I recently purchased a Sigma SA-9 and two SA-7n's. These are SLR (film) cameras. With the kit lens that came with it, they are all registering two to three stops under for just about any scene in good, constant light (i.e. no backlighting). I checked it against two known working cameras (one film, one digital) and a Sekonic light meter. All three of those agree within 1/3 stop. I installed fresh batteries and checked all the settings (ISO, EC, metering, etc.). I can't understand how three different camera bodies could all have the same problem.
Unfortunately I only have the one SA-mount lens that I mentioned -- a Sigma 28-80 AF lens, and I tested across the entire aperture range. Could the lens be the culprit here? And if so, is there a way to test the cameras' meters without the lens? I don't have another SA-mount lens, but I do have some Pentax and Olympus lenses.