Could anyone provide their experiences about lens length vs. distance vs. f number?

For instance, I have heard of many photographers using a 400mm lens to take a picture. However, by looking at the photo, I have no idea how far away the subject was and what f-number was used.

If I had a 400mm lens, how close would I need to get to an animal to get a good photo? How fast of a lens would I need to buy? How far away can I stay for a 300mm lens?

Any rules of thumb would be appreciated. Thanks!

i have a 300 2.8 and a doubler i have used it to photograph eagles. it seems no matter how much lens you have you always want more. i dont know what the rule of thumb is for distance. it sure gets expensive when you start buying big lenses. there are some companies that rent big stuff you might consider doing that. i am thinking of getting rid of the 300 it is a nikon afs

Hi Scott,

Are you saying that you are effectivly using a 600mm lens? How close (in yards) do you need to get to an eagle to fill the frame with a 600mm? How close to fill the frame with a 300mm? I am a beginner at all this. Thanks!

pretty damn close even at 600mm. i cant remember exactly, but it seems like 30 or 40 sorry i am probably not helping much i took phots of it in the nest which was probably like 80 or 100 yrds and the nest was in the center of a 3x5