If you have zoomed with a zoom lens open to full aperture, you may have noted a drop in video level at the telephoto end. This is called the F drop or "ramping". The "entrance pupil" of a zoom lens changes in diameter as the focal length is changed. As you zoom toward the telephoto end, the entrance pupil gradually enlarges. When the entrance pupil diameter is equal to the diameter of the focusing lens group, it can not become any larger, so the F-stop drops. That is the reason for the F drop.
To eliminate F drop completely, the focusing lens group, (the elements in the front of the lens), has to be larger than the entrance pupil at the telephoto end of the zoom. It has to be at least equal to the focal length at the telephoto end divided by the F-number. To reduce the size and weight of a zoom lens to make it easy to use for hand held cameras, we have a trade off that makes it common to have a certain amount of F drop or ramping at the telephoto end. For better composition effect, however, in some studio zoom lenses the focusing group is made large enough that no F drop occurs. F drop is a major determinant of the value of zoom lenses used in live on-site sports broadcasts, which require a long focal length and must frequently contend with twilight or inadequate artificial illumination.
As many people know, movie camera lenses are rated by a T-number instead of an F-stop.The F-stop expresses the speed of the lens on the assumption that lens transmits 1OO% of the incident light. In reality, different lenses have different transmittance, so two lenses with the same F-stop may actually have different speed. The T-number solves this problem by taking both the diaphragm diameter and transmittance into account. Two lenses with the same T number will always give the same brightness.