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Digital Camera basics - Part 3: Capturing images

Digital cameras use different methods to capture the image depending on the type of image sensors and colour filters. As the image sensors are only capable of recording the intensity of light (i.e. brightness or darkness), digital cameras use colour filters to produce a full colour image. Thus, digital cameras use the combination of sensors and filters in different ways to record the image.

Capturing the brightness of image
What happens, when the shutter opens (though in fractions of a second) and light enters through the aperture? The light falls on CCD sensor whose pixels (in fact they are light sensitive diodes) convert the light into electrically charged particles. Each pixel, depending upon the brightness of the light that falls on it, becomes charged with different magnitude.  Brighter the light highly electrically charged it becomes. In other words, pixels that record the brighter portion of the image are highly charged while the pixels that record the darker areas of the image will be less electrically charged.  Thus, the image is wholly converted into electrons of various magnitudes on a CCD sensor.

Capturing the colours of image
The image sensor in a digital camera records only the brightness of the image in a series of various intensities. To produce a wide range of colours, the digital camera uses filters when the sensor is exposed to the light (image). The principle behind the colours dates back to the late nineteenth century, when James Maxwell discovered that a full colour photograph can be formed by using three filters of primary colours – red, green and blue. To recreate the natural colour tones of the image, all one has to do is to capture the image using these colour filters in quick succession and put them together on a screen or a print. Digital camera uses the same principle to capture colours, however, it differs from a conventional camera by the way it captures the image.

Single exposure of image sensor to the subject
Some digital cameras use single-shot method, in other words, the sensor is exposed to the image only once. To produce colours, two mehods are used – one way is to use three individual sensors of red, green and red and another way is to use a single sensor with a Bayer filter which has all the primary colours (RGB).

The first method uses beam splitter which splits the light and makes it pass through image sensors of the primary colours (RGB). The Image captured using this method usually will have more vivid, very precise colours and some of the high-end cameras use this type of image capturing.  However, these cameras usually come in bulky models and with very high price tags.
 
The second method is better suited to compact digital cameras. This method uses a single sensor and a colour filter which is placed over it. This colour filter (mostly Bayer filter) helps to produce all natural colours that make up the image. The Bayer filter mosaic is a colour filter array (CFA) made up of red, blue, and green filters each sitting on every pixel or photosite of the sensor. CFA will usually have more green elements as half of the array is made up of it. Red and blue will constitute 25% each. According to this Bayer filtering method, each pixel will represent only one colour of the primary colours (i.e. RGB) and all the pixels need to be interpolated to produce the complete colour details of the image. For interpolation or deducing the complete colour information of each pixel and to produce the full image, a set of demosaicing algorithms are used by a digital camera. The raw data provided by the Bayer filters produce a jpeg or Tiff file depending on the type of demosaicing algorithm used.

Multiple exposures of image sensor to the subject
In some digital cameras, the sensor is exposed to the subject (image) three or more times in succession. Most multi-shot cameras use a single sensor with RGB filters which move over the sensor to get the colour values of each pixel so that the image with true colours can be captured. In another variation, a CCD sensor and a Bayer colour move on the plane where the image is focused to produce sharper picture quality. Apart from these two methods, some digital methods use a hybrid of the two methods, however, the Bayer filter is generally not used.

Scanning technique
This is another method used by digital cameras to capture the image. Here, the sensor scans over the plane where the image is focused. Most sensors used in this type of cameras are either linear or tri-linear which scan a single or three arrays each representing a colour of RGB. Some digital cameras use advanced scanning technique and the camera is rotated while scanning over the focal plane. These cameras usually much more picture details (resolution) than other cameras.

Kodak Linear CCD sensor

When comparing these different techniques of image capturing, multi-shot (multiple exposure of the sensor) and scanning methods offer much better quality in terms of resolution and colour information than a simple single shot technique. Hence, most high-end cameras used for professional use adopt multi-shot and scanning technique while a single shot technique produces decent image quality for amateur use. 

Inside digital cameras : RGB and CYMG colour filters

There are two types of colour filters used in digital photography. They are RGB and CYMG referring to number of coloured filters used in each method. In RGB type colour filter, filters of three primary colours are used – Red, Green and Blue with half of the colour elements are green as the human eye is more sensitive to this colour. In CYMG type colour filter, cyan, magenta, yellow and black filters are used and type of filter is used mostly in colour printers and a few digital cameras.

RGB Filter

RGB uses primary or additive colours (red, green and blue) and mix them in different amounts to produce various colours. When these colours are added in exactly equal amount, the result is you will see white colour. When there is no light hitting on the photosites, you get black. In this method, red and green filters sit over the pixels (photosites) on the sensor arranged one after another. The next row of pixels is covered by similarly arranged array of green and blue filters and so on. This is how the filters are arranged in a Bayer filter or in display devices like televisions and computer monitors. Recently, improved RGB patters are used in digital cameras to produce better results.

CYMG Filter

CNMG colour filter uses four colours - cyan, magenta, yellow and black to capture the colour information of the image. In this method, colours are produced using reflective and subtractive technique and this type of filters use slightly light colour tones when compared with RGB. Since, they use lighter colour tones, they allow much more light to fall on pixels of the sensor and work well with sensors that are prone to noise during capturing of the image.

Digital Camera basics - Part 2: Image Sensors

A conventional camera and a digital camera differ in the way they capture and store an image. The earlier cameras (even now they are not completely replaced by digital cameras) use film to capture the images while a digital camera uses an image sensor instead. The image sensor is made up of millions of pixels which are photosensitive and covert the light that falls on them into electrically charged particles, i.e. electrons. In other words, the resolution of a digital camera is the number of pixels or tiny photosensitive spots present the sensor. For instance, a 5 mega pixel digital camera will have a sensor having 5 million pixels.

CCD sensors and CMOS sensors

Digital cameras use two types of image sensors to capture the images - charge-coupled devices (CCD) and Complimentary Metal Oxide Semi-conductor devices (CMOS). Both the sensors have photodiodes or photosites which capture the light and convert it into electrically charged particles (electrons). Both the technologies produce excellent pixels quality, however, they differ from each other in terms of processing the image and the hardware aspects.

CCD sensors (Charge-Coupled Devices)

Being the older of the two technologies and long been used in various digital cameras like camcorders, compact digital cameras, scanners and astronomical telescopes, CCD is considered better suited to capturing of images in digital cameras and most digital cameras use this technology. Since CCD has passed through various advancements during its evolution, it is capable of producing better resolution of an image as it has large number of pixels that can produce much more picture details than its counterpart.

A CCD sensor, when it is exposed to light, its pixels get charged with different magnitudes depending upon the intensities of the light that strike each pixel. The register in the sensor reads the charge of the pixels row by row. The pixel values are fed into an analog-to-digital converter which convert them into digital format (in 1s and 2s strings). This is done for each and every row of pixels on the surface of the CCD chip.

CMOS Sensors (Complimentary Metal Oxide Semi-conductor devices)

CMOS sensor has been recently used in digital cameras for a few advantages like lower cost of production and less consumption of power. They are made up of small circuits and other devices like transistors knitted together on a silicon chips. Since CMOS chips are used in PCs as processors and memory chips, they are generally produced in large scale resulting in lower cost of production, thus cutting the price of digital cameras when they are used instead of CCD sensors. Another significant advantage that a CMOS sensor has over CCD sensor is that it consumes less power, just one-hundredth of what a similar sized CCD chip will consume. On the negative side, CMOS sensors generally produce lesser pixels quality in comparison with CCD sensor as their photo sensitivity is much lower than CCD sensors. This is because the light strikes the network of transistors arranged near the pixels (photodiodes). CMOS sensors also produce noisy pictures as the processing circuits are present on the same chip.

Major Differences between CCD Sensors and CMOS sensors

• CCD sensors are well known for their superior pixels quality. They are highly photo sensitive. In comparison, CMOS sensors are less sensitve to the light and produce slighltly inferior picture quality.
• CCD sensors need a separate amplifier and Analog to Digital Converter (ADC) to process the image while CMOS sensors come with integrated transistors which amplfy the charges and hence do not need ADC.
• CCD sensors generally do not produce noisy image processing while CMOS sensor are quite prone to noisy pictures.
• CMOS sensors are less expensive and consume much less power. However, a CCD usually costs much higher and consumer many times the power a similar sized CMOS sensor consumes. 

In spite of these differences, both CCD and CMOS sensors are being used by the leading digital camera makers. Despite their advantages and disadvantages, both produce excellent picture quality.

Pixel counts of Digital cameras

What is resolution?
The resolution of an image generally refers to the sharpness of visible objects that the image displays. In other words, it refers to how detailed the image is. Sharper, more detailed images have high resolution. To be capable of producing images of high resolution, a digital camera must have large number of pixels, the unit that measures details on an image. How come a digital camera can record and reproduce images of very high resolution? What are the pixels standards for digital cameras? How they can help the digital camera produce different picture qualities? This article provides you extensive study into this area.

How resolution affects the image quality?
First, let us know the factors that attribute to the image quality. Undoubtedly, the resolution must precede other things like colour quality, contrast etc. Low resolution pictures often lack clarity and sharpness making them unappealing to eyes. On the contrary, pictures with high resolution look clear, sharp and catching our eyes instantly. Most digital cameras come with very high resolution (millions of pixels) and high-end digital cameras can capture pictures at 12 million pixels rate.

Resolution of digital cameras
In a digital camera, the image sensor is the component that determines the resolution of the camera. The sensor or the CCD (Charge Coupled Device) records the image, processes it and changes it to digital information. This semiconductor device is made up tiny units called pixels with a colour filter over each pixel. Each pixel is made up of three numbers which denote three colours (RGB) and the combination produces complete colour.

Digital cameras come with resolution of mega pixels. As the old conventional camera has pixels below one mega pixels (1 mega = 1 million), the images are of low resolution and picture quality continuously degrades with so much grains over them, when you enlarge those images. In contrast, the latest digital cameras come with pixel counts ranging between 2 – 20 mega pixels. This is due to the constant enhancement of CCD chips that make up sensors of digital cameras with every new digital camera that hits the market.

Different pixel counts of digital cameras
Digital cameras for amateur use are available with pixel counts - 1.3 mega pixels, 2.1 pixels, 3.3 mega pixels and 4 mega pixels. Digital camera of 4 mega pixels is considered ideal for everyone – cost-wise as well as quality-wise. Currently, many compact digital cameras come with 8 mega pixels CCD sensors and are capable of producing excellent picture quality and help you to make enlarged prints with quality comparable to prints that you get from studios.

There are, however, cameras with higher pixel counts – 11.1 mega pixels, 16 mega pixels and 20 mega pixels. These cameras are largely used for adding professional quality to the pictures/videos. A 20 mega pixel digital camera can deliver the typical picture quality of a 35 mm movie.

Some facts about pixel counts

Pixel Count Up to 2 Mega pixels
Suitable for thumbnails, online photo sharing, for use in websites. Low resolution cameras produce inferior picture quality.

Pixel Count Up to 2 Mega pixels 3 or 4 Mega pixels
Amateur use. Improved picture quality. Up to 6 x 7 size prints are fine.

Pixel Count Up to above 5 Mega pixels and up to 8 Mega Pixels
Suitable for larger print outs, enlargements. Superb over-all image quality. Expensive.

Pixel count above 8 Mega pixels. 16 or 20 Mega pixels
Suitable for professional photographers. Image quality comparable with motion picture. Very expensive.

Although higher pixel count produces better picture quality, the quality may degrade when the digital camera supports too many pixels that its lens could not handle. This results in so called noisy pictures which show poor shadow details. Overall image quality is also not up to mark as the pixel size becomes tinier and tinier which leads to holding less charged particle.

Digital Camera basics - Part 1

What is digitizing of an image? Conventional cameras use film to record images and chemicals to take prints of the images. Digital cameras which do not require films capture and process the images electronically and store the visual & audio data in digital format. Digitizing information is not new as we have already seen it penetrating all electronic storages like compact discs, DVDs and MP3s. This technology has enabled us to compress vast information or data and store it on a small disk or magnetic tape. It is also much more convenient to handle digital information than analog. You can use a PC or some software to work with digital information. As the images captured by digital cameras are in digital format, they are PC compatible and you can transfer them to your PC or upload them to internet. Conventional cameras still produce good results in terms of resolution and colour quality, however, digital cameras have become one of the most popular gadgets as they have following advantages over conventional cameras: Digital photography does not require films or developing thus becoming one of cheapest hobbies. Only cost you have to bear is the cost of a digital camera and its accessories. Digital cameras can store limitless stills and many video clips in their storage devices – tapes or disks. Images and video clips captured by digital cameras can be directly directed to your PC and you can enhance the picture quality using software like Adobe Photoshop. Most modern digital cameras produce superior picture quality and high resolution images. This is one of the reasons why they have become so popular nowadays. Digital cameras are capable of storing audio which a conventional camera can not do. Digital cameras are constantly improving with time and digital technology adds new features to the high-end cameras that hit the market. How a digital camera converts image into digital information? A conventional camera uses a film (or light-sensitive paper) to record the image. Processing of film and printing the images are purely chemical with a conventional camera. However, a digital camera has an electronic device inside it to record the images in digital format, i.e. in 1s and 0s and each combination of these two digits represent the colours that make up the image. This digital data is actually coded and decoded by the digital camera with the help of an in-built computer. What are the differences between a conventional camera and a digital camera? 1. A conventional camera needs a film to record images whereas digital camera does not require it as it is an electronic device. 2. Images captured by a conventional camera are in analog format as they are result of chemical reaction of film to different intensities of light. Whereas, the digital camera captures the image using its electronic components and converts the images into digital format, in other words, in strings of 1s and 2s. 3. Since you do not need films, digital cameras are ideal for taking numerous still images or video clips. With a conventional camera, there is always a problem of running out of film. However, there are no such problems with digital cameras. 4. Images captured using digital cameras do not require chemical processing and you can review the pictures instantly. It also saves money. 5. You can transfer the images from your digital camera to your PC or send them to anybody via internet. In case of conventional camera, the prints of the images need to be scanned before you send them via internet. However, there are certain similarities between digital cameras and conventional cameras: 1. Both require a different set of lenses to focus the light on the specified area ( photographic film or a semiconductor component) 2. High-end conventional cameras produce excellent results like their counterparts – high resolution and superior colour quality. 3. Images from both cameras can be printed on papers specially made for that purpose.

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