> Subject: Change in stop down mechanics K vs. M42 was RE: SMC-A 20mm 2.8 > diaphragm > > > What you say is true. If you stick with Pentax(Takumar) you're all set. > > My only point on the K mount change is that it is probably easier to > engineer
Right and build, Right therefore simpler with fewer moving parts Right > so it > 'should' be less expensive Right > to manufacture Right > with equal reliability. > WRONG! The K design is more likely to fail to stop down ( Actual picture taking ), while the Takumar design is more like to fail to reopen ( Pictures will still be fine). Both cases caused by a weakened old spring. JCO > At 11:38 AM 12/13/2002 -0500, you wrote: > >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 > > > > > > > > >
