Good point. But since you still get the A/M switch on every auto aperture PENTAX screwmount lens, I like that system better. The aperture ALWAYS stops down fully unless it's REALLY fowled with oil because its forced hard by the body and isnt relying on a spring that may weaken with age. JCO
> -----Original Message----- > From: Peter Alling [mailto:[EMAIL PROTECTED]] > Sent: Friday, December 13, 2002 10:57 PM > To: [EMAIL PROTECTED] > Subject: RE: SMC-A 20mm 2.8 diaphragm > > > 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 > > > >
