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What is a Camera ?

What is a camera?

Essentially, a camera is a device consisting of a light-tight box which has an opening covered by a lens in one wall. This lens forms a real image image of the object upon a film of light sensitive material placed on the opposite side. The film is chemically processed to make a negative.

• A pinhole camera, instead of a lens, has a hole of 0.016 inch in diameter which produces the image. But the image is blurred. Being a very small hole, the quantity of light passing through is very little and need very long time to expose the film.

• In a box camera, either the shutter speed is fixed with variable aperture or with a fixed aperture and variable shutter speed.

• Bellows type camera are usually used for large size films .

• Small format (35mm and below) are often of the tube type. They are earlier versions to SLR type. High-class models are quipped with coupled rangefinders (for focusing) interchangeable lenses and shutter speeds up to 1/1250^th of a second.

• The medium format ‘Twin-lens Reflex’ camera has two compartments, one above the other. The lens of the two parts have exactly the same focal length. A mirror positioned at 45º inclinations reflects the image to the viewfinder above in the top compartment while the lower one takes the picture. Both lenses move simultaneously.

The viewing image remains visible all the time, but the viewpoint difference (parallax) of the two lenses means that the framing on the top screen is not exactly identical with that on the film. Hence, this poses a problem while capturing faces or portraits – the head portion is eliminated and the neck is very prominent.

PARTS OF AN SINGLE LENS REFLEX (SLR) CAMERA

1. Shutter speed dial/film speed set ring
2. Shutter release button
3. Film advance lever
4. Frame counter
5. Strap hook
6. Hand grip
7. Self –timer lever
8. Lens release button
9. Hot shoe
10. Film rewind crank
11. Film rewind knob and film compartment opening knob
12. Film speed setting window
13. Aperture setting ring
14. Focusing distance scale marked in meter and feet
15. Focusing ring
16. Depth-of-field scale
17. Depth-of-filed preview lever
18. ME (Multi exposure) lever
19. Exposure measurement button (correct/over/under)
20. View finder eye-piece
21. Rewind shaft
22. Film loading chamber
23. Battery compartment cover
24. Tripod fixing socket
25. Film Guide rails
26. Sprocket
27. Back cover
28. Film pressure plate
29. Film rewind button
30. Film take-up spool
31. Flash synchro contact
32. Data back contact
33. Window to view film data

* Image from uni. victoria website

SINGLE LENS REFLEX CAMERA (SLR)

Most of the modern 35 mm (small format) and medium format cameras are all of this type. A mirror in the camera body is pivoted at an angle of 45 degrees just in front of the focal plain shutter. The light rays passing through them is reflected above the focusing screen. The focused image on this screen is seen again through a prism placed at the top.

The prism is called ‘penta prism’ since it has five sides and it turns the light rays to 90 degrees with the view finder without reversing the image. The light rays take a total internal reflection inside the prism.

The stop value (f) or the aperture is set before the exposure is made. When the shutter is released the deviating mirror is automatically swung up above and the focal plane shutter is mounted directly in front of the film, momentarily exposes the film.

The advantage of an SLR camera is that the photographer is able to see the image exactly what the lens is viewing, (the field of coverage).

But the disadvantage is that (it is not once you get used to it) there is a black out in the view finder at the exact moment of exposure.

The set aperture closes to the set value only just before the exposure is made. Otherwise it is fully opened to allow maximum light through the lens for convenient sharp focusing.

WORKING OF AN SLR CAMERA

Most of the modern 35mm (small format) and medium format are all of this type. A mirror in the camera body is pivoted at an angle of 45⁰ just in front of the focal plane shutter. The light rays passing through it are reflected above to the focusing screen. The focused image on this screen is seen again through a prism placed at the top. The prism is called ‘Penta Prism’ since it has 5 sides and it turns the light rays to 90⁰ to the viewfinder without reversing the image. The light rays take a total internal reflection inside the prism.

The stop value (f) or aperture is set before the exposure is made. When the shutter is released the deviating mirror is automatically swung up above and the focal plane shutter mounted directly in front of the film, momentarily exposes the film

*** Diagrams visible at www.howstuffworks.com

HANDLING THE CAMERA

• While focusing, see that your finger is not blocking the lens.
• When taking a picture from top angle, sometimes your camera case or the sling will get into the field of view.
• In standing posture, more stability is achieved by butting the fore- arm firmly against chest.
• Always use the camera sling. Put it around your neck to protect your camera from falling.
• Use lens hood to protect your lens. It also blocks stray light entering the lens.

APERTURE

In optics, an aperture is a hole or an opening through which light is admitted.

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.

Diagram of decreasing aperture sizes (increasing f-numbers) for "full stop" increments (factor of two aperture area per stop)

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 largest aperture opening is f/2.8 an the smallest is f /22

The Aperture Controls Light and Depth of Field

Aperture is referred to the lens diaphragm opening inside a photographic lens. The size of the diaphragm opening in a camera lens regulates amount of light passes through onto the film inside the camera the moment when the shutter curtain in camera opens during an exposure process.

Aperture is the ratio of the focal length of the lens to the diameter of the actual opening of the lens.

Aperture size is usually calibrated in f-numbers. i.e. those little numbers engraved on the lens barrel like:

f22 (f/22),16 (f/16), f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8

Each of this value represents one time the amount of light either more or less in quantity. Meaning to say, f/16 will let in 1X the amount of light than a diaphragm opening of f/22 and so forth; while on the other hand, an aperture of f/4.0 will let in 1X lesser than that of f/2.8 etc.

Area of aperture opening:

If f: 50mm and you set the aperture at f4 then

Ratio: 50/d= 4 = d: 50/4: 12.5mm Area of the opening: ( d^2)/4

: (3.414x12.5^2)/4

: 122.7 sq.mm

For aperture at f5.6:

Ratio: 50/d= 5.6 =d: 50.5.6: 8.93mm

Area of opening: ( d^2)/4

: (3.414x8.93^2)/4

: 62.63 sq.mm

122.7 is nearly double the area of 62.63 sq.mm.

Hence at f4 the amount of light let in (brightness) is double that of f5.6.

In the same way at f16 : 7.7 sq.mm
f11 : 16.2 sq.mm

f8 : 30.6 sq.mm

f5.6 : 62.63 sq.mm

f4 : 122.7 sq.mm

f2.8 : 250.4 sq.mm

Hence, the squares of the f-numbers are inversely proportional to the amount of light admitted

Aperture and exposure

The aperture can be opened up to let in more light or closed (stopped down) to let in less. Like the shutter speed, the aperture is used to control exposure. The larger the aperture opening, the more light reaches the image sensor in a given period of time. The more light, the lighter the image.

Aperture and depth-of-field

Changing the aperture changes the depth of field, the depth in a scene from foreground to background that will be sharp in a photograph. The smaller the aperture you use, the greater the area of a scene that will be sharp.

For some pictures-for example, a landscape-you may want a smaller aperture for maximum depth of field so that everything from near foreground to distant background is sharp. But perhaps in a portrait you will want a larger aperture to decrease the depth of field so that your subject's face is sharp but the background is soft and out of focus.

* Images from website:uni. of Victoria & www.tpub.com

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.

SHUTTER

A movable cover for an opening. In photography, that opening is the lens - more specifically, the aperture. The shutter blocks the passage of light traveling through the lens to the film when it is closed, and allows light to reach the film when it is open. Shutters are composed of blades, a curtain, a plate or another movable cover. They control the amount of time that light is allowed to pass through the opening to reach the film.

Shutter and shutter speed

The shutter keeps light out of the camera except during an exposure, when it opens to let light strike the image sensor. The length of time the shutter is open affects both the exposure of the image and how motion is portrayed in it.

The shutter controls the duration of an exposure. The camera's shutter speed is a measurement of how long its shutter remains open as the picture is taken. The slower the shutter speed, the longer the exposure time. When the shutter speed is set to 1/125 or simply 125, this means that the shutter will be open for exactly 1/125th of one second. The shutter speed and aperture together control the total amount of light reaching the sensor.

Shutter speed is measured in seconds. A typical shutter speed for photographs taken in sunlight is 1/125th of a second. In addition to its effect on exposure, shutter speed changes the way movement appears in the picture. Very short shutter speeds are used to freeze fast-moving subjects, for example at sporting events. Very long shutter speeds are used to intentionally blur a moving subject for artistic effect.

Shutter is a mechanism which controls the duration of light passing the lens unit for correct exposure. In other words it is the timings.

In non- SLR camera they are marked on a ring in the lens barrel.

In SLR camera they are marked on the dial in the body or the LCD (liquid crystal display).

They are referred as shutter speed.

Shutter speed settings

There are a series of settings that have traditionally been used. These shutter speed settings are arranged in a sequence so that each setting lets in half as much light as the next slowest setting and twice as much as the next fastest. The traditional shutter speeds (listed from the fastest to the slowest speeds) include

1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, and 1 second.

Although speeds faster than 1 second are fractions of a second most cameras display them without the numerator. For example, 1/2 second is displayed as 2.

Types of shutters

Principal present-day shutters are the leaf shutter and the focal-plane shutter.

a. The leaf shutter

The leaf, or diaphragm, shutter consists of a series of blades or leaves fitted inside or just behind the lens. The shutter opens by swinging the leaves simultaneously outward to uncover the lens opening. The leaves stay open for a fixed time – the exposure – and closes again. A combination of electromagnets or electromagnets and springs drives the mechanism, while an electronic circuit—often coupled with a light metering system—or an adjustable escapement in mechanical shutters controls the open time. This is typically between one second and ¹/₅₀₀ second.

b. Focal-plane shutter:

The focal-plane shutter consists of two light-tight fabric blinds or a combination of metal blinds moving in succession across the film immediately in front of the image plane. The first blind uncovers the film and the second blind covers it up again, the two blinds forming a traveling slit the width of which determines the exposure time: the narrower the slit, the shorter the time. The actual travel time is fairly constant for all exposure times. A mechanism or electromagnet and control circuit triggers the release of the second blind. Focal-plane shutters are usually adjustable for exposure times between one second (or longer) and ¹/_1,000 to ¹/_4,000 second.

The shutter and exposure

Slower shutter speeds let more light strike the image sensor making an image lighter. Faster shutter speeds let less strike it and make the image darker.

The shutter and motion

In addition to controlling exposure, the shutter speed is the most important control you have over how motion is captured in a photograph. Understanding shutter speeds is vital if you want to anticipate if a moving subject will appear in your image sharp or blurred. The longer the shutter is open, the more a moving subject will be blurred in the picture Also, the longer it's open the more likely you are to cause blur by moving the camera slightly.