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From Wikipedia

Thin lens

In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the focal length of the lens. Lenses whose thickness is not negligible are sometimes called thick lenses.

The thin lens approximation ignores optical effects due to the thickness of lenses and simplifies ray tracing calculations. It is often combined with the paraxial approximation in techniques such as ray transfer matrix analysis.

The focal length, f, of a thin lens is given by the Lensmaker's equation:

\frac{1}{f} \approx \left(n-1\right)\left[ \frac{1}{R_1} - \frac{1}{R_2} \right],

where n is the index of refraction of the lens material, and R1 and R2 are the radii of curvature of the two surfaces. Here, R1 is taken to be positive if the first surface is convex, and negative if the surface is concave. The signs are reversed for the back surface of the lens: R2 is positive if the surface is concave, and negative if it is convex. This is an arbitrary sign convention; some authors choose different signs for the radii, which changes the equation for the focal length.

For a thin lens, in the paraxial ray approximation, the object (s) and image (s') distances are related by the equation

{1\over s} + {1\over s'} = {1\over f}.

In scalar wave optics a lens is a part which shifts the phase of the wave-front. Mathematically this can be understood as a multiplication of the wave-front with the following function:

\exp\left(\frac{2\pi i}{\lambda} \frac{r^2}{2f}\right).

Camera lens

A camera lens (also known as photographic lens, objective lens or photographic objective) is an opticallens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.

While in principle a simple convex lens will suffice, in practice a compound lens made up of a number of optical lens elements is required to correct (as much as possible) the many optical aberrations that arise. Some aberrations will be present in any lens system. It is the job of the lens designer to balance these out and produce a design that is suitable for photographic use and possibly mass production.

There is no major difference in principle between a lens used for a still camera, a video camera, a telescope, a microscope, or other apparatus, but the detailed design and construction are different.

A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths, apertures, and other properties.

Theory of operation

Typical rectilinear lenses can be thought of as "improved" pinhole lenses. As shown, a pinhole lens uses a tiny aperture to block most rays of light, ideally selecting one ray to the object for each point on the image sensor. Pinhole lenses would be excellent except for a few serious limitations:

  • A pinhole camera with a large aperture is blurry because each pixel is essentially the shadow of the aperture stop, so its size is no smaller than the size of the aperture (below left).
  • Making the pinhole smaller improves resolution (up to a limit), but reduces the amount of light captured.
  • Diffractionlimits the effectiveness of shrinking the hole, so at a point, making the hole smaller makes the image blurrier as well as darker (below center).

Such lenses can be thought of as an answer to the question "how can we modify a pinhole lens to admit more light and give higher resolution?" A first step is to put a simple convex lens at the pinhole with a focal length equal to the distance to the film plane (assuming the camera will take pictures of distant objects). This allows us to open up the pinhole significantly (below right). The geometry is almost the same as with a simple pinhole lens, but rather than being illuminated by single rays of light, each image point is illuminated by a focused "pencil" of light rays. Standing in front of the camera, you would see the small hole, the aperture. The virtual image of the aperture as seen from the world is known as the lens's entrance pupil; ideally, all rays of light leaving a point on the object that enter the entrance pupil will be focused to the same point on the image sensor/film (provided the object point is in the field of view). If one were inside the camera, one would see the lens acting as a projector. The virtual image of the aperture from inside the camera is the lens's exit pupil.

Practical photographic lenses include more lens elements. The additional elements allow lens designers to reduce various aberrations, but the principle of operation remains the same: pencils of rays are collected at the entrance pupil and focused down from the exit pupil onto the image plane.

Construction

A camera lens may be made from a number of elements: from one, as in the Box Brownie's meniscus lens, to over 20 in the more complex zooms. These elements may themselves comprise a group of lenses cemented together.

The front element is critical to the performance of the whole assembly. In all modern lenses the surface is coated to reduce abrasion, flare, and surface reflectance, and to adjust color balance. To minimize aberration, the curvature is usually set so that the angle of incidence and the angle of refraction are equal. In a prime lens this is easy, but in a zoom there is always a compromise.

The lens usually is focused by adjusting the distance from the lens assembly to the image plane, or by moving elements of the lens assembly. To improve performance, some lenses have a cam system that adjusts the distance between the groups as the lens is focused. Manufacturers call this different things. Nikon calls it CRC (close range correction), while Hasselblad and Mamiya call it FLE (floating lens element).

Glass is the most common material used to construct lens elements, due to its good optical properties and resistance to scratching. Other materials are also used, such as quartz glass, fluorite, plastics like acrylic (Plexiglass), and even germanium and meteoritic glass. Plastics allow the manufacturing of strongly aspherical lens elements which are difficult or impossible to manufacture in glass, and which simplify or improve lens manufacturing and performance. Plastics are not used for the outermost elements of all but the cheapest lenses as they scratch easily. Molded plastic lenses have been used for the cheapest disposable cameras for many years, and have acquired a bad reputation: manufacturers of quality optics t


From Yahoo Answers

Question:I already got the thickness, converge and diverge light rays passing through 'em, used for short and long sightedness, and focal length. What's more?

Answers:Go here http://www.globalshiksha.com/content/10-differences-between-concave-and-convex-mirrors Goodbye

Question:I don't understand the difference between a convex lens and a convex mirror. Although they are the same shapes, they yield different results. The same goes for concave lens and a concave mirror. Can you explain why this is? Also for converging and diverging lens and mirror. why is a converging lens convex while a converging mirror is concave. What is the difference.

Answers:Look at what they do to the light hitting them. A convex mirror makes the light fan outwards on reflection. A concave lens makes the light also fan outwards as it passes through the lens. And a Concave mirror reflects light "inwards" by reflecting it and a convex lens, by being fatter in the middle makes the light bend inwards. The two inwards benders make the same sort of images and the two outward benders also make images similar to each other.

Question:Can someone explain to me how a magnifying glass is a convex lens but not a concave lens?

Answers:A convex lens magnifies, a concave lens makes the image smaller.

Question:A camera is filtered with a lens having a 200 mm focal length. Is the lens convex or concave? Is the image real or virtual? If your object is 1 m away from the lens, how far must the lens be from the film so a well focused image falls on the film? Explain your work. Thank you!

Answers:explain my work? ok clients ring me, i go meet them and complete a "brief sheet" - then go and do the job. as for convex or concave, lenses have elements in them that are both for your 1 meter question try this place for the answer: http://www.dofmaster.com/dofjs.html you havent given enough info for it to be answered..... a

From Youtube

3 Rules of Geometrical Construction for Convex & Concave Lens :

Lab Report: Finding a Focal Length of a Convex Lens :relationship between the convex lens and the focal point