Audio problem: When Jack says "Anyway, the conch doesn't count on this side of the island", he is clearly biting his banana and eating it, however the dialouge is heard perfectly clear, even though he is eating.
Audio problem: In the scene when the boys are going up the mountain to search for the beast, Roger is standing slightly down the slope in relation to Ralph. After Ralph says something about not leaving the littluns with Piggy, Roger's mouth moves but no sound comes out. He probably had a line there, but they decided to take it out when they were recording the sound.
Answer: The key factor there is the focus of the light over distance. The light coming through the glass is refracted and focused on a single point. But it's bent like a ribbon. There is a "sweet spot" so to say where you have to hold the magnifying glass or lens at just the right distance and angle from the object to focus the center point of the light on it. Typically, this means holding the glass out a good several inches or even a foot or so away from what you wish to ignite to get the focal point of the light on it. Someone wearing glasses has them pretty much right up to their face. And so the light can't reach a focal point. Also keep in mind that for focusing the light through a lens, it needs to be angled just right for the light to go through it at the optimal angle and focus. Usually this means facing the sun directly. Typically people don't look up directly at the sun, at least not for more than a second. Especially with glasses on.
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