Single-lens Reflex Camera

Sutton ushers in modern photography with a new camera.

The modern era of of photography began in 1861 with the invention and patenting of the world's first single-lens reflex (SLR) camera y photography expert Thomas Sutton (1819-1875). His prototype led to the creation of the first batch of SLR cameras in 1884, with a design that is still in use today. Sutton also assisted James Clerk Maxwell in his successful demonstration of color photography in 1861.

In noon-SLR cameras, light enters the view finder at a slightly different angle to that at which it enters the lens, so the resulting photo can appear different to the intended composition. In SLR cameras, a mirror is positioned in front of the lens and directs light up into a pentaprism. The light bounces between its edges until it enters the viewfinder with correct orientation, as if the viewer is looking directly through the camera lens. When a photograph is taken,the mirror moves out of the way allowing light to reach the film or, with digital SLrs (DSLRs), the imaging sensor. 

Now the most popular popular professional camera format, the SLR camera was the culmination of decades of photographic innovations that began with the production of Louis Daguerre's daguerrotype and josef Maximilian Petzval's lens systems, which led to the first mass-produced cameras.

Although no record of the first production model exist, the camera was first commercially produced in the mid-1880s. By the 1930s it was undistorted view of the subject from the correct perspective. DSLRs have all but replaced the traditional SLR, but the principle that Sutton poineered is still used today.   

How Cameras Work

Photography is undoubted one of the most important inventions in history it has truly transformed how people conceive of the world. Now we can "see" all sorts of things that are actually many miles-- and years--away from us. Photography lets us capture moments in time and preserve them for years to come.

The basic technology that makes all of this possible is fairly simple. A still film camera is made of three basic elements: an optical element (the lens), a chemical element (the film) and a mechanical element (the camera body itself). As we'll see, the only trick to photography is calibrating and combining these elements in such a way that they record a crisp, recognizable image.

There are many different ways of bringing everything together. In this article, we'll look at a manual single-lens-reflex (SLR) camera. This is a camera where the photographer sees exactly the same image that is exposed to the film and can adjust everything by turning dials and clicking buttons. Since it doesn't need any electricity to take a picture, a manual SLR camera provides an excellent illustration of the fundamental processes of photography.

The optical component of the camera is the lens. At its simplest, a lens is just a curved piece of glass or plastic. Its job is to take the beams of light bouncing off an object and redirect them so they come together to form a real image-- an image that looks just like the scene in front of the lens.

But how can a piece of glass do this? The process is actually very simple. As light travels from one medium to another, it changes speed. Light travels more quickly through air than it does through glass, so a lens slows it down.

When light waves enter a piece of glass at an angle, one part of the wave will reach the glass before another and so will start slowing down first. This is something like pushing a shopping cart from pavement to grass, at an angle. The right wheel hits the grass first and so slows down while the left wheel is still on the pavement. Because the left Wheel is briefly moving more quickly than the right wheel, the shopping cart turns to the right as it moves onto the grass.

The effect on light is the same-- as it enters the glass at an angle, it bends in one direction, It bends again when it exist the glass because parts of the light wave enter the air and speed up before other parts of the wave. In a standard converging, or convex lens, one or both sides of the glass curves out. This means rays of light passing through will bend toward the center of the lens on entry. In a double convex lens, such as a magnifying glass, the light will bend when it exists as well as when it enters.

This effectively reverses the path of light from an object. A light source-- say a candle--emits light in all directions. The rays of lights all start at the same point -- the candle's flame-- and then are constantly diverging. A converging lens takes those rays and redirects them so they are all converging back to one point. At the point where the rays converge, you get a real image of the candle. In the next couple of sections, we'll look at some of the variables that determine how this real image is formed.

We've seen that a real image is formed by light moving through a convex lens. The nature of this real image varies depending on how the light travels through the lens. This light path depends on two major factors :

• The angle of the light beam's entry into the lens.
• The structure of the lens.  

The angle of light entry changes when you move the object closer or farther away from the lens. You can see this in the diagram below. The light beams from the pencil point enter the lens at a sharper angle when the pencil is closer to the lens and a more obtuse angle when the pencil is farther away. But Overall, the lens only bends the light beam to a certain total degree, no matter how it enters. Consequently, light beams that enter at a sharper angle will exit at a more obtuse angle, and vice versa. The total "bending angle" at any particular point on the lens remains As you can see, light beams from a closer point converge father away from the lens than light beams from a point that's farther away. In other words, the real image of a closer object forms father away from the lens than the real image from a more distant object.

You can observe this phenomenon with a simple experiment. Light a candle in the dark, and hold a magnifying glass between it and the wall. You will see an upside down the image of the candle on the wall. If the real image of the candle does not fall directly on the wall, it will appear somewhat blurry. The light beams from a particular point don't quite converge at this point. To focus the image, move the magnifying glass closer or farther away from the candle.

This is what you're doing when you turn the lens of a camera to focus it-- you're moving it closer or farther away from the film surface. As you move the lens, you can line up the focused real image of an object so it falls directly on the film surface.  

You now Know that at any one point, a lens bends light beams to a certain total degree, no matter the light beam's single of entry. This total "bending angle" is determined by the structure of the lens.

In the last section, we saw that at any one point, a lens bends light beams to a certain total degree, no matter the light beam's angle of entry. This total "bending angle" is determined by the structure of the lens.

A lens with a rounder shape (a center that extends out farther) will have a more acute bending angle. Basically, curving the lens out increases the distance between different points on the lens. This increases the amount of time that one part of the light wave is moving faster than another part, so the light makes a sharper turn.

Increasing the bending angle has an obvious effect. Light beams from a particular point will converge at a point closer to the lens. In a lens with a flatter shape, light beams will not turn as sharply. Consequently, the light beams will converge farther away from the lens. To put it another way, the focused real image forms farther away from the lens when the lens has a flatter surface.

Increasing the distance between the lens and the real image actually increases the total size of the real image. If you think about it, this makes perfect sense. Think of a projector: As you move the projector farther away from the screen, the image becomes larger. To put it simply, the light beams keep spreading apart as they travel toward the screen.

The same basic thing happens in a camera. As the distance between the lens and the real image increases, the light beams spread out more, forming a larger real image. But the size of the film stays constant. When you attach a very flat lens, it projects a large real image but the film is only exposed to the middle part of it. Basically, the lens zeroes in on the middle of the frame, magnifying a small section of the scene in front of you. A rounder lens produces a smaller real image, so the film surfaces sees a much wider area of the scene (at reduced magnification).

Professional cameras let you attach different lenses so you can see the scene at various magnifications. The magnification power of a lens is described by its focal length. In cameras, the focal length is defined as the distance between the lens and the real image of an object in the far distance (the moon for example). A higher focal length number indicated a greater image magnification.

Different lenses are suited to different situations. If you're taking a picture of a mountain range, you might want to use a telephoto lens, a lens with an especially long focal length. This lens lets you zero in n specific elements in the distance, so you can create tighter compositions. If you're taking a close-up portrait, you might use a wide-angle lens. This lens has a much shorter focal length, so it shrinks the scene in front of you. The entire face is exposed to the film even if the subject is only a foot away from the camera. A Standard 50 mm camera lens doesn't significantly magnify or shrink the image, making it ideal for shooting objects that aren't especially close or far away.

 Writer: Mr. Krishan Kumar Saini 

Today we have learnt Basics about Single-lens Reflex Camera. Hope this lesson is helpful for you.