what does the "mm" relate to?

Can someone explain to me what the mm refers to when talking about lenses?

18mm 200mm? What is it measuring exactly? Does it refer to the thickness of the lense? And assuming it measures distance is it like a road map (as an example) where 1in=1 mile type thing? Where 18mm= 18 feet? or the likes?

sorry for such a profound question.

distance between the lens and the back of the camera where the image is formed.
ie: in a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor.

mm = milimeters, "For an optical system in air, it is the distance over which initially collimated rays are brought to a focus."

Check this out:
http://en.wikipedia.or​g/wiki/Focal_length
The parts immediately after the part I quoted above explain how a smaller focal length (mm) is a greater optical power, and thus less magnified, view than a longer focal length provides (of course, theoretically - some wide angle lenses focus much closer to their front than some telephoto lenses and so can actually magnify an object more, but this is only at their extremes - at infinity focus or other long distances a wide angle gives a less magnified, i.e. objects appear smaller, view than a telephoto or long lens would).

Ok so much like a magnifying glass in reference to your eye.... the further you pull the magnifying glass away from your eye (distance) the closer the [grass] will become through the lense.

Ahhhh ok.. I get it now.

What it is referring to is the focal length of the lens. Take the example of a single thin lens (like in your reading glasses):
[\] () -
Thing lens focal point

The focal point is the point where all light rays coming through the lens intersect. In the case of a camera lens with different elements, this is more complicated. The number measures a sort of average focal length. The sensor is put near the focal point.

See this link:
http://www.photoxels.c​om/tutorial_focal_leng​th.html

Apparently my 3 minutes to type was too long. . .

aussiedee wrote in post #13073988
Ok so much like a magnifying glass in reference to your eye.... the further you pull the magnifying glass away from your eye (distance) the closer the [grass] will become through the lense.

Ahhhh ok.. I get it now.

Bingo! The other consequence, of course, is that as you get more magnification, the part of the page (grass) you can see is lesser, and this is the part photographers usually choose first: Magnification is important but generally photographers are looking for typical angles of view, because at typical distances at which most all lenses are used they can magnify an object to a useful (and normally used) size.

When magnification is the primary concern, then you should look carefully at macro lenses (if you want more), and look at minimum focus distance and/or rated magnification power - the MP-E 65mm f/2.8 is a 1X-5X life size lens - or wide angle lenses (if you want less magnification, though generally this is not wanted, people instead generally are thinking in terms of how much of the surroundings they can fit in).

Ok.... only reason I ask is because I am enrolled in an online photography course and my first assignment has me choosing lenses for certain situations.

I got a bit "lost" somewhere and needed some clarification.

pkim1230 wrote in post #13073980
distance between the lens and the back of the camera where the image is formed.
ie: in a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor.

I first thought this must be the back of the lens, since the front is (usually?) much further away. But checking a lens, it looks like the back is not that distance either. I'm checking my 15-85 lens. Zooming from 15mm to 85mm I can definitely see the back of the lens moving more from the sensor. But e.g. from 70mm to 85mm it doesn't move at all anything close to 15mm. The front moves mroe though (and possibly things in the middle). Even at 15mm where the back is closest it doesn't look like the lens is 15mm from the sensor. So...

Edwin Herdman wrote in post #13073986
mm = milimeters, "For an optical system in air, it is the distance over which initially collimated rays are brought to a focus."

So I guess this point doesn't necessarily relate to a physical distance? Maybe a very old and "regular" lens design it is the distance too, but with newer designs there are ways to control that but the physical way the lens is built is different?
If I compare my 15-85 at 15mm and my 60mm prime, neither the front or back are that different from each other. Also the lens moves when focusing to a different distance, not just the focus length in mm.

So I don't know

aussiedee wrote in post #13074047
Ok.... only reason I ask is because I am enrolled in an online photography course and my first assignment has me choosing lenses for certain situations.

I got a bit "lost" somewhere and needed some clarification.

If you are enrolled in a photography course it may help you to know that the word is spelled "lens". It'd be mighty embarrassing to hand in an assignment with it spelt wrongly - a bit like a medical student writing that a hande is found on the end of an arme.

pkim1230 wrote in post #13073980
distance between the lens and the back of the camera where the image is formed.
ie: in a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor.

Then explain to me why the 24L is bigger(longer) than the 50L

Isnt it denoting the distance between te back of the lens and its focal POINT? that would make a lot more sense regarding FoV.

In practice, guessing the focal length of a lens by size isn't quite accurate because lenses are not as simple as a diagram may show you, here's a good tutorial on the subject: http://www.bobatkins.c​om …asuring_focal_l​ength.html

The complex optical formulas of modern lenses really screw around with these things, so it's something you'll have to calculate yourself, or ask the lens engineer who designed it. Also, the focal length of lenses changes as you focus, unless there is a built-in system to compensate for this as in those high-end lenses used for video and film.

xarqi wrote in post #13074264
If you are enrolled in a photography course it may help you to know that the word is spelled "lens". It'd be mighty embarrassing to hand in an assignment with it spelt wrongly - a bit like a medical student writing that a hande is found on the end of an arme.

is correct.....

"choosing lenses" -or- "choosing a lens"....

I believe was talking about your first post:

you have an "e" on the end there. Same way some people seem to thing "color" is more correct than "colour"

Kolor-Pikker wrote in post #13074396
I believe was talking about your first post:

you have an "e" on the end there. Same way some people seem to thing "color" is more correct than "colour"

Well that would make more sense.... LOL.... nope, honest mistake in that post, but he quoted the other post.

Kolor-Pikker wrote in post #13074396
Same way some people seem to thing "color" is more correct than "colour"

Which of course it's not!