Focal Length

The focal length of a lens refers to the distance from the image sensor required from the image to be focused. In simple terms, people refer it as "zoom in" or "zoom out". A short focal length is regarded as a wide angle or "zoomed out", while a long focal length refers to being "zoomed in".


For example, the lens above has a focal length range of 18 - 55mm. This means that the focal length when it is "zoomed out" is 18mm, while the focal length when it is "zoomed in" is 55mm.

A short focal length would be known as wide angle
A long focal length would be known as telephoto


Focal Length vs How many x Optical Zoom


Most compact camera on the market nowadays boast of 5 x optical zoom, some even 8 x optical zoom. How is optical zoom related to focal length?

Optical Zoom is given by :
(Longest Focal Length - Shortest Focal Length) / Shortest Focal Length.

For example , the 18 - 55mm lens above has an optical zoom of (55 - 18) / 18 = 2 times

However, that is where the common misconception arises. Many sales promoter market their camera based on the amount of optical zoom they have, 5 times ... 8 times... 10 times...

However, it is the focal length range we are really interested in, not the amount of optical zoom.

An example to demonstrate my point. A Sony Cybershot T90 claims to have a 4 times optical zoom, and so does a Casio Exilim Z280. Which is better? Or are they the same? If we compare the focal length range, the T90 has a range of 35 - 140mm, while the Z280 has a range of 26 - 104mm.

Clearly the Z280 features a wider angle lens compared to the T90. Most compact cameras are used for group photos, and a wide angle setup would come in handy in order to squeeze everyone into the photo.

An extreme case to point, the Sony Cybershot H10 boast a 10x optical zoom, with focal length range of 38 - 380mm. Not wide angle compared to most compact, and too telephoto for the camera to handle.