Lens question

[Fastmarc]

In my question to learn as much as I can, there is still something I am not sure about. Lens.
For example, with a 12-24 lens, what does the 12 and the 24 represent?

 

[Warren_L]

A lens that is referred to by two numbers like that is a zoom lens (lens that can change it's focal length between a range of focal lengths. In this case, it is from 12mm to 24mm. The smaller the focal length, the wider the angle of the view at that focal length. Focal length is the distance from the optical centre of the lens to its focal point (where an object at infinity is in focus - usually the focal plane where the film is or sensor in a dSLR).

Why is this? Think of this analogy as it relates to angle of view. When you are looking out a window, the closer you are to the window, the greater the angle of view. If you (the focal plane) moves closer to the window (the optical centre of the lens), you are able to see more. As you adjust your distance from the window (focal length), you will be able to see more or less depending on whether you move closer to the window (shorten the focal length) or move further from the window (longer focal length).

 

[Fastmarc]

Oh....
... and the fixed ones like 50mm is a single focal lenth?

 

[rjsimp]

Yeap! You got it.!

 

[Charlie99]

Keep in mind that the effective zoom or magnification is a combination of the sensor (or film) size and the focal length of the lens.

For a 35mm film camera (or CCD of the same 24mm x 36mm size) a 12 to 24 mm focal length lens is a wide angle lens.

For a digital point and shoot with a small sensor (like my Olympus Stylus 410), a 17mm focal length is roughly a 3x zoom.

 

[stiebs]

Charlie, your last statement doesn't make much sense:

For a digital point and shoot with a small sensor (like my Olympus Stylus 410), a 17mm focal length is roughly a 3x zoom.

The notion of a "3x zoom" for a "17mm" lens doesn't work. The x3 (or x5, or whatever) zoom terminology is something as was introduced by marketting departments of point and shoot cams for average joe to measure genital extenions.

In fastmarc's original question, his "12-24mm" lens is a x2 zoom. Ie, the long end (24mm) is two times the focal length of the wide end (12mm).

A fairly standard 3x zoom on compact camera usually refers to a 35mm equavalent focal range of a 35-105 zoom. (35 x 3 = 105).

 

[kapula]

Thanks guys!
This is a very informative thread. As I kinda have figured out what different focal lengths mean in practise it's very nice (for a nerd such as myself) to hear those definitions aswell.

 

[Fastmarc]

Though these replies helped, I still a little confused by it.
I assume that the experienced can look at the 17-55 for e.g. and know what it will be suitable for?

 

[parabolic]

A lens that is referred to by two numbers like that is a zoom lens (lens that can change it's focal length between a range of focal lengths. In this case, it is from 12mm to 24mm. The smaller the focal length, the wider the angle of the view at that focal length. Focal length is the distance from the optical centre of the lens to its focal point (where an object at infinity is in focus - usually the focal plane where the film is or sensor in a dSLR).

Why is this? Think of this analogy as it relates to angle of view. When you are looking out a window, the closer you are to the window, the greater the angle of view. If you (the focal plane) moves closer to the window (the optical centre of the lens), you are able to see more. As you adjust your distance from the window (focal length), you will be able to see more or less depending on whether you move closer to the window (shorten the focal length) or move further from the window (longer focal length).

Warren your explanation is outstanding! Clear and concise. Thanks, ++Ken++

 

[Fastmarc]

Why is this? Think of this analogy as it relates to angle of view. When you are looking out a window, the closer you are to the window, the greater the angle of view. If you (the focal plane) moves closer to the window (the optical centre of the lens), you are able to see more. As you adjust your distance from the window (focal length), you will be able to see more or less depending on whether you move closer to the window (shorten the focal length) or move further from the window (longer focal length).

With the camera, does going to a longer focal length makes the object closer (larger, fill the frame, zoom it in)?