This brings back the old discussion of full aperture metering vs. stop down metering. Some people thought stop down metering was more accurate because it measured the light coming through the lens rather than the setting based on the f/stop set on the lens. Full aperture metering has been around for a long time. Some cameras had it earlier than others. The Minolta SRT-101 had it in 1966. The Konica Autoreflex T had it in 1968. The Canon F-1 and FTb cameras had it in 1971. Pentax did not have it until the ES of 1971. Before the Micro Nikkor P Nikon made the 55/3.5 Micro Nikkor Auto which had a mechanical exposure compensation in the close-up range. This made things easier for someone who was using a separate meter and wanted to get proper exposure up close. To use it with a camera with TTL metering you would need to stop down to meter. If you are using a camera like the Minolta X-700 and are set to aperture priority, the camera will read the actual amount of light coming through the lens and will not depend only on the f/stop the lens is set at. Your finder may show a shutter speed of 1/250 but the actual shutter speed at the moment of exposure may be faster or slower than that. This would cancel out any minor differences between the marked f/stop and the actual T stop.