An A lens is enough for pentax to switch to matrix metering. A type lenses only transfer the max aperture and eject a electric contact from it's mount when the A setting is selected. Sigma and others have the contact permanently "ejected" and probably use an internal electrical switch. I never did real tests but matrix metering should at least know how to handle backlight correction.
It should be possible to convert any type of lens to A type. Bojidar's KMP site has the coding table online and drilling some holes and inserting some isolating epoxy should be enough. I never found a good solution for the lens based contact. The contact in the K mount is mounted below the "surface" of the K mount. I tried inserting some metal foil and voila the body sees a A lens. On 10/22/06, Godfrey DiGiorgi <[EMAIL PROTECTED]> wrote: > On Oct 22, 2006, at 8:24 AM, Shel Belinkoff wrote: > > ...While it might have been nice to know how to make the lenses > > provide multi > > segment metering, I really didn't care very much to waste time > > looking for > > a solution. Others here are so much better at fiddling around with > > such > > things. If it were an issue, I'd have just asked the list, and > > guys like > > Roberts would probably have an answer. > > > > BTW, isn't matrix metering what Nikon calls their system, and doesn't > > Pentax use the term "multi segment metering?" Are the two the > > same, or is > > there some difference between them? ... > > Nikon was the first to market with "matrix metering" in the FA model > and I guess the name is theirs, but it's stuck around as being a > generic term. Pentax calls it 'multi-segment metering'. Both are > implementations of the same idea, which I think is more precisely > called "evaluative exposure estimation": > > - Measure the scene at multiple points using independently sensitive > zones. > - Compare the relationships of the zones, weighted appropriately, > against a library of scene types to identify known exposure > evaluation issues. > - Take that type analysis plus the total average brightness of the > scene, along with the weighted segment values, to produce a good > guess at best overall exposure setting. > > For the simple, early generation systems like this, the information > required for open aperture metering is max aperture and exposure time > aperture setting. > > This can become arbitrarily trickier with more sophisticated > information and higher power processing in the metering system. > Factors that help aid scene type identification can be focal length > and focus setting, factors that help aid exposure setting can be > color balance, you can include color information (as Nikon does with > RGB matrix metering bodies). These later systems require lenses that > provide the relevant additional information for the specific metering > system in question. > > Ancient lenses that do not have chips in them to provide this > information electronically are not compatible with this metering mode > on the Pentax DSLRs no matter what you do, due to the way the > implementation was integrated with the rest of the camera's real time > control system. (The same is true for Nikon's D200 ... except that > they've provided a way to input some of the required data for a > specific lens that you mount and enable one of the simpler forms of > the metering mode.) > > In my experience, matrix metering evaluations with the Pentax *ist DS > resolve to be arbitrarily close to Center Weighted Averaging readings > UNLESS I set the option to link the AF and AE point and use the full > AF sensor array. Then I see some variations in the selected exposure > settings. > > Godfrey > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
