For DSLR beginner, we always see the lens with the number such as 16-35mm, 24-70mm, 70-200mm. After that I know that’s call ‘Focal Length’, but as a beginner, i don’t even know what is focal length. 
What is Focal Length?
According to the description from wikipedia, the focal length of an optical system is a measure of how strongly it converges (focuses) or diverges (diffuses) light.
The picture above i “borrow” from Understanding Camera Lenses. From this picture, we can know that:
- as the focal length shorter, as the angle of view wider
- as the focal length longer, as the angle of view narrower
Angle of view wider, means that the object can be focus is nearer. So normally those lenses with short focal length (below 35mm) is for wide angle use, focal length with 35mm to 70mm is normal use (example: portrait), and the 70mm and above is for telephoto (example: sports).
| Lens Type | Focal Length | Best Use |
| Wide Angle | below 35mm | Landscapes and Interiors |
| Standard | 35mm to 70mm | Portraits |
| Telephoto | 70mm to 300mm | Portraits and Sports |
| Super-Telephoto | 300mm and higher | Wildlife and Sports |
For more information about DSLR Lens, may refer Understanding Camera Lenses.