[StarDust]

Just at this very moment, bored at the computer I was going to take a picture of my desk. The camera meters it to ISO 100, F19, 20 sec. But as soon as I half-press the shutter button (auto focus for those who have that turned off) it starts fluctuating between 15"-10". I release the button and it goes back to 20". This happens every time I repeat the procedure

So here's the question: What happens when you auto focus? There are no moving parts involved, no mechanical sounds when it's in focus. Is there some power consumption involved?

Any ideas?

If there's any more info needed, please ask. This is quite odd behavior, don't you think? The best metering was 20 secs according to me.

[ivanp]

The display in viewfinder also slightly affects the metering. When you press the shutter and AF-confirmation appears, the camera meters more light, thus decreases exposure. If at the same time the flash-sign flashes on the display, it would explain your metering fluctuation.

[KjetilH]

My guess it's that the custom setting (no. 7 for the K20D) " Link AF Point and AE" that mess with the exposure. Basically, with matrix metering, the priority on light metering is given to where the camera selects the AF point.

Your desk is probably evenly lit. In that case, it is probably not the AF-AE setting.

[kittykat46]

Just at this very moment, bored at the computer I was going to take a picture of my desk. The camera meters it to ISO 100, F19, 20 sec. But as soon as I half-press the shutter button (auto focus for those who have that turned off) it starts fluctuating between 15"-10". I release the button and it goes back to 20". This happens every time I repeat the procedure

So here's the question: What happens when you auto focus? There are no moving parts involved, no mechanical sounds when it's in focus. Is there some power consumption involved?

Any ideas?

If there's any more info needed, please ask. This is quite odd behavior, don't you think? The best metering was 20 secs according to me.

I've had the same behaviour from the camera before. Yes, its the "Link AF Point and AE" custom setting for Matrix metering. It kicks in only when you do AF.
If you turn it on (default is OFF), it can be a help - and a problem at other times, depending on the lighting situation.

If the scene has some bright lights or brightly lit portions which are not your focus subject, it helps bias the exposure system towards exposing your focus subject correctly (kind of like focus-point weighted metering).

On the other hand, it can lead to inconsistent exposure of the same scene from shot to shot due to changes in your focus subject.
I had a minor disaster the other day, a 10-shot series of pretty much the same scene location was exposed differently each time because I switched focus points each time.

So use the "Link AF Point and AE" custom setting wisely - it can be a help, and a hindrance.
My suggestion now is to switch it off, as the K-7's metering is anyhow more accurate than previous Pentax bodies.

[StarDust]

Yeah, the "Link AF Point and AE" is on. I see, so when I don't focus, the camera meters the entire picture, and when I auto focus the camera meters more towards the specific focus point. Good to know.

Thanks guys!

My suggestion now is to switch it off, as the K-7's metering is anyhow more accurate than previous Pentax bodies.

I'm using a good old K10D