See you're picking it up already Tom!
A lower iso setting will give you a better quality picture than something around 1600. The transparency film i used to use was iso 50, on a light box it looked like you could literally walk into it.
It's all about finding a happy medium. If your using a long lens then you will need a shutter speed that is quite fast to eliminate camera shake, so that has to take a priority. On Canons there is a shutter or aperture priority setting (Tv and Av) BUT the camera meter is still just reading light from the center of the view finder. That could be anything and on evaluated metering it just uses an average based on the brightest and darkest areas, so the whole thing just looks wrong. So say your using a 70/200 MM Zoom, put the camera on manual select a shutter speed around 250/1- 320/1 Sec, try and stop down the aperture at least a bit so your not wide open, then adjust the iso to suit based on a spot meter reading from a mid tone, grass is best.
Again you should have a read on how light meters work, it will really help. There are two types Reflective, which is the type that are built into cameras and Incedent (Not sure of the spelling) which measure light falling on to the subject, like the ones you see studio photographers holding in front of peoples faces when they test the flash. They usually have a opaque white dome on them. The advantage these have over the meters in cameras is they can't be fooled into giving incorrect readings by colour or very bright tones like snow. All light meters see everything in 18% grey so you have to help them by taking readings that are mid tones like grass or skin.
