I have already written posts on the working of film cameras, digital cameras and also on the image sensors. To know more about them click on the links below.

TAKE A LOOK : WORKING OF FILM CAMERAS

TAKE A LOOK : WORKING OF DIGITAL CAMERAS

Digital cameras are mainly classified according to their use, automatic and manual focus, and also price. Here are the classifications.

1. Compact digital cameras

Compact cameras are the most widely used and the simplest cameras to be ever seen. They are used for ordinary purposes and are thus called “point and shoot cameras”. They are very small in size and are hence portable. Since they are cheaper than the other cameras, they also contain fewer features, thus lessening the picture quality. These cameras are further classified according to their size. The smaller cameras are generally called as ultra-compact cameras. The others are called compact cameras.

Here are some features of this camera

• Compact and simple.
• Images can be stored in computer as JPEG files.
• Live preview can be seen before taking photos.
• Low power flashes are available for taking photos in the dark.
• Contains auto-focus system with closer focusing ability.
• Zoom capability.

Although these features are available, their magnitude may be less compared to other cameras. The flashes may be available only for nearby objects. The preview of the picture to be taken will have less motion capability.  The image sensors used in these cameras have a very small diognal space of about 6mm with a crop factor of 6.

Compact Digital Cameras

2. Bridge cameras

Bridge cameras are most often mistaken for single-lens reflex cameras (SLR). Though they have the same characteristics their features are different. Some of its features are

• Fixed lens
• Small image sensors
• Live preview of the image to be taken
• Auto-focus using contrast-detect method and also manual focus.
• Image stabilization method to reduce sensitivity.
• Image can be stored as a raw data as well as compressed JPEG format.

Though they resemble SLR in many ways, they operate much slower than the latter. They are very big in size and so the fixed lenses are given very high zooming capability and also fast apertures. The autofocus or manual focus is set according to our necessity. The image preview is done using either a LCD or an Electronic View Finder (EVF).

Bridge Cameras

3. Digital single lens reflex cameras (DSLR)

This is one of the most high end cameras obtainable for a decent price. They use the single-lens reflex method just like an ordinary camera with a digital image sensor. The SLR method consists of a mirror which reflects the light passing through the lens with the help of a separate optical viewfinder.

Some features of this camera are

• Special type of sensors is setup in the mirror box for obtaining autofocus.
• Has live preview mode.
• Very high end sensors with crop factors from 2 to 1 with diagonal space from 18mm to 36mm.
• High picture quality even at low light.
• The depth of field is very less at a particular aperture.
• The photographer can choose the lens needed for the situation and can also be easily interchangeable.
• A focal plane shutter is used in front of the imager.
• 

  Digital single lens reflex cameras (DSLR)

4. Electronic viewfinder (EVF)

This is just a combination of very large sensors and also interchangeable lenses. The preview is made using an EVF. There is no complication in mechanism like a DSLR.

Electronic View Finder

5. Digital rangefinders

This is a special film camera equipped with a rangefinder. With this type of a camera distant photography is possible. Though other cameras can be used to take distant photos, they do not use the rangefinder technique.

Digital rangefinders

6. Line-scan cameras

This type of cameras is used for capturing high image resolutions at a very high speed. To make this mechanism possible, a single pixel of image sensors are used instead of a matrix system. A stream of pictures of constantly moving materials can be taken with this camera. The data produced by a line-scan camera is 1-dimensional. It has to be processed in a computer to make it 2-D. This 2-D data is further processed to obtain our needs.

Line-scan cameras

I have already described about the working of a camera. Almost all the basics of this post have been explained there. Now let us know more about a digital camera, its working, and also advantages.

The digital camera can be considered as an alteration of the conventional analog camera. Most of the associated components are also the same, except that instead of light falling on a photosensitive film like an analog camera, image sensors are used in digital cameras. Though analog cameras are mostly dependent on mechanical and chemical processes, digital cameras are dependent on digital processes. This is a major shift from its predecessor as the concept of saving and sharing audio as well as video contents have been simplified to earth.

TAKE A LOOK : WORKING OF AN ANALOG CAMERA

Digital Camera Basics

As told earlier, the basic components are all the same for both analog and digital cameras. But, the only difference is that the images received in an analog camera will be printed on a photographic paper. If you need to send these photos by mail, you will have to digitally convert them. So, the photo has to be digitally scanned.

This difficulty is not seen in digital photos. The photos from a digital camera are already in the digital format which the computer can easily recognize (0 and 1). The 0’s and 1’s in a digital camera are kept as strings of tiny dots called pixels.

The image sensors used in an digital can be either a Charge Coupled Device (CCD) or a Complimentary Metal Oxide Semi-conductor (CMOS). Both these image sensors have been deeply explained earlier.

TAKE A LOOK : CHARGE COUPLED DEVICES (CCD)

TAKE A LOOK : CMOS ACTIVE PIXEL SENSOR (CMOS APS)

TAKE A LOOK : CCD V/s CMOS

The image sensor is basically a micro-chip with a width of about 10mm. The chip consists arrays of sensors, which can convert the light into electrical charges. Though both CMOS and CCD are very common, CMOS chips are known to be more cheaper. But for higher pixel range and costly cameras mostly CCD technology is used.

A digital camera has lens/lenses which are used to focus the light that is to be projected and created. This light is made to focus on an image sensor which converts the light signals into electric signals. The light hits the image sensor as soon as the photographer hits the shutter button. As soon as the shutter opens the pixels are illuminated by the light in different intensities. Thus an electric signal is generated. This electric signal is then further broke down to digital data and stored in a computer.

Pixel Resolution of a Digital Camera

The clarity of the photos taken from a digital camera depends on the resolution of the camera. This resolution is always measured in the pixels. If the numbers of pixels are more, the resolution increases, thereby increasing the picture quality. There are many type of resolutions available for cameras. They differ mainly in the price.

• 256×256 – This is the basic resolution a camera has. The images taken in such a resolution will look blurred and grainy. They are the cheapest and also unacceptable.
• 640×480 – This is a little more high resolution camera than 256×256 type. Though a clearer image than the former can be obtained, they are frequently considered to be low end. These type of cameras are suitable for posting pics and images in websites.
• 1216×912 – This resolution is normally used in studios for printing pictures. A total of 1,109,000 pixels are available.
• 1600×1200 – This is the high resolution type. The pictures are in their high end and can be used to make a 4×5 with the same quality as that you would get from a photo lab.
• 2240×1680 – This is commonly referred to as a 4 megapixel cameras. With this resolution you can easily take a photo print up to 16×20 inches.
• 4064×2704 – This is commonly referred to as a 11.1 megapixel camera. 11.1 megapixels takes pictures at this resolution. With this resolution you can easily take a photo print up to 13.5×9 inch prints with no loss of picture quality.
• There are even higher resolution cameras up to 20 million pixels or so.

Color Filtering using Demosaicing Algorithms

The sensors used in digital cameras are actually coloured blind. All it knows is to keep a track of the intensity of light hitting on it. To get the colour image, the photosites use filters so as to obtain the three primary colours. Once these colours are combined the required spectrum is obtained.

For this, a mechanism called interpolation is carried out. A colour filter array is placed over each individual photosite. Thus, the sensor is divided into red, green and blue pixels providing accurate result of the true colour at a particular location. The filter most commonly used for this process is called Bayer filter pattern. In this pattern an alternative row of red and green filters with a row of blue and green filters. The number of green pixels available will be equal to the number of blue and red combined. It is designed in a different proportion as the human eye is not equally sensitive to all three colours. Our eyes will percept a true vision only if the green pixels are more.

The main advantage of this method is that only one sensor is required for the recording of all the colour information. Thus the size of the camera as well as its price can be lessened to a great extent. Thus by using a Bayer Filter a mosaic of all the main colours are obtained in various intensities. These various intensities can be further simplified into equal sized mosaics through a method called demosaicing algorithms. For this the three composite colours from a single pixel are mixed to form a single true colour by finding out the average values of the closest surrounding pixels.

Take a look at the digital camera schematic shown below.

Parameters of a Digital Camera

Like a film camera, a digital camera also has certain parameters. These parameters decide the clarity of the image. First of all the amount of light that enters through the lens and hits the sensor has to be controlled. For this, the parameters are

1. Aperture – Aperture refers to the diameter of the opening in the camera. This can be set in automatic as well as the manual mode. Professionals prefer manual mode, as they can bring their own touch to the image.

2. Shutter Speed – Shutter speed refers to the rate and amount of light that passes through the aperture. This can be automatic only. Both the aperture and the shutter speed play important roles in making a good image.

3. Focal Length – The focal length is a factor that is designed by the manufacturer. It is the distance between the lens and the sensor. It also depends on the size of the sensor. If the size of the sensor is small, the focal length will also be reduced by a proportional amount.

4. Lens – There are mainly four types of lenses used for a digital camera. They differ according to the cost of the camera, and also focal length adjustment. They are

• Fixed-focus, fixed-zoom lens – They are very common and are used in inexpensive cameras.
• Optical-zoom lenses with automatic focus – These are lenses with focal length adjustments. They also have the “wide” and “telephoto” options.
• Digital zoom – Full-sized images are produced by taking pixels from the centre of the image sensor. This method also depends on the resolution as well as the sensor used in the camera.
• Replaceable lens systems – Some digital cameras replace their lenses with 35mm camera lenses so as to obtain better images.

Digital Cameras v/s Analog Camera

• The picture quality obtained in a film camera is much better than that in a digital camera.
• The rise of technology has made filming the help of digital techniques easier as well as popular.
• Since the digtal copy can be posted in websites, photos can be sent to anyone in this world.

Since posts on Charge Coupled Device and CMOS Active Pixel Sensor have already been posted, it is time to know their comparison, advantages and disadvantages. Though both of them are equally used in cameras, there are some differences in parameters like gain, speed and so on.

Comparison –  Charge Coupled Device and CMOS Active Pixel Sensor

• Both the devices are used to convert light into electric signals and are used for the same applications.  After converting the signals, they have to be read from each cell. This process is different for both the devices.

• The charge from each chip is taken to the end of the array and then read in a CCD. This is then converted into a digital signal with the help of an analog to digital converter (ADC). The process of reading the signal by CMOS Active Pixel Sensor is done by using transistors and amplifiers at each pixel and then the signal is moved using traditional wires.

Difference – Charge Coupled Device and CMOS Active Pixel Sensor

• CCD image sensors create super quality pictures. They also produce lesser noise than CMOS APS.

• In a CMOS all the transistors are kept right next to each pixel. As a result, all the photons that hit the device actually get scattered by hitting the transistors as well. Thus, the sensitivity of CMOS Active Pixel Sensor is lesser than that of a Charge Coupled Device.

• The design of the CCD sensors is in such a way that they require more power for its operation. If both the devices of equal reception are taken, the CCD is considered to consume almost 100 times more power than its equivalent CMOS Active Pixel Sensor.

• All the devices have been using Charge Coupled Device devices far more than CMOS Active Pixel Sensor. As a result a vast study has been done on CCD devices. So, they are more mature and also tend to have higher quality pixels.

Active Pixel Sensor (APS) is an image sensor, made up of an array of pixel sensors. In these pixel sensors, each pixel sensor consists of a photo detector and an amplifier. Out of these APS, the most notable is the CMOS Active Pixel Sensor (CMOS APS). CMOS APS has great applications in cameras and also DSLRs. It is called so as it is manufactured by the CMOS process. This type of image sensor is very similar to that of a Charge Coupled Device (CCD). They are also called active pixel sensor imager and also active pixel image sensor.

The CMOS APS uses a photo detector to detect the light and converts it into electrical signal. This signal is then amplified using several transistors and is then moved using traditional wires.

Introduction of CMOS Active Pixel Sensors

The wide use of CMOS Active Pixel Sensors began during the year 1993. The Jet Propulsion Laboratory developed some prototypes which were later commercialized. After knowing its immense potential in the field high speed, low power motion capture cameras many companies quickly adopted and developed this technology.

During the early 1960′s, before the discovery of active pixel sensors, there were only passive pixel sensors. In this mechanism, the pixels were designed in a 2D structure, with access enable wire shared by pixels in the same row, and output wire shared by column. Each pixel did not have an amplifier and so an amplifier was connected ar the end of each column. High power consumption, greater noise and also slow output were some of its disadvantages. In 1969, the active pixel sensor was first introduced by adding independent amplifiers for each pixel. Thus in 1970, the Charge Coupled Device was invented. Thus they were very useful in the working of cameras. During the early 1990′s the CMOS process was well developed and was considered to be the base for all types of logic devices as well as microprocessors. This further led to the invention of CMOS APS.

Architecture of Active Pixel Sensors

For describing the architecture of a CMOS APS, there are mainly three different parameters. They are

• Pixel

The pixel of a CMOS APS mainly consists of photo detectors like a JFET photogate or pinned photodiode. The whole pixel will be called a 4T (4 transistor) cell. The 4T cell mainly consists of a transfer gate, reset gate, selection gate and also a source follower input transistor connected to the photo detector.

The photo detectors used in this 4T cell were first used for Charge Coupled Devices. But, when it was further connected it to the transfer gate and other components the charge transfer was done at a greater speed and also low noise was generated.

There are 3 transistor (3T) cells used now as well. As they are very simple in fabrication they are more commercially produced.

• Thin Film Transistor APS

APS has applications in the field of X-ray as well. For taking digital X-rays, thin film transistors (TFT) is also used. But, its larger size and low gain makes the number of TFTs limited.

• Array

A 2-D array of pixels is organized into rows and columns. The reset lines are connected to the rows so that when RESET occurs the whole row gets reset. Similar is the case of the select lines. The outputs of each pixel in any given column are tied together. Take a look at the architectural diagram given below.

CMOS active pixel transisitor

Applications

The applications of CMOS APS includes web cameras, motion capture cameras, digital radiography, endoscopy cameras and also X-ray imaging. They are mainly known for their application in filmless cameras.

The work of a camera – photography is considered to be one of the greatest inventions of mankind. It has not only helped us see the entire world through a click, but has also transformed how people conceive the world. They can also be kept as a remembrance for the rest of our life.

Camera can be defined as a device that is used to capture and record photos or videos.

Early use of camera

Nowadays we see a lot of advanced cameras that are used to capture motion as well as images from a very far distance. During the time of its invention images could be taken only in a room and could not be portable. The instrument should be kept in a dark chamber or box and the room should function as a real-time imaging system. Thus the camera was earlier called “camera-obscura” which meant “dark chamber”. The first of this kind was invented by a scientist called Johannes Kepler. But this apparatus was very huge and could be portable only as a tent. For this instrument to work the light was passed onto it through a convex lens. Thus an image consisting of external objects would be formed which was subjected to the surface of a paper or glass, placed at the focus of the lens. A much compact and portable camera was introduced in 1685 by Johann Zahn.

After years of work by many prominent people the first colour photo was invented by the famous physicist James Clark Maxwell along with Thomas Sutton. Then came the invention of the video made in cameras during the early 1920s. This technology has eventually grown to such heights that in this 21st century, these ordinary film cameras have been replaced by digital cameras.

Parts of a camera

A camera has mainly three parts. They are

• Mechanical part or the camera body

• Optical part or the lens section

• The chemical part or the film

The way in which these three parts are connected represents the different types of cameras. Thus by combining these three parts and using them under the correct calibration produces a correct picture. They are capable of working in both the visible spectrum as well as in other portions of the electromagnetic spectrum. The basic shape of a camera needs an enclosed hollow chamber with an opening at one end. This opening, also called aperture helps in the entrance of light. This light is the actual image that has to be captured. So a capturing mechanism is set at the other end. All cameras have the lens assembled in the front. This lens helps in capturing the light, which is in turn captured and stored by the recording surface. Most ordinary cameras can take one image at a time. Most video cameras can take a maximum of 24 film frames/sec.

Mechanism of a camera

To know the complete mechanism of the camera, it is better to know each and every parameter of the camera.

1. Focus

A camera’s focus greatly depends on the clarity of the picture taken. But the focus can be limited only to a certain distance. This range is limited to the range of the lens. This range when adjusted to get a perfect image is called the focus of the camera. For accurate focussing of cameras, the device is comprised of a fixed focus and also consists of a wide-angle lens and a small aperture in front of the camera. The range of focus will be clearly indicated in the camera with symbols like two people standing upright, mountains and so on. For a simple camera, a reasonable focus of about 3 meters to infinity is available. The focus available on each camera is different. Single-lens reflex (SLR) cameras have a focus that can be changed according to our like. This is done by providing a objective lens and a moving mirror so as to projecting the image to a ground glass or plastic micro-prism screen. Similarly each camera has different settings which will be explained briefly later.

• The focus of a camera depends on two main features. They are

• The structure and position of the lens.

• The angle in which the light beams enter into the lens.

Consider a pencil kept at a short distance from the lens. When the distance is altered, that is kept near and then farther away from the lens, the angle of entry of the light changes accordingly. This light is hit on the film surface kept inside the camera. The angle becomes sharper when the image is close to the lens and will become narrower when the image is kept far away. Thus when the lens is focused farther and then nearer from the pencil, the image is actually moving closer or farther away from the film surface. The correct image will be obtained when the focus is adjusted in such a way that you can line up the focused real image of an object so it falls directly on the film surface.

Camera Focus

2. Camera Lens

The quality of the photograph taken largely depends on the type of lens used. The precision of a lens depends on a factor called “bending angle”. This in turn, depends on the structure of the lens. If the lens has a flat shape, the bending angle is less. Thus the light beams will converge a little distance farther away from the lens. Thus the image is also formed farther away. Thus when the distance increases, the size of the image also increases, though the size of the film is constant. If the lens has a round shape, the bending angle will be high. Thus the image will be formed a lot more nearer to the lens.

Costly cameras have a lot of lenses, which are replaced or combined according to the magnification required. This magnification power of a lens is called the focal length. Greater the focal length, greater the magnification.

3. Camera Film

For an image to be recorded and viewed it must be stored in a film. When an image is captured, it is actually being “chemically” recorded onto a film. The film mainly consists of millions of light-sensitive grains, which are suspended on a plastic strip. These grains chemically react, when exposed to light. This reaction causes the image to be recorded on the film. This film is then developed by reacting it with other chemicals. For black and white films, the chemicals cause the grains to appear darker when exposed to light. Thus, the darker areas appear lighter and the lighter areas appear darker. This is reversed while printing out the photos.

For producing colour films, the film consists of light sensitive materials that respond to colours red, green and blue. When they are washed and chemically reacted, you get a negative of a colour photo.

Different camera designs

There are a lot of types of cameras like Plate camera, large format camera, medium format camera, folding camera, rangefinder camera and so on. Out of these the most used ones are the single-lens reflex camera (SLR) and the point and shoot camera. The difference comes in the manner in which the photographer visualizes the scene. In a point and shoot camera, you do not see the real image through the camera lens. Instead, you get to see only a blurred vision of the image.

In an SLR camera, you can see the real image of the scee you are about to capture. It has the same configuration as that of a periscope. When the image is seen from the lens, it hits the lower mirror and bounces from there. It then hits the prism. This prism flips the image to form the original image. The mirror and translucent screen help in providing the exact image to the photographer. Thus, you can focus and compose the image so as to get the exact picture you have in mind.

With upcoming technology, the point and shoot cameras are nowadays fully automatic. SLR is built with both manual and automatic controls. The only difference between the manual and automatic cameras is that the former will be controlled by a central processor, instead of the photographer.

The focus system and the light meter transmit the signals to the microprocessor and thus activate all the motors accordingly. These motors control the adjusting lens and also open and close the aperture.