17th Mar 2005

The Digital Camera

Analogue to Digital Converter (ADC)

When a picture is taken, the sensor device sends an analogue signal to the analogue to digital converter (ADC) to specify how much light was captured by each pixels on the sensor. Different cameras have ADCs with different capabilities. Whereas some lower-end of the range cameras will be able to process up to 256 brightness values on the sensor, more professional SLR cameras will be able to pick up to 4095.

AF Assist Lamp (Automatic Focus Assist Lamp)

Commonly referred to as the ‘automatic flash’ on a camera, this function aims to illuminate a photograph’s subject in low light. It is usually positioned above and to the side of the lens barrel and can help the auto focus operate within approximately 4 metres of the subject.

Some more modern cameras use a system involving infra red light instead of white light. This is far better for taking photos of subjects that you don’t want to surprise or frighten.

AF Servo (Automatic Focus Servo)

This feature is usually found on the professional SLR cameras rather than the average digital camera. The function allows the camera to stay focussed on a moving object and is particularly useful for photographing wildlife or sports.

The way in which AF Servo mode is switched on will depend on the model of camera but is usually activated by partially pressing down the shutter release button. The camera will then automatically focus and refocus on the moving object until the shutter release button is fully depressed (when you take a picture).

Camera Batteries

Battery life can be a huge problem for digital photographers. Many older digital cameras (1999-2000) take AA batteries as their source of power. Although these batteries are cheap to buy and easy to find, they usually do not sustain nearly enough energy to keep the camera running long enough for you to fill the memory card with pictures.

However, newer models of digital camera (2000+) tend to supply rechargeable alternatives. These are either charged in a separate battery charger, or inside the camera using an external adaptor.

NiCD (Nickel Cadmium) Batteries

These are the most common form of rechargeable and have an average life of about 700 recharge and discharge cycles. However, to work at their optimum level, they must be completely discharged before recharging. Gas bubbles can build up on the battery cell plates if it had not been properly discharged and can seriously affect the performance of the battery – this is known as memory loss. For this reason, NiCD are normally only used as backup batteries

NiMH (Nickel Metal Hydride) Batteries

These are the most common batteries for use in digital cameras; they have a larger capacity and do not suffer from memory loss. However, even when not in use, NiMH batteries gradually lose their charge and their average life is only about 500 recharge and discharge cycles.

Lithium-Ion Batteries

Lithium-Ion batteries have a much larger capacity that both NiCD and NiMH batteries but they require their own charger and can be quite expensive. These types of batteries usually come with the camera as they can not be bought in AA format.

Camera Buffer

The buffer acts as the cameras random access memory (RAM) and stores the photo before it is written to the storage card. This improves the efficiency of the camera and allows for a burst (or continuous) function to take place.
There are two different types of Buffer, which are explained below:

Before Image Processing Buffer

This system holds the data of the picture that has just been taken in the buffer before it is processed. Because of this, no matter what quality or size you choose to take you picture at, the limit of pictures that can be held in the buffer at any one time will always be the same because they have not yet been processed.

After Image Processing Buffer

This system only holds the image in the buffer after it has been processed. This means that the size or quality of the picture that you have set will affect how many images can be held in the buffer at any one time. The larger or greater the image quality, the less images will be able to fit in the buffer thus affect the burst (or continuous) function.

Burst Mode & Continuous Mode

Depending on the model of camera, this feature can be called ‘burst’ or ‘continuous’ mode. It means that instead of taking one picture at a time, the camera is able to take a set of photos within a short space of time. This is particularly useful for taking action shots where the subject is moving and you want to catch it at several points.

The amount of pictures that can be taken at any one time depend largely on the size of the memory buffer in the camera. Pictures are taken too quickly to be stored individually on the memory card so the memory buffer stores them until the camera has more time to write them to the memory card.

Digital Camera Connectivity

Connectivity describes how well a digital camera can connect to other devices i.e. a computer – to download images that have been taken.

Image transfer

The vast majority of modern digital cameras use a USB connection to connect to external devices. This is a relatively quick method of data transfer (up to 350KB per second) and is compatible with a large range of devices such as PCs, Macs and a selection of PDAs.

However, Firewire (iieee1394) connections are become increasingly popular because of the speed in which then can transfer data (up to 500KB per second). However, Firewire requires a special connection that is only recently coming as standard with a PC or Mac.

Remote Control

Very recent digital cameras have started to be supplied with remote control applications which allow pictures to be stored straight onto the hard disk of your PC or Mac through a USB connection.

Video Output

Most digital cameras also have a video output facility to connect your camera to a TV, to view your pictures on a larger screen. You can also connect your cameras to a VCR, to record any small video clips onto video cassette using this method.

Effective Pixels

As part of the specification, many digital camera manufacturers use the total number of pixels as a representation of the cameras resolution. However, the pixel count of a camera should really be calculated by the number of recorded pixels rather than the actually number of pixels. A camera’s sensor assesses four aspects of the pixel count:

  • Total number of pixels
  • Number of read pixels
  • Number of active pixels
  • Recommended recorded pixels

The camera only records a crop of the total number of pixels. This is because some of the outside horizontal and vertical columns around the edge if the sensor is coated with a video signal shading (a black dye). In order for a digital camera to know what black is, it needs to take a reading from a pixel, which it knows is black.

The small gap around the outside shows the read pixels that do not record the image. The smaller area usually then leaves the output of the image in a 4:3 ratio format (the standard photo size).

EXIF (Exchangeable Image File)

This image format was created by JEIDA (Japanese Electronic Industry Development Association) towards the end of 1995 but has been reviewed several times since then. Most digital cameras store images in EXIF compressed format, which uses the same compression as a JPEG image. This means that each images downloaded from the camera can be viewed by any application supporting JPEG format.

EXIF also contains additional information into the header of the image, which include information on how the picture was taken i.e. aperture and shutter settings, the date and time that he picture was taken and sometimes other information about the actual camera. Although software to read these headers is usually included with the camera, there are some free packages that can be downloaded from the internet.

Digital Camera Lag Time

Lag time is a common criticism amongst digital camera users since it can take a number of seconds between the shutter release button being depressed and the camera actually taking the photo. This time varies immensely among different models of camera but the issue has been recognised by digital camera manufacturers and total lag time in newer models of camera seems to be getting shorter.

Auto Focus Lag

This is the time that it takes the camera to automatically focus. The time can be affected by a number of factors including, available light, position and activity of the subject.

Shutter Release Lag

Assuming that the subject is already in focus, this is the time that it takes for the shutter to be opened and the photo to be taken.

Total Lag

This is the total time that it takes for a picture to be taken. As well as the Auto Focus Lag and the Shutter Release Lag, it allows time for the photographer to find the shutter release button and make a personal decision on when to take the photo.

LCD – Liquid Crystal Display

The LCD screen acts as a large view finder so that you can see your subject without having to peer through a small hole (although the small view finder also features on the majority of digital cameras). It also acts as a place to view and set menu options, and view previously taken photos. The TFT screen is usually range from between 1.5” and 2” diagonally from corner to corner depending on the camera model.

Sensor (CCD / CMOS)

In the absence of a film a digital camera needs a way of capturing an image. This job is given to the sensor (CCD – charge-coupled device/CMOS – Complementary Metal Oxide Semiconductor). The sensor records light exposed to it, these are then read and analysed by the pixels (or photosites). It is sometime helpful to compare pixels to small containers, which are able to capture the light particles. The light captured is then analysed when the ‘containers’ are emptied. The actual sensor itself can only recognise black and white, therefore a colour filter array is needed to distinguish colours and create a colour photograph.

The amount of light that the ‘containers’ are able to capture determines the dynamic range of the sensor. Professional SLR cameras tend to have more of a dynamic range than the more common consumer grade digital cameras.

Interline Transfer Sensor

The vast majority of consumer range digital cameras use an interline transfer sensor. Using an electronic shutter system, it automatically starts and stops recording the light hitting the sensor. This information recorded by the sensor is then transferred onto ‘shift registers’ before being outputted as an image. This type of sensor has many advantages – it can produce video feed and although usually used alongside a mechanical shutter, it doesn’t need one because it can be controlled by the camera’s software.

Because each pixel requires a lot of electronics which surround it, the actual surface area of the pixel becomes quite small. For this reason, a microlense is placed over the pixel to increase the area.

Full Frame Sensor

Full frame sensors are mainly used in professional cameras. Full frame sensors do not used electronic shutter systems, they only use mechanical shutters (like those in film cameras) to allow light onto the sensor. The sensor then transfers the data it has recorded onto the ‘serial register’ to be processed. The image is then outputted into RAW format (for more information on RAW.

Digital Camera Storage Cards

Digital Cameras use a form of memory storage instead of a film – that’s what makes them digital. In most cases, the memory storage is removable but some cameras use a built-in flash unit. In this case, a cable is used to transfer the data from the camera to an external device such as a printer or PC.

There are several removable storage solutions in use at the moment and they are explained below:

PCMCIA PC Card

This is a more professional type of media storage and are relatively large in size. In fact they use the same PCMCIA cards in laptop computers. They are primarily used in more specialised SLR cameras as PCMCIA are too big to fit more compact versions.

Compact Flash Type 1

To date, these are the most popular types of memory card and can be shared with other small devices, such as Personal Digital Assistants (PDAs). They may store up to 256mb.

Compact Flash Type 2

The compact flash type 2 has the same advantages as the type 1 but they are slightly larger. This is to allow for extra storage as they can store up to 1GB of data.

SmartMedia

SmartMedia cards are significantly slimmer than Compact Flash but have a more limited storage amount of either 64MB or 128MB. However, SmartMedia cards only contain the flash memory chips so the camera’s own controller chips need to be configured in order to recognise this format.

Sony MemoryStick

The Sony MemoryStick is one of the smallest types of digital storage. Although, on its launch many people were sceptical as to its success, many devices are now Sony MemoryStick compatible. They can also be purchased with up to 256MB worth of storage.

Thumbnail Format

The majority of digital cameras let you view the photos that you have taken in thumbnail format for easy navigation. The term ‘thumbnail’ describes a very small version of the image – not large enough to pick out any detail, but enough to recognise the photo. This mode allows you to select a picture to be viewed on full screen or in some cameras, change minor settings or delete an image.

Viewfinder

The view finder is the window that allows you to view your picture composition before a photo is taken. The largest problem with viewfinders is the difference in what it sees and what the actual lens sees. This is because of their difference in position. For this reason, LCD view finders have become more popular.

There are three main types of view finder:

Optical Viewfinder

This is the small window at the back of the camera, the same as the viewfinder you would find on a film camera. There are some issues with this type of viewfinder as what you see through it is slightly different to what the lens sees – this causes more trouble with close up subjects because the different views are more noticeable.

TTL (Through the lens) Optical Viewfinder

This is the most advanced type of optical viewfinder, found on SLR cameras. By passing the image through a prism or mirror, a small screen inside the viewfinder actually shows the image that the lens sees. This system is only used in very professional SLR cameras.

Electronic Viewfinder (LCD Viewfinder)

An LCD viewfinder simply relays the image that would be seen on a larger LCD screen on the back of the camera. But instead it is positioned just inside the small viewfinder window. Because of the way this system works, it doesn’t experience the same problems as an optical viewfinder.

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