Sun Oct 18, 2015 3:43 pm
Welcome to the world of using still camera zoom lenses for video, they are variable in there f/stop, unless you stop them way down. Even a constant f/stop lens like,themPanny 12-35 f/2.8, will shift a little when zoomed at f/2.8, as the light transmission of the lens changes, along with the focal length in relegation to the Iris opening. F/stop is Based on the ratio of the iris opening to focal length, which works for non zoom lenses.
Cine/Video lenses use T scale, which is based on the actual light transmission through the lens (not a math formula), and zoom lenses are designed to keep the light transmission constant, which is why they are more expensive than still zoom lens counterparts. That said,,their are a few still zooms that behave more like a Cine/video zoom (constant f/stop and parfocal) the Sigma 18-35 f/1.8, Tokina's designed Cine zooms, based on their still"lenses like the 11-16, and Olympus' Four Thirds Pro f/2.0 Zoom lenses.
These issues with "still" zooms, is why I do not use them for video work anymore, as they can mess up a job very quickly. You have to use still zooms like a variable focal length lens, rather,than as a true zoom. Set the focal length you want, reset the f/stop to where you want it, then Shoot/Roll camera.
Denny Smith
SHA Productions