Fixed Aperture Only.
The more advanced lenses sold by Canon, Nikon, Pentax and Sigma, have built-in electronics that along with the camera, allow for fine control over the aperture and focus. The Samyang lens does not have such electronics. This means that you will need to have a good working knowledge of controlling the exposure with ISO and shutter settings on the camera. The fixed aperture will appear smaller with increased focal length. As you slide out the focal length adjustment tube (zoom), the aperture or the F-number is marked next to the focal lengths. They are calibrated as follows:
These are only marked on the lens, to help the photographer decide the appropriate exposure and shutter speed on the camera.