. The Canon FD mount lasted less then twenty years, and is incompatible with modern Canon bodies; this also seems to be the case with the earlier FL and R mounts. While the EF mount has lasted longer than the FD lenses, I have a 15-year-old Sigma lens which does not work correctly on modern Canon bodies due to electronic incompatibilities.

Additionally, there’s the issue of Canon’s EF-S (APS-C only) lenses, which are incompatible with their full-frame DSLRs. If one wants to upgrade to a full-frame body in the future, any money spent on EF-S lenses is wasted.

The situation seems to be better with Nikon’s gear. It’s my understanding that current Nikon bodies are largely backwards-compatible with lenses dating back to the ’60s.

Answer

In talking with a number of working pros, the general attitude tends to be that you buy lenses to keep and you buy bodies to upgrade. My personal planning mirrors this; I've tried to invest in higher quality lenses that i expect to own for a while (10-15 years) while given how body technology is changing, upgrading a body every 2-3 years doesn't surprise me. When I started, I bought the Canon 100-400, an inexpensive wide angle, and a Rebel XT body. I've upgraded the body 3 times (now I have a 30D and a 7D, and I'm starting to think about upgrading the 30d), and I've upgraded the inexpensive lens to a better but not high end lens, and honestly, I'm also planning to upgrade my wide angle lenses to "keeper" lenses over the next couple of years as I can.

So while "last a lifetime" might not be strictly true (but a co-worker of mine collects old camera gear, and we were working on an 8x19 lens made around the turn of the century a week or so ago; it still works...) it is true that if you buy more expensive lenses and less expensive bodies, it'll be a better long-term investment and you won't upgrade your lenses nearly as often as the bodies, adn they'll tend to last a lot longer with some care and maintenance. My upgrade model is 3-4 years for bodies and 10-15 years for quality lenses (such as IS style). Some regular maintenance doesn't hurt; many pro photographers I know send their lenses in once a year for pro cleaning and calibration...