Determining F Stop on Cine Lenses

[sk85]

Since Cine lenses are marked in T-Stops how does one determine what the F-Stop is? I assume that the focus puller would want to know exactly what the F-Stop is so he would know how much of the picture is in focus.

Thanks for any help.

 

[Moseph]

I assume that the focus puller would want to know exactly what the F-Stop is so he would know how much of the picture is in focus.

No, never really heard of this. Maybe in some rare circumstances the exact depth of field might matter but the difference between the T stop and the actual F stop isn't likely to make much of a dent in the DOF.

 

[Gabel]

No, never really heard of this. Maybe in some rare circumstances the exact depth of field might matter but the difference between the T stop and the actual F stop isn't likely to make much of a dent in the DOF.

If one was shooting with a very long lens, then yes. But most of the time, if you want to compensate, I usually just count as it being opened up 1/3rd of a stop. Say your T-stop is 2, then you could count that as 1.4+2/3rds. But most of the time I use the same as the T-stop, as said the difference is usually too small to notice.

 

[Noel Evans]

Seems to be some confusion. OK so the variant of F stop and T stop is that t stop includes the factor for the light loss from transmission through the lens. However, in the OP's question its utterly irrelevant as regardless of any variance between t stop and f stop, or that numbers relative effect on depth of field, the focal distance remains the same.

In pulling focus, the focus puller need only know distances to focus to and if these are not marked and are fluid, he may get angry should the t or f stop be too open and will be very aware as to the fact of what the aperture is - he / she will have that number right in front of them.

 

[sk85]

Thanks. So the focus puller will always be pulling for a specific distance regardless of the aperture.

 

[j1clark@ucsd.edu]

Thanks. So the focus puller will always be pulling for a specific distance regardless of the aperture.

Depth of Field only changes the 'apparent sharpness' of the image. There is still the plane of critical focus, which will change if the subjects move. DoF being 'deeper' gives a little cheat room.

 

[Mitch Gross]

This is not entirely true. Some lenses have breathing and if there is enough objects at various distances then the constant riding of focus can be distracting. Many ACs (or under direction from their Operators or DPs) choose to work within their depth of field rather than always nail the focus point directly. A conversation between two characters can feel like focus ping pong with a distracting Bokeh change.

That said, T-stop and f-stop are quite close, and no one wants to risk riding too close to the edge of focus and falling off. Giving oneself an acceptable allowance for depth will take take of any differences between the mathematical f-stop and the transmitted t-stop.