I know about DOF, but my question is about Back-focus & Front-focus. I realize the total DOF is the result of many factors (i.e. lens length, f stop, focal distant etc), and DOF changes as the variables change, but I get a little confused with BF & FF problems. Is it possible to have no BF or FF problems at one focal point but as the subject you are focusing on, gets closer (or farther), BF or FF can show up?? Or is it safe to say if you test your lens at a short distance (let say 3 feet) and it shows no problem of BF or FF, then it is safe to say you will not have any problems at any range of focus with that lens? I might not be saying it correctly, but can BF or FF come and go as you change the distance of your subject? Can you hide BF & FF by stopping down?