Confused by ramping on zoom lenses

Raymond Zrike · June 7, 2022

A number of the best super-16 zooms have a ramping maximum T-stop. For instance, the Angenieux 7-81mm is T2.4 until around 50mm when it becomes T3.4 (as marked on the lens itself). The Canon 6.6-66mm is similar.

My question—if I were to shoot the Angenieux at T3.4 or slower, and I zoomed from 7mm to 81mm, would I see the exposure drop after 50mm? Or would it stay the same (i.e. the exposure drop would only occur if the lens is set to T3.3 or faster)?

What is causing this drop in T-stop?

Anecdotally, has anyone noticed a drop in exposure during a shot when zooming a lens like that?

I assume a ramping maximum T-stop is not equivalent to a ramping f/stop? As in, I assume it only affects light transmission rather than depth of field.

Brian Drysdale · June 7, 2022

If you zoom at f3.4 or stopped down beyond that, you won't see the ramping.

The ramping is usually a design trade off to reduce the size and weight of the zoom lens. To have the same stop at longer focal lengths, the lens would need to be larger. Some Angenieux film zooms have it and the Canon is based on a broadcast zoom lens, so see below.

Since it's a common feature on broadcast zoom lenses, this extract from this site ( https://members.tripod.com/camera_dave/lens-theory-2.htm )explains the reasons,

"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."