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"I used to be able to set aperture and shutter speed and the camera would choose the correct exposure time, let's say 8 seconds."
If you are selecting aperture and shutter speed, you are shooting in manual, not automatic. Your shutter speed is your exposure time. About exposure: There are only three adjustments that you can make: ISO, aperture and shutter speed. If you are using film, you usually set the ISO at the speed of the film, although you can adjust this to give an exposure that is slightly over or under if your shutter will only allow for whole stop adjustments. Otherwise, aperture and shutter speed are all there is to it. If you are selecting both, you are shooting in manual, because there is nothing else for the camera to select.