Originally Posted by
revi
Well, easiest way to see the effect would be to take one of your pictures where you know the focal lenght used. Let's say it's a 100 mm.
Then, to see the effect of a 200 mm, crop so that you use half the height and width of the original (so from a 3000x2000 pixel image, take a 1500x1000 section). To see the effect of a 400 mm, take a quarter (still starting from a 100mm original focal length).
That exercise will show you what the lens sees, although with a real telephoto lens you would of course have the full resolution of your sensor.
What it doesn't show is the larger weight of a 400 mm lens (or the much smaller max. aperture); also, a longer lens is more difficult to use in general (vibrations, aiming, etc). And there is the cost as well.
Perhaps OP could give some more information about the reason forhis question?