[glasbak]

I noticed that I got serious overexposed images, when using my A-2X-S converter with my A 200mm/2.8.
Without converter, the exposures are OK.

I tracked down that the problem has the same root cause as the metering error with non-A lenses and the green button on the K20D.
Not only the amount of light, but also the angle of the licht cone affects the lightmeter of the K10D/K20D.

That should not be a problem with A lenses you might think.
What happens when you use a 2X converter, the 200/2.8 becomes a 400/5.6, but the lenscontacts tell the camera it is 2.8 !
No problem for a decent lightmeter, as in my *istD, but for the flawed K10D/K20D meter it is.
The light cone is now equivalent to a 5.6 lens, but the camera thinks it is a 2.8 light cone, and applies the wrong lightmeter correction.
Lets change the lens contacts with aluminum foil and tape, so the camera reads 5.6 as largest aperture from the 200mm/2.8 + 2X converter.
As I expected, the overexpose problems are gone...

What about people who use the *istDs screen to get correct metering with non-A lenses and metering with the 'green button'.
It is unclear to me how this could be a solution, if the *istDs screen makes the metering less sensitive for the angle of the lightcone,
it should make faulty exposures in combination with A lenses, because the lighmeter is still corrected for the aperture it reads from the contacts.

George

[Adam]

I was quite confident that the A converters applied a corrective factor to the aperture, but since they apparently don't (at least for A lenses), why not just use an EV compensation equivalent to the loss of light?

BTW, you'd expect it to underexpose rather than overexpose, right?

[Lowell Goudge]

This was a hotly debated issue 2 or so years ago

The actual culprit is the focusing screen

As Adam pointed out you can set the ev compensation it needs about 1.3 stops of correction

Do a search for exposure and my name and you will find a curve showing the exposure error

[zinj]

Interesting. I had a similar problem, but using the 2x-s with a DA 300. Ended up either doing as Adam suggested and setting EV comp, or just shot manually after adjusting the initial exposure to get it correct. For some reason, I don't think it was an issue in all lighting conditions, but unfortunately I don't have my K20d any longer to double check. Intersting to hear a reason for it.

Andrew

[glasbak]

, why not just use an EV compensation equivalent to the loss of light?

BTW, you'd expect it to underexpose rather than overexpose, right?

Loss of light is not the problem, any proper lightmeter only measures the amount of light entering the camera.
The flawed K10D/K20D meter also changes readout when the shape of the light cone changes, and thats the problem.
Once determend, you can apply a fixed EV compensation for every pentax-A 2.8 lens in combination with the 2X converter (and for a F4 lens probably another compensation), but the cone of light also changes with close focussing.
Probably only Pentax knows how the lighmeter meters wrong as function of the angle of the light cone.

[glasbak]

This was a hotly debated issue 2 or so years ago

The actual culprit is the focusing screen

Do a search for exposure and my name and you will find a curve showing the exposure error

I find way too many posts with your name and 'exposure', but with a google search I found a curve.

Yes I know that the screen of the K10D/K20D is probably the source of this, but the camera system assumes this screen is present, and applies a correction factor to the metering, based on the aperture it reads from the lens contacts.

Replacing the screen solves it for non A lenses, but when you use A lenses, you still get the correction based on the lens contacts.

[Lowell Goudge]

There is no solution for the metering on the K10/20. I went as far as asking pentax if they would recalibrate my metering with an *istD screen but they said no they do not customize cameras. At least the k20mhas ev comp in manual. The k10 if you shoot manual you need to consider a correction every time you meter.