I bought a used 135mm EF 2.8 SoftFocus lens on eBay for a reasonable price. If I am reading the date code correctly, the lens is nine years old. The lens focuses fine and works fine, but there is one problem, when I shoot pictures with that lens, the metering seems to underexpose by almost a stop. This doesn't happen with any of my other lenses (all of which I bought new).
The underexposure happened on both of my bodies (5D Mark II and T2i). Any advice on what to do to correct the problem?
Thanks for your help.