Among other things the lens automatically works correctly in stop down mode on un-coupled devices and when mounted reversed.
It can be a pain to try to use certain 3rd party m42 lenses on un-coupled extension tubes or for that matter a K to M42 adapter since they lack auto manual switches. Pentax saved a few yen per lens by removing the need for those switches on the K-mount. At 07:52 AM 12/13/2002 -0500, you wrote:
Anybody know why pentax flip-flopped their aperture actuation when they went to k-mount? On the screwmount lenses the aperture is normally held OPEN by a spring and is forced closed by the camera at the time of exposure. On the kmounts the spring holds the aperture CLOSED, and the camera forces it open except at time of exposure it lets go and the spring does the work. At least when the spring gets weak with the screwmounts, the lens can still be used manually. Not so with the kmounts. JCO > -----Original Message----- > From: Mark Roberts [mailto:[EMAIL PROTECTED]] > Sent: Friday, December 13, 2002 7:44 AM > To: [EMAIL PROTECTED] > Subject: Re: SMC-A 20mm 2.8 diaphragm > > > Anton Browne <[EMAIL PROTECTED]> wrote: > > >Would some of you SMC-A 20mm 2.8 owners check the action of the > diaphragm via the lever on the lens please. I have just bought > one and it seems very sluggish. Most of my other Pentax lenses > snap shut, but they do vary in their snappiness. This 20mm 2.8 > glides shut and it seems scarcely fast enough to fully close when > set at f22 with a fast shutter speed. I'm wondering if it's faulty. > > I think you have a problem. > The diaphragm in my A20/2.8 is just as zippy as any other lens I have. > > -- > Mark Roberts > Photography and writing > www.robertstech.com >
