Trying to get into photography. How and why does aperture affect focus?
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[This diagram shows it pretty nicely.](https://www.vision-doctor.com/images/stories/optik/grundlagen/Fundamentals_depth_of_field.png)
When a point of an object is in focus, it means that all the light rays emanating from that point converge to a single point on the sensor. But once they converge, those light rays will continue to go in a straight line and end up diverging. If you move your sensor back (bring things out of focus), you will capture those diverging rays and end up with a blurry “bokeh” circle instead of a point. The further back you move your sensor, the larger that circle becomes, because the light rays just keep diverging further apart from each other.
If you make your aperture smaller, you block the outermost light rays, which means that the light rays coming in at the steepest angle will get blocked. Only the ones coming in straight will get through. The straight ones simply don’t diverge as much. So with a small aperture, the only light rays that make it through are coming in quite parallel to each other, which means they can’t diverge as much, which in turn means your bokeh circles will be smaller.
Imagine you have a spray can of paint. You press the nozzle and make a large spot on the wall. But if you hold a piece of paper with a hole in it in front of the nozzle, the spot on the wall will be smaller, because the particles of paint that go more sideways simply get stopped by the paper.
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