It's also got something to do with the angle of incidence of the light on the metering screen. The K10D (and later cameras) have a screen that's quite different from the one in the D bodies. The K10D compensates for the non-linear response of the system, but it assumes metering is being done wide open (I don't know what it assumes for a pre-A lens, but I'd guess f/2.8 or f/4.0). But if you're using stop-down metering at a small aperture the K10D calculates the exposure incorrectly.
I'm glad I decided, back when I got my PZ-1p in 1995, to stick with a guideline of "A or later" for lens accquisitions. On Fri, May 09, 2008 at 11:03:57PM -0500, Bob Sullivan wrote: > Bill, > > I've found some exposure error as well with K and M series lenses and > the green button. > Somebody else mentioned it, and I don't understand it? > > I thought the green button closed the aperture down and measured light > exposure values. > I can't imagine how this is off, but here is an example with K35/2 lens. > (The lens was set at different apertures and green button pressed > before exposure.) > > http://picasaweb.google.com/rf.sullivan/K352ApertureTestOnK20d > > As you move away from wide open (f2) to smaller apertures (f12.8), > light moves toward overexposure. > (You can see more light in the big shadow on the right.) > This did the same with an M400/5.6 although I didn't post the details. > > All I can figure is that it has something to do with the aperture > lever being linear. > Since the A lenses (and later), the movement has been linear. > With the earlier M and K lenses, the movement is non-linear. > Is it possible that the K20D measures the right exposure, and then > sets a matching A lens aperture, > not closing the lens all the way down to the f12.8 we have set? > > Regards, Bob S. > > > > On Fri, May 9, 2008 at 8:56 PM, William Robb <[EMAIL PROTECTED]> wrote: > > One of the guys on the Pentax Dysfunctional Web Forum chalenged me to test > > my K20 exposure > > system with manual aperture lenses (green button manual). > > The camera failed miserably, over exposing by quite a large amount at all > > f-stops but maximum > > and minimum. > > Would it be possible for some others of us to run the following test: > > > > Put a non A series lens onto their camera doesn't matter which, but it > > would be nice to have > > details, and run a series of exposures from maximum aperture to minimum > > aperture to test the > > linearity of the metering and perhaps report back. > > > > Sorry if this has been discussed before. > > > > I ran the test with several lenses, and then put an original equipment istD > > screen into the K20 > > as buddy thought that the screen itself was at fault, and insisted that it > > had been a problem > > with every DSLR since the istD. The screen was not the problem. > > > > Thanks > > > > > > -- > > PDML Pentax-Discuss Mail List > > [email protected] > > http://pdml.net/mailman/listinfo/pdml_pdml.net > > to UNSUBSCRIBE from the PDML, please visit the link directly above and > > follow the directions. > > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
