How does fixed aperture zoom lens work? Normally as you zoom to tele the aperture will get smaller (ie bigger F number), Constant F-number lens get fixed aperture no matter how you zoom...how it is acheived? anyone? :dunno: :dunno:
Actually, the aperture doesn't get smaller. In non-pro lens, the aperture moves outwards as you zoom to tele, so the aperture goes further away from the film/sensor. As things further away looks smaller, the aperture becomes smaller. In pro-lens with constant aperture, the aperture does not move when you zoom.
In fixed aperture zooms, the thing that is fixed is the ratio of the focal length to the diameter of the aperture openning. To maintain the fixed ratio throughout the entire zoom range, the aperture is mecahnically linked to the zoom such that it changes its diameter with resect to the focal length setting.
The more complex mecahnism is the main reason for the higher price of "fixed" aperture zooms.
For modern SLRs where the lens is able to transmitt its zoom range and actual focal length setting to the camera body, the camera can actually maintain the fixed f-number for you instead of relying on the lens mechanism, so you can effectively have fixed f-number throughout the entire zoom range. However, the max aperture (min f-number) is limited.
For example, for a f/4 ~ 5.6 lens, if you set the camera to maintain fixed f-number when you zoom, you can get max f/5.6 throughout, but not f/4.