Open-aperture metering
The phrase "open-aperture metering" contrasts with an older stop down metering scheme, used in the first SLR cameras to offer TTL exposure metering. In open-aperture metering, a linkage inside the lens communicates the currently-selected f-stop to the camera's exposure meter, while leaving the lens at its widest aperture for a bright viewfinder image. (An automatic aperture then closes the lens to the selected aperture in the instant before the shutter opens.) This avoids the need to dim the viewfinder while taking meter readings.
Open-aperture metering has been standard for SLR and DSLR cameras since the mid-1970s. Earlier SLRs (notably, ones using 42mm screw lenses) may be assumed to lack open-aperture metering, although there are exceptions.
