Sensor Dust
These diagrams are supposed to show why you need a small aperture when checking for dust on your sensor.
The vertical red line represents the sensor and the vertical grey line represents the AA filter. The black blob is a dust particle. The top diagram shows light coming in at a narrow angle, corresponding to use of a small aperture. The blue area shows the region that is in shadow at all angles, and since it reaches the sensor the resulting image will show a dark spot there.
The bottom diagram shows light coming in at wide angles corresponding to use of a large aperture. The green area shows the region that is in shadow at all angles. Since it does not reach the sensor the resulting image will not show a distinct spot from the dust particle. Some light can get around the dust particle. There will be an area of reduced brightness on the sensor, but it will not be as noticeable as it would be when the small aperture is used.
Sensor Dust
These diagrams are supposed to show why you need a small aperture when checking for dust on your sensor.
The vertical red line represents the sensor and the vertical grey line represents the AA filter. The black blob is a dust particle. The top diagram shows light coming in at a narrow angle, corresponding to use of a small aperture. The blue area shows the region that is in shadow at all angles, and since it reaches the sensor the resulting image will show a dark spot there.
The bottom diagram shows light coming in at wide angles corresponding to use of a large aperture. The green area shows the region that is in shadow at all angles. Since it does not reach the sensor the resulting image will not show a distinct spot from the dust particle. Some light can get around the dust particle. There will be an area of reduced brightness on the sensor, but it will not be as noticeable as it would be when the small aperture is used.