andre... what u r talking about is a magnification. there is many other aspects in optics. one of the most important (for what michael askes) is angle of view.
lets take... 24mm lense (on small format) and 50 mm lense. and lets divide our negative (horizontally) into three parts (for ease, but we can make it it in tenths and more). so we are talking about 24x36 fromat (very popular, rite?) and we talk of the 36mm base divided in 12+12+12.
now im taking a photograph of say.. some person.
1. 24mm lense, distance=70cm, magnification=0.034
2. here im going to keep the magnification, which means - i will have the same frame of person.
so: 50mmlense, distance=145cm, magnif=0.034.
so i have just the same frame - person on white clear wall or black clear cloth (so taht the background will not confuse us)
what is the defferance...
the differance is thaht the 24mm lense has angle of view of about 84degrees, while the 50mm lense has about 45degrees. (sorry that the numbers are not totally acurate, since when u move very close the focal lenght will not stay 24mm but nominally slightly longer, but it is not that important now)
now, we have made dividation of negative surface in three parts (for ease). what happens is taht the center part has angle of view about the same on both 24mm and 50mm, while the sides has totolly diferent perspective (but symetrical).
if we consider the 50mm lense, the perspective of the centere and the sides is about the same (on longer lenses like 90mm is even more equal). on the 24mm lense it is very far from being the same - (metphorically) it is as if the left side looks at left and rite side looks at rite.
of course, add to it the front and back differnce.