AF points and exposure

[candleman]

when changing AF points does it change the exposure?

i noticed something off on the weekend whilst snapping some pics of my boys digging a hole.
i was in portrait position and aperture priority.
when i switched from the centre af point to the lowest right af point it changed the exposure aby abuot 2 or 3 stops(just a guestimate)

I know what you're thinking "its the spot metering dummy"
nope as i was using pattern "matrix" metering and the amount i re-composed was so slight it was hardly noticable. so hence my surprise at the difference in exposure.
i havn't performed a test since as its slipped my mind

has anyone had anything similar happen? and is this normal?
OR
did i just have too much for lunch that day and i was halucinating.

[OsmosisStudios]

If they were digging, then they were moving. It could have been creating shadows or highlights, causing the camera to adjust. it usually changes the shutterspeed more than anything.

Cant say it has anything to do with the AF points, unless you were in spot or area metering

[gh patriot]

Ive noticed the same thing in live view when Im focusing manually. It says the default (and only) metering mode in live view is evaluative metering, however when I move the box that appears in the live view screen, the exposure changes even though the camera and subject are not moving.

[OsmosisStudios]

It would seem that the metering is done with a bit of a bias towards the focus point (on my D80, at least).

So it meters for the entire scene but with an emphasis on whatever happens to be in the focus point.

[Cuchulainn]

You are correct. Matrix Metering does meter the entire scene. It actually divides the scene into segments (a matrix), reads each segments brightness and analyzes the differences between them. The pattern which is determined by the matrix is then compared with a database of image patterns stored in your camera as well as other inputs. Mostly Autofocus Information. It is actually a pretty nifty system, particularly in the higher end bodies which use a a 1005 cell matrix vs. the 420 used by the D90 and lower bodies.

Originally Posted by OsmosisStudios

It would seem that the metering is done with a bit of a bias towards the focus point (on my D80, at least).

So it meters for the entire scene but with an emphasis on whatever happens to be in the focus point.

[candleman]

aaaah i see.. the makes sense.
thanks Cuchulainn & OS
if it ever becomes an "issue" the exposure bias is a quick fix i suppose

[Mr Guy]

As a side note, on my Pentax K110D, there's specifically a setting to link the auto focus and the metering. I'm not sure how the weighting works whether that setting is off or on, but you may see if you can dig up a similar settings.