F Ratio

The F Ratio is the ratio between the focal length $$f$$ and the diameter $$d$$ of the lens. Strangely, $$f/6$$ means that $$f/d = 6$$

Fast Optics
A small F Ratio corresponds to fast optics because $$d > f$$ which means that more light will pass through the apperture, reducing the time it takes to make an image.

Slow Optics
A large F Ratio corresponds to slow optics because $$f > d$$ 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.