I bought an adaptor to use my older lenses with my Canon t5i. The exposure using 2 similar lenses is off by at least a mile. Using my t5i and the Canon 50mm lens in AV, I get an good exposure of 1/13 at f5.6. With the t5i and the older 55mm lens the t5i in AV exposes for 1 sec at f5.6. Setting the t5i in manual to 1/13 at f5.6 gives me the correct exp with the older lens. The adaptor holds in the M/A pin on the old lens which puts the aperture into manual mode, so the aperture stays at whatever I set it for. Why can't the t5i meter correctly with the old lens?