Why only objects within a limited range of distances from a camera will be reproduced clearly?

Question

From http://en.wikipedia.org/wiki/Camera

Due to the optical properties of photographic lenses, only objects within a limited range of distances from the camera will be reproduced clearly.

What are "the optical properties of photographic lenses"?

Does "the optical properties of photographic lenses" mean that the range of possible distances between a len and a film in a camera is limited? My understanding is that given distance d1 between the target and the len, and distance d2 between the len and the film, and the focal length f of the len, 1/d1 + 1/d2 = 1/f. If we have a limited range of adjustable possible values for d2 due to the limited size within the camera, then there is also a limited range for d1.

Thanks.

Answer 1

Due to the optical properties of photographic lenses, only objects within a limited range of distances from the camera will be reproduced clearly. The process of adjusting this range is known as changing the camera's focus.

The next sentence makes it clear that they are saying that not everything is in focus: only a fixed distance and a tolerable depth on either side. They are not explaining why focusing is necessary, only that a lens works that way.

It's not saying that the lens has a limited focusing range. It is saying that a focusing adjustment mechanism needs to be included so you can choose the focal distance.