What causes this blurring? Also, why would a small aperture cause less blurring? Shouldn't allow more light lead to a better picture?
It reminds me that when we take pictures with phone, we can choose which point/object we want to focus on by clicking that area on the screen. Then, that object will be more clear than the background.
@philippe-eecs Assume the focal length is constant, then the smaller the aperture, the larger the F-number. By the equation between the depth of field and the F-number in https://cs184.eecs.berkeley.edu/sp21/lecture/15-95/cameras-and-lenses, a larger range of objects can have a circle of confusion with a diameter less than C. This is why small aperture leads to less blurring.