The Rokinon 35mm T1.5 Cine Lens for Canon EF with geared, manual focus and aperture control rings, is optimized for digital cinematography. The aperture ring has been “de-clicked” for smooth iris pulls, and depth of field markings provide an essential aid to focus pullers. *Optically, this lens is identical to the Rokinon 35mm f/1.4, which is known for its excellent sharpness, contrast and bokeh.
A T-Stop represents light transmission rather than aperture ratio (ratio of aperture to focal length). F-Stops don’t always transmit 100% light transmission at the corresponding f-stop since lenses absorb some amount of the light passing through them. In cinema, since images are seen in rapid succession, even small amounts of exposure inaccuracy can be pretty problematic. Cinema lenses are tested for actual light transmission and calibrated in T-stops for exact accuracy in exposure. This way, you can swap out fixed focal length lenses without altering your exposure the way photography lenses are effected. Now again, since all lenses absorb some light, the T-Stop of any given aperture on a lens will always be greater (less light transmission) than the F-Stop. The T-Stop corrects exposure for absorption of light.
There are no hard clicks when changing apertures on the Rokinon 35mm T1.5 Cine Lens. Aperture can be manually set to a very specific number not a hard set number f/1.7, f/2.8, etc. as well as the absence of hard clicking on the aperture barrel. This also makes for smoother focus pulls.
Focusing on a cinema lens typically has a longer and smoother travel, not like the short focus of a photo lens. This equates to more degrees of turning when focusing. The new Rokinon Cine lens has a slightly longer and smoother travel than the f/1.4 Photography lens. Additionally, the focal distance numbers on this Cine Lens are on the side of the lens, rather than on the top, for easier focus pulling.
With a lens that has internal focus, there is no back and forth movement as you focus in on your subject. In other words, the front lens element does not move. Both the 35mm Cine and Photo lens has internal focus.