Originally posted by luftfluss It's an interesting thought exercise.
I say put a Pentax lens - perhaps the D FA* 50/1.4 - in a steel box and wait for the Pentax brand to go bust.
Is the D FA* 50/1.4 then worth more, or less... or both?
You could draw on the experience with cars. Just check the price for which a fairly luxurious car (the most likely analogy to a Pentax lens) was sold, then assume that in - say - fifty years only x% of those lenses survive (just as only x% of these cars survived) and that's how you find out that there is this unpredictability about the desirability of something after fifty years.
I'd just use the money to pay the mortgage and allow the kids to study. Seems a more sensible investment.
If you do put a lens in a steel box and wait for an unpredictable event, make sure that there is plenty of desiccant in there so there's no chance of fungus or anything.