The lens determines the magnification of the image or the angle of view, depending on how you want to look at it (pun intended). We may select a lens to make a subject appear smaller or larger and this is sometimes due to a physical barrier, we can't get close enough or far back enough. Often however, this is not the case and we select the lens merely out of convenience to avoid walking around to look for the strongest composition. Shameful.
Besides, lens selection should be based more on artistic ideas than on necessity of our position.
So, selecting a lens is (or should be) a little more involved than just how big the image appears, after all we can often walk away or towards a subject to set the image size. Say we want an image of a tree to fill the frame top to bottom. From where we stand it does just that with our trusty 50mm lens but if we move closer it fills the frame just the same with our 28mm, or if we move away it does the same with our 100mm. So in this case how (and why) do we choose a lens?
It comes down to the perspective we wish to portray. Even though the tree is the same size image in our three examples the other elements of the scene are changing dramatically. The hill behind the tree (perhaps twice as high) is the same image height as the tree with our 50mm, much smaller than the tree with our 28mm, and taller than the tree with our 100mm. That's not all, the apparent distance of the objects changes too, from the hill appearing twice as far away, to appearing much farther away, to appearing close. The change is due to the relative distance between us and the two objects changing with the lens we use as we get closer or further from the tree. So the lens we use might depend on how we want the image elements to relate to each other, in size; importance; focus; clarity; and more.
One example is the "portrait" lens, usually in the range of 70-100mm, chosen to keep the eyes, nose, and ears all in proportion. Take that same portrait with a 22mm and you might do Jimmy Durante proud! Try a 200mm and the facial features flatten out too much.
A note here, the lens does not change perspective, only relative distance does that. A series of photographs taken from the same spot with a 28mm, 50mm, and 100mm will have the identical perspective. The reason we say the lens changes perspective is that we also usually change our distance from the subject when we change the lens.
Thanks for any explaination you guys can share.
