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Your eye and brain adjust for the different light levels.
But note that your "field of vision" is exactly the same. It's just that part of what you're seeing is your hand blocking stuff. When you change the aperture of the camera, the field of view doesn't change any more than the field of view of your eye changes when your pupils contract and dilate to account for different light levels.
Oh, umm, you asked WHY doesn't it change? I don't know.
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