A lens with an automatic aperture diaphragm that can be kept wide open, at its widest aperture, until the shutter is released, regardless of the aperture setting for the exposure. Such a lens makes for a brighter viewfinder view and eases focusing with through-the-lens cameras. When the shutter is released, the aperture automatically stops down to its pre-set opening so that proper exposure is made, and instantaneously returns to its widest aperture. This lenses also allow for metering when wide open with a compatible camera with metering capability.
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