Learn Photography
Let's say you have a powerful telephoto zoom.
You zoom from 400 mm to 800 mm.
It's a change of 100%.
Let's say you put a wideangle zoom on your camera.
You zoom from 12.5 mm to 24 mm.
Even though you didn't move many numbers (12.5 to 25), it's the same percentage change as going from 400 to 800: 100%.
So, the takeaway is:
• Wideangle focallength numbers are small numerically—but represent a large change in magnification.
• Telephoto focallength numbers are large numerically—but represent a small change in magnification.
The chart below is another way to make the above fine point.
The focal lengths below are for fullframe digital SLR cameras.
50 mm is considered the normal focal length, i.e., the scene is not magnified.
Not magnified means that the 50 mm focal length isn't wideangle or telephoto.
The power value for the 50 mm focal length is therefore 1x.
You can see below that going from 12.5 mm to 25 mm is a change of 100%.
You get a big change in magnification by moving very few numbers.
Going from 400 mm to 800 mm is the same change, 100%, but its a bigger change in the numbers.
Power Change From 50 mm 
% Change Between Rows 

12.5 mm  1/4x  100% 
25 mm  1/2x  100% 
50 mm  1x  0 
100 mm  2x  100% 
200 mm  4x  100% 
400 mm  8x  100% 
800 mm  16x  100% 