The F-Stop number is the ratio between the camera’s aperture and the camera’s focal length:
Camera Focal Length divided by F-Stop equals Camera Aperture
So, increasing the F-Stop number while keeping the focal length constant, means that you divide the focal length by a higher number and the result is a smaller aperture. A smaller aperture means less light hitting your film strip or camera sensor and blurring your image. Increasing the F-Stop number on a real world camera to the next higher value means reducing the amount of light entering the camera to the half.
