Consider light leaving a point on a source placed on the
principal axis, and relatively close to
a converging lens, as shown
below. 

The light is converged to a real image point on the axis. 

We now move the source further from the lens, as shown in the
next diagram. 

If we assume that the same lens will produce the
same deviation of the light, then, clearly, the rays will meet
nearer to the lens than in the first case. 
We therefore conclude that as the object distance increases,
the image distance decreases. 
This is confirmed by experiment. 

Now imagine moving the object much
further away 
Imagine that the three rays on the next
diagram come from a point on a source 25m away or more. 
Those rays will be very nearly
parallel. 
Also, if the object is then moved, say, ten
times further away, no change would be expected in the
image distance. 
Once the object is far enough for the rays
to be considered parallel then the image distance will
reach a fixed value. 
This constant, minimum value of image
distance (for a given lens), is called the focal length of the
lens, f. 



The point on the axis where the parallel rays meet is called the
principal focus (or principal focal point), F. 

Rays which are parallel to each other but not to the principal
axis of the lens are converged to meet at other points the same
perpendicular distance from the lens as the principal focus, as
shown below. 

These points define the focal plane of the lens. 

The Power of a Lens 
In everyday speech, if one mentions a
"powerful magnifying glass" one thinks of a converging (convex) lens
with strongly curved surfaces. 
For example,of the two lenses shown below,
the second "fatter" lens would make the better magnifying glass. 


We notice that the lens with the more strongly curved
surfaces has a shorter focal length. 

We define the power of a lens as follows 

where, if the focal length is in metres, the power is in
Dioptres (D) 

Thus, a converging lens of focal length 50cm
has a power of 2D. 
Similarly, a diverging lens of focal length 25cm, has a power of
4D (see here for explanation of the negative sign). 
