1. ## Mmmm...what is 'mm'?

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...

2. ## Re: Mmmm...what is 'mm'?

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

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

3. ## Re: Mmmm...what is 'mm'?

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

millimetre

mei mei

6. ## Re: Mmmm...what is 'mm'?

Originally Posted by creampuff
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.

7. ## Re: Mmmm...what is 'mm'?

Picture worth a thousand words.

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

http://www.gutenberg.org/files/16593.../image_072.jpg

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