Other mistake: After there has been the vote for chief, behind one boy you can see a piece of wood reading "Anthony M," a marker for showing where the actor (Anthony McCall-Judson) should sit. (00:13:25)
Other mistake: Take a look at the size of the rock that the boys push onto Piggy. A rock of that size, falling from that height would've exploded Piggy's head like a ripe melon. It also would've crushed his body into a mangled heap.
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.
Factual error: Throughout the film, sea grape vines are seen all along the shore. Sea grapes do not grow in the Pacific, where the story takes place, only in the Caribbean where the film was made.
Continuity mistake: In the scene when Sam and Eric are sleeping by the fire, Sam is on the left side of the screen and Eric is on the right, and Eric has a rip all the way down the back of his shirt. Later, Eric gets up and moves away and calls for Sam to join him. In the shot of Sam moving towards Eric, his back is to the camera and you can see that Sam's shirt now has the rip - not Eric's.
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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