i void warranties
I’ve been very happy with my Sony A500, but when I bought my first F2.8 lens, something was amiss. The focus just wasn’t sharp. At first I thought it was the lens, so I exchanged it for another. Same problem.
Like all digital troubleshooters, I turned to the interwebs for answers, and started researching how the autofocus system on DSLRs works. I won’t bore you with the details (en.wikipedia.org/wiki/Autofocus#Phase_detection), (1) DSLRs generally don’t use the sensor for auto focus, and (2) cameras have a chip in with specs for each lens that helps with the AF. “For this reason, some 3rd party lenses may not focus as well…”
My new lens is a Tamron f2.8 28-75. Could it really be that this “3rd party lens” doesn’t focus well on a Sony body?
So I did a little more experimenting. I shot at a 45 degree angle at a sheet of paper covered with text. And what I found was that it was focusing, just on a spot about ½ inch in front of where I was aiming.
More research uncovered information that cameras can have a slight "front focus" or "back focus". I.E. The camera auto focus system is working fine, but the sensor is ever so slightly too far forward or back, and so the spot in focus on the sensor is in front of or behind the spot that’s in focus to the AF system.
This generally is only apparent with fast lenses that have a very narrow DoF, which is why it was never an issue until I had a 2.8 lens.
Sony DSLRs have 3 screws on the bottom of the camera, hidden under a sticker, and you apparently void the warranty if you touch them (www.dynaxdigital.com/sony-alpha-crop-sensor-camera-discus...).
So off I went with my Allen wrench, and in a few minutes, I had tack sharp AF. My camera's out of warranty any ways.