How does peering through your eyelashes allow you to see things in focus that are so close your eyes can’t normally focus on them?
In: Biology
Any lens has a property called “depth of field”. This is the range of the field of view which is in focus. This property is a function of two parameters, the curvature of the lens, and the diameter of the lens. The more the curvature the smaller the depth of field and the smaller the diameter the larger the depth of field. It seems a little surprising, but with a small enough diameter any lens can have focus from near to infinity. This is how the “pinhole camera” works. When you squint, or look through your eyelashes, or look through a pinhole, you are changing the lens diameter from the normal size (the pupil) to something smaller. This makes dark things harder to see, but it increases the depth of field if there is enough light.
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