APERTURE

The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.

A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the iris of the eye—it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.

The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor. The photography term "one f-stop" refers to a factor of √2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity.

Aperture priority is a semi-automatic shooting mode used in cameras. It allows the photographer to choose an aperture setting and allow the camera to decide the shutter speed and sometimes ISO sensitivity for the correct exposure. This is sometimes referred to as Aperture Priority Auto Exposure, A mode, Av mode, or semi-auto mode.

Maximum and minimum apertures
The specifications for a given lens typically include the minimum and maximum apertures. These refer to the maximum and minimum f-numbers the lens can be set at to achieve, respectively.A typical lens will have an f-number range from f/16 (small aperture) to f/2 (large aperture) (these values vary). The maximum aperture (minimum f-number) tends to be of most interest (and is always included when describing a lens). This value is also known as the lens speed, because it is proportional to the square of accepted light, and thus inversely proportional to the square of required exposure time (i.e. using a lens with f/2, one can take pictures at one quarter of the exposure time necessary using a f/4 lens). Lenses for 35mm cameras can have f-numbers as low as f/1.0, while professional lenses for some movie cameras can have f-numbers as low as f/0.75 (very large relative aperture). These are known as "fast" lenses because they allow in more light and therefore reduce the exposure time. Stanley Kubrick's film Barry Lyndon has scenes with the largest relative aperture in film history: f/0.7.
Prime lenses (non-zoom lenses) have a fixed focal length (FFL) and large aperture and are favored by professionals, especially by photojournalists who often work in dim light, have no opportunity to introduce supplementary lighting, and need to capture fast breaking events.
Zoom lenses typically have a maximum aperture (minimum f-number) of f/2.8 to f/6.3 through their range. A very fast zoom lens will be constant f/2.8 or f/2, which means the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable relative aperture, since it is harder and more expensive to keep the effective aperture proportional to focal length at long focal lengths; f/3.5 to f/5.6 is an example of a common variable aperture range in a consumer zoom lens.

f/32 - narrow aperture and slow shutter speed

f/5.6 - wide aperture and fast shutter speed

History of photography

Photography is the result of combining several technical discoveries. Long before the first photographs were made,Ibn al-Haytham (Alhazen) (965–1040) invented the camera obscura and pinhole camera,[2] Albertus Magnus (1193–1280) discovered silver nitrate, and Georges Fabricius (1516–1571) discovered silver chloride. Daniel Barbaro described a diaphragm in 1568. Wilhelm Homberg described how light darkened some chemicals (photochemical effect) in 1694. The fiction book Giphantie, by French author Tiphaigne de la Roche, described what can be interpreted as photography.
Photography as a usable process goes back to the 1820s with the development of chemical photography. The first permanent photograph was an image produced in 1826 by the French inventor Nicéphore Niépce. However, the picture took eight hours to expose, so he went about trying to find a new process. Working in conjunction
with Louis Daguerre, they experimented with silver compounds based on a Johann Heinrich Schultz discovery in 1724 that a silver and chalk mixture darkens when exposed to light. Niépce died in 1833, but Daguerre continued the work, eventually culminating with the development of the daguerreotype in 1837. Eventually, France agreed to pay Daguerre a pension for his formula, in exchange for his promise to announce his discovery to the world as the gift of France, which he did in 1839.
Meanwhile, Hercules Florence had already created a very similar process in 1832, naming it Photographie, and William Fox Talbot had earlier discovered another means to fix a silver process image but had kept it secret. After reading about Daguerre's invention, Talbot refined his process so that it might be fast enough to take photographs of people. By 1840, Talbot had invented the calotype process, which creates negative images. John Herschel made many contributions to the new methods. He invented the cyanotype process, now familiar as the "blueprint". He was the first to use the terms "photography", "negative" and "positive". He discovered sodium thiosulphate solution to be a solvent of silver halides in 1819, and informed Talbot and Daguerre of his discovery in 1839 that it could be used to "fix" pictures and make them permanent. He made the first glass negative in late 1839.
In March 1851, Frederick Scott Archer published his findings in "The Chemist" on the wet plate collodion process. This became the most widely used process between 1852 and the late 1880s when the dry plate was introduced. There are three subsets to the Collodion process; the Ambrotype (positive image on glass), the Ferrotype or Tintype (positive image on metal) and the negative which was printed on Albumen or Salt paper.
Many advances in photographic glass plates and printing were made in through the nineteenth century. In 1884, George Eastman developed the technology of film to replace photographic plates, leading to the technology used by film cameras today.
In 1908 Gabriel Lippmann won the Nobel Laureate in Physics for his method of reproducing colours photographically based on the phenomenon of interference, also known as the Lippmann plate.

thank, http://en.wikipedia.org/

TIPS HOW TO TAKING GOOD PHOTOS WITH YOUR DIGITAL CAMERA

The Rule of Thirds.
Using the steps outlined previously will help to tighten up your composition. Now we will look at a few techniques you can employ to help improve your composition. If you are taking photographs for your own pleasure, as I assume you are, then you only have to come up with pictures that please you. You may be able to overlook the huge empty spaces or people with their heads cut off but no-one else will. That cute kid looks really cute it's just a pity that you need a magnifying glass to see him. Producing pictures that are pleasing to someone other than yourself will make your photography much more rewarding.

What is the Rule of Thirds?
The rule of thirds is an imaginary tic-tac-toe board is drawn across an image to break it into nine equal squares. The four points where these lines intersect are strongest focal points. The lines themselves are the second strongest focal points.

The Rule of Thirds.

One of the most popular 'rules' in photography is the Rule Of Thirds. It is also popular amongst artists. It works like this:Imaginary lines are drawn dividing the image into thirds both horizontally and vertically. You place important elements of your composition where these lines intersect. I've even made a little diagram for you.



As well as using the intersections you can arrange areas into bands occupying a third or place things along the imaginary lines. As you can see it is fairly simple to implement. Good places to put things; third of the way up, third of the way in from the left , you get the idea. Duff places to put things; right in the middle, right at the top, right at the bottom, away in the corner.


Using the Rule of Thirds helps produce nicely balanced easy on the eye pictures. Also, as you have to position things relative to the edges of the frame it helps get rid of ' tiny subject surrounded by vast empty space' syndrome.






One last thing about the Rule of Thirds for the time being. Once you have got the hang of the Rule of Thirds you will very quickly want to break it ! This is fine. As I said earlier these 'rules' are best used as guidelines and if you can create a better image by bending or ignoring rules then fire away.



The Rule of Thirds is fairly structured but there are a great many methods you can employ which rely on your ability to 'see' things and incorporate them into your composition. Next up we will look at some, but by no means all, of them.