A lot of times I hear people saying that you can simply get the effect of the different focal lengths simply by going closer or further to the subject. Not only do you get different perspective of the subject, but for outdoors, the background perspective changes with different focal lengths. Attached here is just a casual sample of the difference between a Nikon 50mm F1.4 shot at F2.5 and a Nikon 180mm F2.8 shot at max aperture(f2.8). Even though the subject fills about the same area in the frame, notice not only the difference in subject perspective but also the background, where in the 180mm, the background appears more isolated, closer and also with less Depth of Field. That is the usual consideration I have when I think of focal lengths. Cheers!