I am new to using the older manual focus lenses. I recently bought this 180mm lens. It takes beautiful photos but will not meter correctly on my D7100 or D700 in aperture mode (where my 135mm Ai lens does). At f2.8 the photo is underexposed with exposure increasing as the f stop number increases to the point where at F8 photos are drastically overexposed. At the moment I get around this by working in manual mode and reviewing the photos each time to get the exposure right but I didn't think I would have to do this with an Ai-S lens. I thought is was supposed to meter correctly in aperture mode on the D7100 and D700. What might be causing this to happen? Is there an easy fix?