Kids ask the best questions, and I didn't know how to answer one that I got from my teenage son the other day. He asked, "Why does it take the same amount of time to expose a photo with a little 8-inch (200mm) Rapid Rectilinear lens that has a diameter the size of a quarter, as it does to expose a photo with a big 8-inch (200 mm) Petzval lens that has a diameter the size of a can of soup, assuming they are set at the same f stop? Doesn't the bigger lens let in a lot more light? So why doesn't it need a shorter exposure?" he asked.

Um, well, hmm... I told him that I really hadn't thought of that before, and now that I had, I honestly couldn't explain why. So how might I explain this to a bright 17 year old (and myself as well)?