TYPES OF LENSES

What is focal length?

The focal length of any lens is the distance between the optical center of the lens and the point at which it focuses an image.

However, a given focal length lens may be a wide angle lens on one camera and a telephoto lens on another. This is because descriptions such as "wide-angle" or "normal" depend on the size of the film or image sensor being used. As these get smaller, a given focal length lens magnifies more.

1. Standard Lens:



A "normal lens" for a 35mm camera usually refers to a lens with a 50mm focal length.

Negative size: 36 x24mm
Diagonal distance BD: 43.26mm

AC (axis of the lens) is perpendicular to the plane of the film and it bisects the diagonal BD at A.
BD: 21.63mm

CAB is 90º
CAB is a right angled triangle.
Tan θ : AB/AC
: 21.63/50
: .433
: 23.30º


BCD = 47 which is nearer to 50º

If the focal length of a lens is equal to the diagonal distance of the negative then it is called the normal (standard) lens for a camera.

If a camera has standard lens, it means that the angle of field of coverage is the same as what a human eye would see, with the other eye closed ≈ 50º




2. Zoom Lens:

A zoom lens lets you choose any focal length within the range the lens is designed for. When you change focal lengths by zooming the lens, two important effects are immediately obvious in the lens’ angle of view and its magnifying power.
Zoomed out, you have a wide-angle of view that captures a wide expanse of a scene. As you zoom in, the field of view narrows and you can isolate small portions of the scene without moving closer to the subject.
Magnification is related to the lens’ angle of view. Since zooming out includes a wide sweep of the scene, all of the objects in the scene are reduced to fit into the image. Zooming in gives a much narrower angle of view, so objects in a scene appear larger.
Note: A zoom lens is an excellent portrait lens, especially for head-and-shoulders portraits. When zoomed in you can keep your distance and still fill the viewfinder frame with the subject. Keeping at a distance eliminates the exaggerated perspective caused by working very close to a subject with a shorter focal length lens. It also helps relax your subjects if they get uneasy, as many people do, when a camera comes close.

3. Wide-Angle Zoom:

A lens zoomed to a wide-angle also has great depth of field. This great depth of field makes short lenses good for street or action photographs. When out to capture quickly unfolding scenes, keep the lens zoomed out to a wide angle so you’ll have maximum depth of field when you respond quickly to a photo opportunity.
Short lenses also let you focus very close to your subject, and the effect this can have on the perspective in your images can be dramatic. Objects very close to the camera loom much larger than those farther in the background. This distortion in the apparent size of objects can deliberately give emphasis and when carried to an extreme, give an unrealistic appearance to a scene.


4. Telephoto Zoom:

A lens zoomed in on a subject acts somewhat like a telescope: It magnifies the image of your subject. This is especially useful when you can’t get close to your subject—or don’t want to. Zooming in like this is ideal for wildlife, portrait, and candid photography, whenever getting close to a subject might disturb it.
When you zoom in on a subject, depth of field gets shallower so you must focus carefully. Also, zooming in visually compresses space, making objects in the scene appear closer together than they actually are.
The primary drawback of zooming in is that it gives you a smaller maximum aperture. This smaller maximum aperture may require a longer shutter speed and since a long lens magnifies movement, just as it magnifies the subject, you may have to use a tripod instead of hand-holding the camera.
For a telephoto view, you can zoom the lens all the way in. For even more magnification, some cameras have optional lens converters that give you even longer focal lengths.

5. Fish-eye lens:

For image angles greater than 110°, it becomes difficult to bring the lens close enough to the film to allow the rays between the lens and film to diverge sufficiently. The fish-eye lens overcomes this difficulty by making the rays diverge less behind the lens than they do in front. The resulting image shows appreciable distortion, with image details near the edges and corners progressively compressed. Fish-eye lenses usually cover angles between 140° and 210° and are used for unusual wide-angle effects where the distortion becomes a deliberate pictorial element. They also have certain scientific applications, for instance, to cover a horizon-to-horizon view of the sky in recording cloud formations.