Problem is Im a bit of a noob, havent a clue what stop down metering is!
What is stop-down metering?
Normally your EOS camera will meter the light using the widest aperture possible for the lens. (ie: the aperture diaphragm will be fully open - f/1.8 or f/3.5 or whatever the lens supports) This lets you look through the viewfinder with the maximum amount of light coming in. When you take a photo the camera quickly stops the lens down to your selected aperture just before opening the shutter. Canon EF lenses rely on electronically-controlled aperture diaphragms, not mechanical ones operated by the camera body via a cam or lever. As soon as the shutter has closed again and the picture has been taken, the lens diaphragm pops back to the widest setting once more.
There are two exceptions to this mode of operation. First, if your camera has depth of field preview and you have it enabled, the camera will stop down the lens aperture to whatever you’ve set it to be. And second, if you have a lens installed that doesn’t have an electronically-controlled aperture diaphragm the camera will operate in stop-down metering mode - it’ll meter whatever the lens is currently stopped down to. Hence “stop-down metering.”
All Canon EF lenses contain a computer chip and various electrical connectors that allow the lens computer to communicate with the camera’s computer. Most third-party lenses for EOS also have EF-mount compatible electronics. However, if you buy a lens designed for another camera system and attach it to your EOS camera using a lens mount adapter you’re obviously not going to get the full electronic control you’d get with a true EF lens. In particular, if there are no compatible electronics then neither autofocus nor electronically-controlled apertures will work.
http://photonotes.org/articles/eos-manual-lenses/I yahooed, easier than writing it all out myself
Basics The lens is always closed to the aperture you select so the viewfinder will be dim in low light. IF you want to manually focus you should open the aperture, focus and then close it down again before taking the photo.