Mmmm...what is 'mm'?

[dimsim]

Ok...one of those things I took for granted until someone asked me recently...


...how to explain the 'mm' used to determine the focal length of a lens? e.g. 50mm - thickness of the glass?

Hmmmm...

 

[Andy Ang]

You have already answered your own question when you ask the question.

How is mm used to determine the FOCAL LENGTH of a lens. e.g. 50mm.

Thus, the FOCAL LENGTH of the lens is = 50mm.

 

[creampuff]

Distance between centre of the optical group to its focal point measured in millimeters.

 

[Lenscapes]

millimetre

 

[morningstar]

mei mei ;p

 

[lsisaxon]

Distance between centre of the optical group to its focal point measured in millimeters.

When parallel rays of light enters the lens, it's the distance at which the rays converges to form a point. For a simple lens, this is the distance measured from the centre of the glass to the focal plane. For compound lenses, the effective focal length is more of a measure of the equivalent angle of view a simple lens of the same focal length would give when brought to focus over the same area.

 

[StreetShooter]

Picture worth a thousand words.

Top is short focal length, bottom is long focal length.

http://www.gutenberg.org/files/16593/16593-h/images/image_072.jpg