This exercise is to appreciate the effect of different focal lengths as to know which is the amount of view of different lenses. The point is, I believe, to understand that focal length only set the angle of view. This mean that if you magnify a part of image taken with a wide angle lens (short focal length) and then compare it to same image taken with a telephoto lens (long focal length), all the relationship between different objects in the scene remain the same. I will prove this in the end of this post.
One should also note that I use Nikon D700 camera body which is so called Full Frame (FF-body). This means that the sensor in this camera body is the same size as 35mm film camera film frame (24 x 36mm). The focal length of lenses is expressed in mm. If a digital camera sensor is smaller than FF sensor it records smaller area of the view with same lens as FF camera body. This difference is called crop factor. For example Nikon APS-C (Nikon DX) camera bodies have a crop factor of 1.5. So, on a DX camera, a lens of 100mm focal length gives the same amount of view as a lens of 150mm of focal length on an FF camera. (more…)