The focus is not accurate on sigma sd15

After several days first touch, I found that the focus system of sigma SD15 is totally inaccurate. With its own lens, auto-focus is less accurate than manual-focus (the focus warning is useless, you need your own eyes). With m42 lens, the focus warning (beep) is greatly inaccurate. Is there any method to adjust sigma sd15"s focus system? Thank you in advance.

PG