I played for 2 months with a 50 cm wide Fresnel lens ... Mind-blowing. Yes it crystallizes the cement in 2 seconds, yes it blows a bottle of dark beer in 5 seconds ... Even a friend welder succeeded in welding metal with it.
But by dint of playing with it, I had to lose at least 2 tenths in the eyes despite my sunglasses, and my eyes became hyper sensitive to ambient light. The resulting light that bounces off the ground or whatever from the lens doesn't hurt, like when there is an eclipse, but it is as strong as a welding light.
Conclusion: get a welder's mask before you are tempted to play with it, and also give it to your friends who are sure to watch you do it.
PS: it's very stupid but don't even think about transporting the naked lens in the car or storing it as it is, it sets everything on fire as soon as a ray of sunlight hits it
A word to the wise.