Stop down metering is a method of through-the-lens (TTL) light measurement in which the camera measures light at the actual aperture that will be used for taking the photograph. Unlike open-aperture metering, where the lens remains fully open during metering and only stops down at the moment of exposure, stop down metering requires the lens to be physically closed to the selected f-stop before measuring light.
This technique provides highly accurate exposure readings because it takes into account the exact light transmission and any filters in use, as well as the depth of field that will affect the final image. Stop down metering is especially useful with older lenses, manual cameras, or specialised optical systems where open-aperture metering is unavailable. While slightly slower than open-aperture metering, it gives photographers precise control over exposure, helping to produce well-balanced images with correct brightness and contrast.