Markers

[wenti]

I'm a rookie in the field of Vision. I want to ask about the meanings of marker on my fixed focal length lens.
What's the meaning of Near~Far (sometimes N~+∞）? If it's the adjustable focal length, my lens has a fixed focal length. If it refer to adjustable work distance(from work to lens), why the camera can only get clear image on a certain distance, not on all distances.

Same question comes up on my adjustable focal length lens. The abbreviation T~W means what?

[BruceAmmons]

Near-Far is the focus control. It is adjusted based on how far away the object is. A fixed focal length only means that the angular field of view is constant. You must still adjust the focus for the proper distance to your object.

With an adjustable focal length, you also have control over the angular field of view. This is commonly known as zoom. The control T-W stands for Telephoto-Wideangle. Telephoto is a small angular field of view, used for making small or distant objects look large. Wideangle is a large angular field of view, which lets you look at larger objects closer up.

Bruce

[wenti]

Angular field of view is equal to FOV? I think it's angle of view. But why the unit is still mm? On the other hand, when angle of view is fixed, because CCD Sensor Size is always fixed to a certain camera, so the focal length is fixed too. Is that a contradiction?

[BruceAmmons]

Think of the angular field of view as a cone which starts at the camera and gets larger as you move away from the camera. If an object is far away from the camera, it will look small. If you move it closer, it will look larger. For a fixed focal length lens, the cone is a fixed shape. You can calculate the FOV at a certain distance by the ratio (CCD size)/(Lens focal length) = (FOV size)/(distance from lens).

For a zoom lens with variable focal length, the cone can be long and thin (telephoto) or short and wide (wideangle). For a wideangle lens, the FOV gets larger much faster as you move away from the camera.

Bruce