Actually, a hand lens for starting fire is not a prominent part of my plans. You need fairly robust sunlight to be successful, and that is not likely to be a situation where fire will be critical. Or to put it another way, when i have really needed a fire, it was after sunset, overcast, cold, etc.

Still, it is a worthwhile option, presuming one can plan ahead. It's been a long time since high school physics, but I daresay you are correct.