Indeed, you can not focus all of the sunlight that would hit a square mile of Earth down to a penny. You just can’t. When you focus the sun, you’re not making a

Indeed, you can not focus all of the sunlight that would hit a square mile of Earth down to a penny. You just can’t. When you focus the sun, you’re not making an infinitely small spot, you are making a tiny image of the sun. The bigger your lens, the larger that image. You can’t get any more focused than “in focus”. When you focus a cutting laser, you are imaging the shape of the laser cavity. The emission is coming from a very narrow spatial region, so you can focus it back down to a small spot. However, a laser that is out of alignment, will often not be able to be focused to a small spot.