The F Ratio is the ratio between the focal length and the diameter of the lens. Strangely, means that
A small F Ratio corresponds to fast optics because which means that more light will pass through the apperture, reducing the time it takes to make an image.
A large F Ratio corresponds to slow optics because which means that the same image will be spread across greater area. To get the same amount of photons per area, we need more exposure time.