I filled the bird feeders and did a photography exercise.

The same view, with different depth of field.

Example #1, aperture set to f2, shutter speed set to 1/5000 seconds, ISO set to 800 (because that is the speed of the film in Dad’s camera right now).

Example #2, aperture set to f8, shutter speed set to 1/500 seconds, ISO still at 800.

Final example, aperture set to f16, shutter speed set to 1/100 seconds, ISO again at 800.

So the lower the aperture number (lower numbers mean wider aperture, so f2 is as wide as my lens gets and f16 is as narrow as it gets) the shallower the depth of field and the less of the image that is in focus. Get it?