Just got a Pan 20 1.7 and noticed something. The lens seems to keep a constant aperture of around 2.8 when at rest but when focusing it opens all the way and then stops back down (regardless of what is set for aperture). When actually taking the picture it stops to what is set like normal. Why would it be set up like this? Wouldn't wide open be the default at all times like an SLR? Can anyone else confirm this?