un-true resolution sensor area?

I asked this in another thread but it got overlooked:

When you take a picture at a less than actual resolution what happens? A/B?


lets say you take a 2MP image with an 8MP camera
http://blacklinux.com/pages/public/cars/skyline/new/CMOS.bmp
which pixels are used on the sensor? a central rectangle of the actual resolution even though it's lower than the whole sensor resolution or.... B?

If it were A wouldn't that change the relative dimensions of the whole optic system?

But if it were B? then if there are multiple incriments lower than the actual resolution then the actual scale/ratio would be inacurate? Does anyone know for sure?

Did I word this question well enough for it to be understood? it sounded kind of stupid when I re-read it :(

This question is technically way beyond me, but when I take a 640x480 picture on my 6.0mp camera, I still get the whole "field of view" that I get with a full resolution picture - I think it just uses far less pixels per inch...

I did not overlook your question last time either but I for one I have no idea if when selecting lower resolusion that native only a pact of the sensor is used or if some sensor sites (i belive they also call them "light wells") are being deactivated.

I wold also like to offer a 3rd option: perhaps the photo is still being taken at full resolution by the sensor and during processing its quality is downgraded for storage. If this is true there might be a big difference among cameras and their "lower rez than native" pictures, based on their reduction algorithms.

EDIT: i just talked to my roomate who is in his PHD program of electrical engineering so he knows his chips. He confirmend that my 3rd option is probably the most likely one since it would be far harder to disable part of the sensor.

As far as I know most simply apply mush strong comresion ratios, and re-sample the image to a smaller size.
Effecitvly you lose a lot of image quality when you shoot at below max resolution.