how is MM standard derived

[candleman]

Where does the mm term for lenses come from?

Is it the distance from sensor to some calculated point in space?
How is the standard derived?

aperture is a fraction of it.. so how is mm derived?

[inkista]

Focal length is the distance from the center of the lens to its foci.

Aperture is the ratio of the focal length to the diameter of the entrance pupil (opening) of the aperture: f/d. If you're at f/2, and your focal length is 50mm, then the diameter of the entrance pupil is 25mm = f/2. where f is the focal length. That's why apertures are written with "f/".

[OsmosisStudios]

Im sure Inkista can answer (oh great goddess of all photography...), but Im pretty sure the mm is a measurement taken from the film/sensor plane to the nodal point in the lens. That is, your lens flips the image onto the image plane (like your cornea to your eye), and where that "crossover" point is.

EDIT:

so I was sorta right. Hurray!

[candleman]

that does make sense.. but if a lens has 15 elements... is that taken from the front element?
(you dont have to answer that)

thanks both of you fine people.
i had a look (briefly) at the wiki link there Inkista, and i'll need to sit an chew on it before i fully understand it.

i was discussing slr cameras with someone at work and i got a question "what do you mean by mm"
so i went "err99... not sure exactly.. "

i was able to explain aperture though.. as you did Inkista.

i understand the crossover stuff.. but how it relates to the sensor is another story..
i'll chew on it and come back when its digested
thanks again.

[OsmosisStudios]

Going from my understanding, this is what it means (see attached).

The focus point moves as the lens extends, thus changing both the focus and the focal length.

Could be wrong, but thats how I understood it.

[jprime84]

Look on top of your camera. Mine actually has an indication on top o the exact point of the focal plane