7-14, 16-35, 14-140 and 8mm etc what does it mean?

[Big fish63]

Hi,

what do the values in mm on a lens actually mean? I understand a 7-14mm would give a wide shot and a 300mm would be telephoto but what do the values mean, where do they come from?

Thanks,

 

[pafindr]

The mm is the distance from the focal plane to the lens.
In a digital camera the focal plane would be the sensor, in a film camera the focal plane would be the film.
So 7mm is the distance from the focal plane (the sensor) to the lens.

A 7-14mm means the lens can go from 7mm to 14mm away from the focal plane.

 

[Big fish63]

Thanks

 

[RonFrank]

The mm is the distance from the focal plane to the lens.

By 'the lens' he means the front of the lens, not the rear. Also there are other factors like IF lens designs that do not follow this standard. There are many websites that provide in depth explanations.

 

[blibecap]

What you are referring here is the focal length. It has nothing to do with the physical length of the lens. My 18-200 does not get 182mm longer and it is much longer than 18mm when it is short. It is explained here, drop down to the photography section.

Focal length - Wikipedia, the free encyclopedia