It is probbably a good time to explain what an fstop means. F stands for fraction. If you use a 100mm lens at f4 then the fstop is 1/4th of th focal length or 25mm. If you set f8 on a 50mm lens then the fstop is 1/8 the focal length or 6.25mm.

The f stop markings are computed with the lens focused at infinity. If you focus at a distance of less th 8 focal lengths away you have to start allowing for the fact that your lens's fstop markings are sufficiently different from infinity and allow for it.

That is enough lesson for the day.