Might be a stupid question but...

[Silverz]

what part of the lens is the mm measurement taken from? for example a 50mm lens... which part of it would be measured as 50mm?

[kirbinster]

Not a stupid question at all. Its one that I have wondered about myself but never thought to ask -- so i am looking forward to the answer.

[nemesis256]

It's the measurement for the focal length, which is the distance between the lens and the focal point (the point where the light converges). In your camera it's the difference between the lens and the camera sensor. Essentially this number tells you how much zoom you have, and how narrow or wide your lens is.

More info: Photography 101 - Lenses and Focus

I think this is right. Someone correct me if I said anything off.

[BryanC]

Originally Posted by nemesis256

It's the measurement for the focal length, which is the distance between the lens and the focal point (the point where the light converges). In your camera it's the difference between the lens and the camera sensor. Essentially this number tells you how much zoom you have, and how narrow or wide your lens is.

More info: Photography 101 - Lenses and Focus

I think this is right. Someone correct me if I said anything off.

Yes, that's pretty much the basic answer. Distance from the lens to the focal point (sensor). It can get more complicated as you add in variables as in amount/types of elements in the lens, sensor size, etc. Pretty soon you would be dealing with formulas such as:

1/f = (n-1) [ 1/R1 - 1/R2 + (n-1d) / nR1R2]....and it starts sucking the fun out of things..

[Doug Pardee]

Originally Posted by Silverz

what part of the lens is the mm measurement taken from? for example a 50mm lens... which part of it would be measured as 50mm?

It's a measure of the magnification power of the lens. It tells you that an object of size x photographed from a distance of that same x will be projected at a size of 50mm on the film/sensor.

Simple multiplication then applies. With that 50mm lens, an object 500 feet across shot from a distance of 1000 feet will project an image 25mm wide on the film/sensor. If you know the size of your sensor, you can figure out how much of the captured image the object will take in. (There will be some variation when focusing on close objects because focal length is stated at infinity focus and almost always changes as you focus.)

Perhaps an easier way to use focal length, though, is to learn what the approximate "normal" focal length is for your camera. For most DSLRs, it's around 30mm. Smaller numbers indicate wide angle, larger numbers indicate telephoto. The smaller the focal length the wider the angle, the larger the focal length the stronger the telephoto.

A number of half the "normal" focal length is about as wide-angle as you're likely to find without going fisheye, and most wide-angles will be more like 60% of normal. A focal length of about twice the "normal" for your camera is a short telephoto good for doing portrait photography. 3x the "normal" is a medium telephoto. At 4x the "normal" you've got a pretty strong telephoto, about as strong as you can hope to control hand-held without some kind of image stabilization.

[Silverz]

Thank you for the answers.

I think that about answers my question