If we have a camera with a thin symmetric lens, then the lens will have two foci placed symmetrically about it. Each focus is the point where parallel light rays approaching from the other side of the lens converge.

Focal length
If from the lens each focus is distance f, the object distance is u, and the image distance v, then it is known that
      1/u + 1/v = 1/f     (Diagram).

How do the object and image sizes relate to the values of u and v?                         Check

 Now think about this relationship for a fixed value of f. If the object is distant (u is large), then the size of the image is

small?   large?    (No guessing!)

What is the focal length of your camera?  Check


We see that the figure contains
two similar triangles (in blue).

This means that

Height (Object) : Height (Image) = u : v .




The focal length of your camera may
be about 50 mm.

My camera features a zoom lens
with focal length varying
between 38 mm and 105 mm.