F-stops Explained P.2

1. How are each F-Stop steps calculated?
2. What does going up or down a stop mean? 

3. What does a fast or slow lens mean?
4. What does a lenses rating mean?

F-stops can be calculated by multiplying the diameter of the aperture by the focal length. In order to double the amount of light available, the diameter must be multiplied by the sqrt of 2. To compensate for the diameter change, the focal length now must be divided by sqrt of 2. Going down in the size of the aperture increases the F-stop number by the sqrt of 2. This is what "going up" or "going down" a stop defines. A fast lens indicates that the aperture is opened wider, therefore letting in more light. On the other hand, a slow lens indicates that the aperture is opened narrower, thus creating a darker image. Lens rating define the largest available aperture on the lens. For zoom lenses, the lens rating may display a range because larger focal lengths with a wide aperture is difficult to manufacture.