Originally posted by i_trax also with old PKA zooms without constant F number, like my Sigma 35-70mm 2.8-4.0.
It has just a single contact and camera body has no way of knowing what focal length was chosen, so at wide end is OK to use all auto metering modes but at tele is better to use Green Button and stop down metering, otherwise camera will use 2.8 instead of 4.0 to do exposure calculations.
Originally posted by Tanzer I don't think this is true, based on some time I had with a PKA zoom (Quantaray f/2.8-4.0). The camera just thinks it got darker outside (or maybe inserted a ND filter) when you zoom to tele, and it recalculates the shutter and/or aperture from there. It doesn't need to know that the "f/2.8" is actually f/4.0, it just tries to stop down the lens by whatever number of stops from where it is when wide open.
And for me, oddly, stop-down metering made things worse. I am learning that this is probably because of the non-linear focusing screen's EV response that some others have described. The wide-open metering on the Quantaray always worked right.
There are truths and half truths in what is being discussed here.
The stopped down metering, / green button etc, was initially introduced in the film era. My PZ-1 had it for example. The function at that time, was a single button push to return you to what the metering considered "correct" exposure. on the PZ-1 the only difference was that we had an uncrippled K mount, so the camera knew how many stops down from wide open the lens was.
Now, come to the DSLR era and no aperture coupling. With manual aperture lenses, the camera stops down to meter, with A lenses it reads the scene wide open and sets the settings accordingly (just like my PZ-1 did)
The real issue with DSLRs is the non-linear performance of the focusing screen at different apertures. There have been a ton of debates over the years about this, and i have even taken the time to test this on several cameras
the latest iteration of my tests with the help of other forum members is attached here
the issue with variable aperture lenses is that some even in MF days had solutions to report the correct aperture through the 5 aperture contacts. sigma for example had a sliding switch that as you zoomed (2 ring zoom) it reconfigured the contacts. others as suggested just assumed that the day got darker, and with the limited range zooms in the MF days, if you were off nominal aperture by +/- 1 stop there was no issue, but with the K10D and later DSLRs this could lead to big errors, same with adding a TC that fed lens data straight through. 1 2x TC om my sigma 70-200/2.8 caused a 1.3 stop over exposure on the K10D (it was obvious why when looking at the chart) and it also plays hell with P-TTL flash, which absolutely MUST know the true open aperture to get things right.