Digital Cameras

Last updated: 18 May 2009
Published in: Digitising analogue media | Creating new digital media
Tags: business & community engagement | cameras | photography | slr

Summary

Digital cameras come in a wide range of shapes and sizes, this document details the different technologies used to capture a digital image. This paper also covers the features that a user should consider before choosing a digital camera.

Contents

1. Introduction
2. Receptor technology
   • 2.1. Imagers
   • 2.2. Which sensor to choose?
   • 2.3. Capture technologies
   • 2.4. Camera type
3. Other camera technology
   • 3.1. Lens quality
   • 3.2. Camera shutters
   • 3.3. Image preview or review
   • 3.4. Filters
   • 3.5. Chip cooling
   • 3.6. Lighting
4. Image transfer
   • 4.1. Memory storage cards
   • 4.2. Image file formats used by digital cameras
5. Choosing a camera

1. Introduction

Digital cameras, like their analogue equivalents, come in a wide range of shapes and sizes making them suited to some tasks but not to others. For example while a compact digital camera is ideal for occasional spontaneous photography on location it would be hopeless when used in a formal photographic studio to capture a painting.

The traditional analogue cameras that are in use today, have taken over one hundred years to gradually evolve, whilst today's digital cameras are being developed at break-neck speed at the forefront of technical innovation.

Development at the cutting edge of technology is always a very costly affair for manufacturers, which has led to digital cameras holding a premium price for a short period before they in turn become delegated to second rate by a new crop of cameras.

The typical production run for a compact digital camera might be 6 months to a year while an entry level digital SLR might have a run of 12 to 18 months and a top of the range professional camera around 3 and a half to 4 years. In contrast Nikon's flagship analogue manual focus camera, the F3 was introduced in 1980 and discontinued 21years later in 2001.

The rapid evolution in digital photography has led to a situation where any recently developed camera is comparatively much more expensive than its analogue equivalent (although these current prices are expected to fall rapidly).

If digital cameras are so expensive and the rate of development so intense as to quickly outdate them, it would be fair to ask in what way are they so different to their analogue siblings and how do they compare? In reality there are differences, as will be explained, but overall the similarities between digital camera systems and traditional analogue cameras can be seen to easily outweigh the differences between the two technologies (See JISC Digital Media advice document Using a Budget Digital Camera for Teaching, Learning and Research.

This document looks at the underlying technologies that drive the digital cameras available today and shows how these technologies influence how the camera works and will hopefully enable you to make the correct choice of digital camera.

We will start by considering the most basic part of the digital camera, found at the very beginning of the image capture process; the image receptor.

2. Receptor technology

A traditional analogue camera is a pretty basic device, with the simple task of accurately exposing a piece of film though a lens and shutter. All of the clever work is done externally, both before the shot is taken, when the film is 'designed' and 'made' and also afterwards when the film is 'processed'. A digital camera is by comparison a very complicated device, with all the image processing undertaken internally by the cameras' electronics. It has to undertake a vast range of tasks in order to capture the image, process it and then store it within a digital form.

Of course, the digital camera still needs all the same physical parts as the analogue camera, such as lens, shutter and focusing mechanism, but rather than simply having film to react to the light that makes up the image, the camera must have internally all of the electronics necessary to capture the light and change it into a digital image file.

2.1. Imagers

The two most common types of sensors used in digital cameras are the CCD - Charge Coupled Device and the CMOS - Complementary Metal Oxide Semiconductor

The digital sensor in an SLR camera

Both CCD and CMOS are pixelated metal oxide semiconductors (photo-diodes) made from silicon. They have basically the same sensitivity within the visible and near-IR spectrum. They both convert the light that falls onto them into electrons by the same process and can be considered basically similar in operation. Both CMOS and CCD imagers can only sense the level/amount of light but not its colour. Coloured images can therefore only be created either by coating each pixel with a coloured filter (red, green or blue) and then interpolating the missing information or making 3 exposures, each through a coloured filter (again red, green and blue).

Neither of the names 'CCD' or 'CMOS' has anything to do with image sensing; 'Charge Coupled Device' is a description of the technology used to move and store the electron charge and 'Complementary Metal Oxide Semiconductor' is the name of the technology used to make a transistor on a silicon wafer.

The fundamental difference between the two technologies is the architecture of the imager within the chip and camera. The difference is less 'what' they are and more 'how' they are used. This is because the imager not only measures the light that falls on it but also undertake a host of other electronic tasks. Where and how these tasks are handled is what differentiates the two types of sensor.

Diagram 1 - Image processing within a CCD imager. Please click the image to open a larger version of the diagram

Within a CCD imager all image processing is done off-chip, away from the sensor, allowing for a versatile approach to sensor and camera design. Only the 'Photon-to-Electron' conversion is done within the pixel providing the maximum amount of space within each pixel to be used for capturing image information. The 'electron-to-voltage' conversion is done on the chip and data therefore still leaves the CCD in an analogue form to be digitised within the supporting camera circuitry before downloading to memory.

Diagram 2 - Image processing within a CMOS imager. Please click the image to open a larger version of the diagram

With the CMOS imager both the 'Photon-to-Electron' conversion and the 'Electron-to-Voltage' conversion is done within the pixel, leaving less room for the light receptive part of the sensor. This means the CMOS chip has less area to actually receive the light and normally some form of micro-lens is needed to capture the light the pixel would otherwise miss.

Diagram 3 - Micro-lens technology. Please click the image to open a larger version of the diagram

Micro-lens technology is used to help capture more light and bend it away from the circuitry on the chip and towards the light sensitive parts of the pixel.

The amount of the pixel that is sensitive to light is called the 'Fill Factor'. Whereas with a CCD this may be as high as 95% with a CMOS it can be much lower, as small as 50-60% and the micro-lens is needed to make best use of the light falling onto it.

The extra sensing area within the CCD imager allows the CCD based camera to capture more light, which will normally provide higher quality than a camera based on the CMOS. On the other hand, the CMOS chip has the advantage of having everything that it needs to work within the chip, thus it is able to provide a 'camera-on-a-chip' whereas the CCD camera needs many other secondary chips (from 3 to 8) as well as the imager to allow it to work. A CCD produces analogue signals that are moved away from the chip before they are digitised and converted to 1's and 0's by an external dedicated chip. On the other hand a CMOS undertakes the digitisation within the chip by undertaking the image-capture process within each pixel.

 

2.2. Which sensor to choose?

The decision really depends on what you want the sensor to do and it should be realised that the camera's final image quality is often more dependent on the overall design of the camera and lenses than on the imager itself.

Up until quite recently the CCD based camera has been considered the 'de facto' standard within the digital market and a great deal of development has been invested in producing these sensors to maximise their quality potential. They can offer high resolution (depending on how they are utilised) and high quality, albeit at a fairly high price.

The CMOS based camera is generally much simpler to manufacture as there are far fewer components due to most of the processing technology being included within the chip. At the time of writing both the CMOS and CCD sensors have their strengths and weaknesses and developers have overcome most of the limitations of the early CMOS sensor. Today both Canon and Nikon use CMOS sensors in their flagship cameras.

Some manufacturers such as Fuji have developed sensors which make more efficient use of the capture area and increase pixel density.

A comparison of CCD and CMOS image sensors:

CCD	CMOS
Long history of high quality performance	Lower performance in past, but now providing comparable quality
High Dynamic range	Moderate Dynamic range
Low noise and best dMax	Noisier, but getting better quickly
Well established technology	Newer technology
High power consumption	Relatively low power consumption
Moderately reliable	More reliable due to integration of chip
Small pixel size (small sensors - best to develop new cameras & lenses)	Larger pixel size (larger sensors - easier to use within current camera technology)
Needs lots of external circuitry	All circuitry on chip
High Fill Factor	Lower Fill Factor
CCD creates analogue signal that is digitised off the chip	CMOS creates a digital signal on chip

So that is a quick overview of the different receptors used to capture digital images, let's now consider how these imagers are used within the camera.

2.3. Capture technologies

There are a variety of different methods that can be used to present the photo-receptor (imager) to the light patterns that make up the image within the camera. Each method has its own advantages and inherent compromises and will be best suited for a specific use. In general these capture methods can be divided into those that give an instant capture, allowing the camera to work with a moving subject and those that take a longer time to capture the image (you can't capture action) but which provide images of a higher quality.

Although the choice of camera is normally a trade off between versatility and quality there are a few specialist camera backs, which can use a single exposure to capture moving subjects and multiple exposure for stationary objects where quality is paramount.

The main capture technologies used within digital cameras at present are these:

2.3.1. Scanning tri-linear array

Diagram 6 - Scanning tri-linear array. Please click the image to open a larger version of the diagram

Copyright AGFA - used with permission

This is basically the same technology that is used within a CCD flatbed scanner and gives these cameras their nickname: 'Scanners on a stick'. In most cases the scanning mechanism is contained within a 'scanning back' that is attached to the back of a traditional professional studio camera in replacement of the more normal 'film back'. The sensor is made up of three parallel lines of CCDs (each coated with either Red, Green or Blue filters) that are gradually moved across the image area by a stepper motor and lead screw. At each step (depending on chosen resolution) a reading is taken, thereby building up a complete colour image (with accurate colour data at every pixel position).

This is a very mature technology (based on scanners) that allows images of an extremely large size to be created (up to 850MB) and as there is no interpolation of image data the quality can also be very high. However cameras using scanning technology need a very controlled environment to work well, which normally restricts their use to the photographic studio. Exposure times are normally very long (think minutes rather than seconds), as the long line of CCDs has to scan slowly across the whole image reading every pixel individually, so you can't capture any action photos at all. As the exposure takes such a long time it is imperative that some form of continuous light is used as any slight variation or flicker in the lighting will ruin the image (which rules out flash-lighting). During exposure the camera must be kept absolutely still as any vibration in camera or of the subject will lead to unsightly artefacts (normally banding) in the image.

Scanning back attached to large format camera

Scanning cameras can capture large sized images of impressive quality, but they tend to be slow in operation. They need both a high level of skill and great care to get the best out of them. They are best suited for studio use and in particular copy work, where the subject does not move (hopefully) and the lighting can be easily and completely controlled.

Scanning arrays are normally sensitive to a wider range of the electromagnetic spectrum than the typical digital camera. They are particularly sensitive to infra red light, this does not present a problem under fluorescent studio lighting, however incandescent  and daylight both contain a lot of infrared which needs to be removed to produce satisfactory results. To remove the infrared wavelengths an infrared cut or 'hot mirror' filter should be mounted in front or better still, behind the lens. While the physical size of digital sensors designed for professional medium and small format camera systems is increasing the scanning back still offers the largest capture area.

2.3.2. Area array

Instead of using a long line of CCDs to progressively scan the image within the camera, a matrix or grid of CCD elements can be used instead. The primary advantage of this system is the ability to capture images all in one go; in an 'instant'. However, reliably creating large pixel dimension matrix CCDs is technologically very challenging and so far it has been impossible to create a single matrix that has a size in any way comparable to what is easily created with a linear array CCD (currently the largest matrix CCDs are 8984×6732 pixels).

The main disadvantage of this system is that each pixel can only capture one colour at a time, so to create full colour information the Matrix must either be exposed 3 times (to capture R, G & B data) or the missing colour information must be interpolated from surrounding pixels.

To capture an image, these 'Area Array' or 'Matrix' CCDs can be utilised within the camera in a range of different ways:

2.3.3. Single matrix - one-shot

Diagram 8 - Single Matrix One-shot. Please click the image to open a larger version of the diagram

Copyright AGFA - used with permission

The single matrix one-shot is the most common technology presently used within digital cameras. All of the digital cameras that are for sale within the consumer and pro-sumer market use this technology. A matrix or grid of CCD/CMOS elements is used to capture the whole image in one go, which provides instantaneous capture. This makes the camera much more versatile and able to capture moving objects as well as working with a wide selection of lighting including daylight, flash and tungsten.

The CCD/CMOS can of course only detect luminance (but not colour) so the colour information must be built up by having alternating coloured coatings on each pixel, normally in a Red-Green-Blue-Green pattern providing twice as many green as blue or red pixels. Every pixel can detect the intensity of the light that falls onto it, but the colour of each pixel must be interpolated from the colour-values of the surrounding pixels. This means that a large amount of the image-data is 'made-up' or interpolated and therefore potentially less accurate. This can lead to visual artefacts that are particularly noticeable around any sharp edge such as text as colour fringing, where the camera has made an incorrect guess about the pixel's colour based upon the colour of its neighbouring pixels. The algorithms used to undertake this colour interpolation have improved dramatically over the last few years and are improving all the time; however the 'colour accuracy' of these cameras can only ever be considered as suspect when such a large amount of the colour has been 'deduced' rather than accurately 'read'.

One side effect of this type of capture system is that this interpolation requires lots of processing, which takes both time and power to accomplish. This can slow the camera down, thereby increasing the time before it is ready again and also goes some way to explain why camera batteries last such a short time.

There are ways around this; most higher-end cameras now offer the ability to export the image in a 'Raw' image file format where all the image data is exported prior to the image processing. This has many advantages; the camera is faster in operation, the files are not compressed, but significantly smaller than an uncompressed TIFF, yet they have all the available image data created by the CCD chip. Of course, at this stage, they cannot be considered finished images until they are processed to interpolate the missing data. So, although the camera-work will be faster, the image-workflow has now gained a further stage in its chain. This workflow will also raise some preservation issues regarding whether it is best to archive the original 'Raw' data as it is closest to original capture (although not yet fully formed), or to archive the image (after it has been fully created through interpolation) in an open 'preservation' format such as TIFF.

 

2.3.4. Single matrix - macro-shift

Diagram 9 - Single Matrix Macro-shift. Please click the image to open a larger version of the diagram

Copyright AGFA - used with permission

One of the main limitations of the single matrix sensor is that a colour image is created by interpolation. The macro-shift sensor offers an alternative approach, in addition to standard single shot capture these sensors also offer 'multi-shot' capture. In this mode the sensor shifts in a square movement at single pixel increments so that the full RGB colour is measured at every point on the sensor. If a macro-shift is used in multi shot mode then, like the tri linear scanning array care should be taken to avoid subject movement between exposures. This technology is highly specialised and only found in the 'digital' backs' produced by a couple of manufacturers.

2.4. Camera type

Each of the capture technologies described above can be seen to have inherent advantages and compromises that make it best suited for a certain type of use. What you want to use the camera for will be largely dependent upon what technology it uses and this will be reflected in what 'type' of camera it is.

Let us therefore consider the types of digital camera available and what capture technology is likely to be used in each type of camera.

2.4.1 'Consumer' compact

The consumer digital compact accounts for the majority of digital cameras being manufactured today. Their design is largely driven by the hard-nosed requirements of commercial success within the consumer market. They have to be relatively cheap to manufacture whilst continually offering an advance in quality, ease of use and additional functionality over their competitors.

These cameras are designed and optimised for normal amateur photography of friends, family, holidays, landscape and other general views. This of course means that it is imperative that the camera is able to capture 'action' images. For this reason, the only capture technology that will work for these cameras is the Single Matrix one-shot type.

Digital compact camera

A few consumer compacts offer image viewing via a direct optical 'glass' viewfinder, but the most commonly used system for composing a scene with this type of camera is via a digital LCD display with direct video feed from the sensor.

The purchase cost of these cameras is really controlled more by marketing than performance and we are unlikely to see them drop in price a great deal (currently from approx. £50 to £500): however it is certain that the performance (quality and functionality) will improve steadily.

2.4.2. 'Bridge' compact camera

There are a few cameras, at the top of the consumer price range, which although still compact cameras, are aimed at the more advanced amateur or the professional wanting an easy and relatively cheap entry to digital capture. Quality and performance of these units is of course better than their cheaper siblings and they provide some of the additional control and functionality that the professional needs. Of course, in the end, how ever much functionality they include, they are still basically compact-cameras with all the limitations that this type of camera naturally has.

2.4.3 35mm SLR

As soon as the photographer leaves the studio, a camera is needed that is able to work hand-held and of course capture action. This means that they (like the consumer-compacts) are limited to using single shot matrix-array image receptors. However, they will also need to be able to choose and use a wide range of differing lenses, whilst seeing exactly what the lens is capturing. This functionality can only be provided by a Single Lens Reflex (SLR) camera system.

Digital SLR camera system

These cameras are effectively a matrix CCD/CMOS set within a 35mm camera type body, allowing the camera manufacturer's range of separate lenses to be used. However it should be noted that only a few camera models have photo-sensors as large as a piece of 135 film, so all the lenses appear to be significantly longer when used on a digital camera (normally between ×1.4 and ×1.6 longer).

Canon and Nikon digital SLRs have evolved from their analogue predecessors, a few lenses made decades ago can be attached to recent camera bodies, though not all of the features offered by modern lenses, such as auto focus will be available.

The digital SLR camera offers the most versatile approach to digital capture and is likely to be the first choice for any photographer wanting to work on location shooting sports, journalism, fashion, people or any other action.

Although digital SLRs aimed at the professional market are very expensive the models aimed at the consumer are now competing on price with 'bridge' and compact cameras. Digital SLRs range in price from two to three hundred to nearly five thousand pounds.

There are two notable snags when working with these cameras:

First, although it is of course, a great advantage to being able to change the lens, it also provides an easy way for dirt and dust to enter the camera and settle on the delicate sensor and degrading the images. With a film camera, this will only normally affect one image, however with a digital camera, it will continue to affect all images until it is removed.

Because the light path through the camera which is used to record the image is diverted by a mirror to the eyepiece to assist in focus and composition, the LCD display normally only displays captured images. A handful of 'flagship' cameras offered by the main manufacturers offer 'Live View'.  In this mode the mirror swings out of the way and the shutter is opened to reveal the sensor, the scene is then displayed on the rear LCD screen prior to capture. This feature is of real value in the studio as it allows the photographer to check the imaging area at pixel level magnification to ensure accurate focus and composition. 

2.4.4. Studio cameras

Some studio cameras have been developed from the ground up to work around a particular capture technology. This was a particularly popular approach amongst early designs, however it leads to cameras that were visually and functionally very different to traditional cameras. It has now become apparent that most professional photographers are more comfortable with cameras that look and handle like the film-based cameras that they are used to. This has led to the latest digital studio cameras being made as 'digital-backs', which can replace the 'film-backs' within a normal professional studio camera system.

Digital camera back (foreground) and medium format SLR camera

These cameras provide very high quality indeed, creating images of large pixel dimensions. Some systems avoid the need for interpolation and the damaging affect this can have on image quality. These cameras can easily compete with analogue derived images from similar professional studio equipment. However, it must be said, they often have prices of a similarly impressive magnitude.

Development of these cameras has been so fast that although a few are beginning to turn up second hand, by the time they do, they are normally considered very low quality in comparison to what is currently available.

3. Other camera technology

Of course, there is more to a digital camera than just a light receptor and some clever electronics. There are also a host of other factors that contribute to the quality of the images.

Here are the main factors:

3.1. Lens quality

The quality of any image, whether made by a digital or an analogue camera, can only ever be as good as the lens that is capturing the light and bending it back towards the light receptor (CCD or film). Often, the biggest single difference between a professional camera and a cheaper alternative is simply the quality of the lens that is used on the front of the camera. However, for the digital camera, the need for high quality optics is even more important. Although some scanning-backs (using a tri-linear array) can provide a larger capture area, most of the matrix cameras tend to have receptors considerably smaller than a 35mm analogue camera, which means that is imperative that the lens is able to accurately resolve very fine detail within this small area. A growing number of digital cameras have been produced with sensors using the full size of 35mm film. While these larger 'full frame' sensors allow photographers to exploit the features of specialist lenses with larger image circles, the cameras with smaller sensors offer better 'reach' at the longer focal lengths. Although at present there are only a few cameras that have a receptor of this physical size, it is likely that there will be more in the future. At present it is only the most expensive SLR type digital cameras made by Canon, Nikon and Sony that do.

3.2. Camera shutters

Shutters traditionally have worked by physically revealing a light receptor (CCD/CMOS or film) to the light and then closing it again after the correct time has elapsed. In an analogue camera, this was done by some form of mechanical blind that opened and shut allowing a known amount of light through to the film or sensor. It is quite possible to use this same system with a digital camera but generally much easier to just 'turn' the receptor on for an established time to receive the light. This is in effect an electronic shutter and has the advantage of no moving parts at all. Exactly how this 'electronic' shutter works and for that matter how long it takes will be entirely dependent upon the type of receptor that is being used; the shutter for a multi shot camera will need to provide 4 exposures, whilst of course the shutter for a scanning back might need to be open anything from a few seconds to many minutes, whilst the image is gradually built up. For one-shot cameras, the shutter must be able to only open for a fraction of a second, allowing the camera to capture a moving image, although this can still be easily done electronically.

3.3. Image preview or review

Like the traditional analogue camera, it is impossible to capture an image and use the direct optical viewfinder simultaneously.

A small number of 'compact' digital cameras have an optical viewfinder with eyepiece in addition to the standard LCD monitor, this uses a separate (glass) viewing system to that which will be used to record the scene.  Most compact cameras however now only offer an LCD display, which is used for both composing the image and viewing the captured shots. While the LCD display provides an effective method to both preview and review images it does require a lot of electricity and can rapidly drain the camera's battery.

The professional SLR type digital cameras have more of a problem, as you cannot view through the camera's eyepiece and take a video feed from the sensor at the same time. This means on most digital SLRs it is impossible to have access to a preview image whilst using the camera. You have to shoot an image and then review it on the LCD screen. The 'Live View' feature offered by a handful of cameras allows the photographer to preview the image with the aperture stopped down to the setting that will be used to exposure the actual shot. With this feature the studio photographer will be able to preview the 'depth of field' and control focus at the pixel level. The 'Live View' feature is beginning to appear in prosummer bodies and may eventually be standard on all SLR cameras.

The clarity of the image on the LCD display can sometimes be an issue, particularly when the subject is in low light or when strong light is shining on the LCD screen.

With studio cameras, there is a wide range of differing capture technologies and although these cameras are often used tethered to a controlling computer (from which many camera operations are undertaken), they do not all offer the ability to see a live preview of the image.

Digital camera back tethered to a computer

As a rule, scanning backs are unable to provide a pre-view of the image without providing some form of extra video sensor, but some matrix-based cameras can provide a preview as long as the normal viewing system is closed down. Either way, the preview image is always likely to be of low quality so therefore it is still normally better to take a 'test' shot first and then make final adjustments based upon the results. If flash lighting is being used to illuminate your subject then the live preview will not give you an accurate representation of the final image.

3.4. Filters

Both CCD and CMOS photo-receptors respond to approximately the same range of colours and both are much more responsive at the red end of the spectrum than at the blue end. Indeed they also respond well into the infrared wavelengths, which can be useful, although is more often a problem in normal photographic use. Dependent upon the filters used, CCDs and CMOS can easily be used for B & W, colour or infrared (IR) photography. However in normal use, it is necessary to filter out any stray infrared light that although invisible to the eye, might still be present and visible within the photograph. This is done by installing an IR-cut-off or 'hot mirror' filter, which totally cuts out the IR, only allowing the visible light to come through and fall onto the image receptor. Hot mirror filters are normally fitted above the sensor by the manufacturer but some sensor types require the user to attach the filter to the camera themselves. These filters come in two strengths, they are not required for images taken under fluorescent lighting, the weaker filter is used for daylight and the stronger one for tungsten lighting.

It is normal to also compensate for the lower blue sensitivity of the receptor by giving the blue pixels a larger exposure or by amplifying the blue signals within the image processing. It is this inherent weakness of the blue channel of any digital image that leads to the blue channel nearly always showing more noise or artefacts than the other channels. (Hint - if you want to visually check the quality of a digital camera image, always look in the shadow areas of the blue channel. If there are any quality problems, this is where they are likely to be).

3.5. Chip cooling

A digital camera is a complex electronic device with a great deal of image processing being undertaken within and around the image sensor. All this electronic 'thinking' can generate a build up of heat within the chip which in turn leads to erroneous charges within the sensor, leading directly to noise within the image. Designers use various methods to get around this problem, from large heat-sinks, to active cooling. Note that this can cause problems if the chip is over-cooled in a humid climate, as this can encourage condensation on the surface of the chip with serious degradation of image quality. One of the main advantages of using CMOS rather than CCD technology is that as a large amount of the processing is done 'on chip' the required voltages are less and the resultant heat build-up correspondingly less also.

3.6. Lighting

Although not strictly part of the camera, it is worth noting that some of the capture technologies that we have discussed are not compatible with all types of lighting. The one-shot matrix cameras have been designed to work in as similar a way as possible to their analogue siblings and will work with most lighting systems. However, some of the other camera technologies (scanning and  multi shot) can be very fussy.

Cameras using a scanning array need a constant light source, if they are used with incandescent lighting, which flickers at 50hz the stepper motor must be synchronised to the alternating current to avoid image banding. Of course it is simply impossible to use flash with any scanning system. So we normally have to use constant lighting such as daylight,  HMIs (Hydrargyrum medium-arc iodide) and Fluorescent Luminaires (See JISC Digital Media advice document - Photographic Guidelines).

Cameras using multi-shot systems will need four matched exposures. Although many different lights sources can be used, it is important that each exposure is exactly the same. This should not be a problem with most constant lighting or even tungsten, but can be a problem with some flash units. This is because some older flash-packs allow the flash to be ready to shoot again after only reaching 80-85% of their full charge. Some manufacturers sell flash lighting they designate as 'digital'; these are modified units that will only consider the flash ready to fire once they have reached their full charge and will therefore avoid this problem.

4. Image transfer

Once the image has been captured within the camera, it needs to be stored. In the first instance most cameras (designed for working on location) are able to store a small number of images within an on-camera 'cache-memory' but as soon as possible all images must be stored either within an on-board storage system or by some form of interface to a tethered computer working alongside.

If the images are stored internally on the camera, in some form of digital storage media e.g. CompactFlash or Secure Digital (SD), then they too will need to be downloaded to a long-term storage area to allow more images to be captured onto the camera's temporary storage media.

There are a number of ways that this can be done, either using some cable interfacing directly to the camera, using a wireless network to transmit the images from the camera to the 'paired' computer or by taking the camera media out and loading it into some other 'card-reader' interfaced to the host computer.

4.1. Memory storage cards

The majority of studio-bound cameras, which includes most of the high-end cameras used by digitisation projects, offer tethered operation. As soon as the image is captured it is immediately downloaded via the interface (FireWire or USB) to the host computer. However most cameras used to capture 'action' photography will need to work without a computer and store the images as they are captured. The images will then need to be downloaded to the computer at a later time.

This temporary storage is normally provided by a memory storage card. So far the industry has unfortunately found it hard to standardise on any one media though the choice is narrowing, with CompactFlash and Secure Digital (SD) as the two most popular types.

4.2. Image file formats used by digital cameras

Many digital cameras offer a selection of different file formats in which to initially store the image. Of course the initial data will depend upon what type of capture technology is used by the camera. A scanning back or multi-shot process will have captured complete colour data for all and every pixel within the image, however a single shot matrix camera will have captured much less information as it has only captured one colour value for each pixel. The rest of the colour information must be created by interpolation and processing will be necessary to build up the full colour data for each pixel.

• TIFF - All high quality and professional digital cameras will allow you to save your images within the lossless TIFF format. This is the recommended 'best practice', however it should be realised that saving directly to TIFF can present some challenges. The file size for these images will be comparatively large and if stored within the camera will use up storage space very quickly. However for the studio-bound camera, this should not be a problem as it will probably be tethered and the image file can be immediately downloaded directly to the computer's hard disk.
• JPEG (EXIF) - Most pro-sumer and all consumer digital cameras are designed primarily for 'action' use outside of the studio. In other words these cameras must work handheld and un-tethered. It is therefore necessary for them to store all their images onboard the camera until such time as they can be downloaded to a PC. This makes the use of some compression at the time of capture useful and in some cases imperative. JPEG is the normal choice of compression for this purpose (although some modern cameras offer the 'RAW' option described below). These cameras will offer a choice of compression rates such as 'Fine', 'Normal' and 'Basic'. For the digitisation project, where quality and archival preservation is of the utmost importance, master archive files must be made without using any form of compression. However if the camera that is being used does not offer an uncompressed option, then JPEG compression might have to be used, although it should certainly only be at the highest quality setting.
  
  Some digital cameras use a modified JPEG file type called EXIF (still with the '.jpg' extension); these are simply JPEG files with a range of technical metadata automatically recorded from the camera within tags in the header. Some of this metadata could be considered pretty mundane such as date and exposure details, however it does offer some interesting possibilities for the future such as indexing the image content at time of capture, linking an audio file or even recording the exact position of camera at the time of capture (using GPS data).
• RAW - All one-shot matrix SLR cameras offer the option of saving all the captured data from the camera in its original 'unprocessed' form before any colour interpolation has been undertaken. The advantage of doing this is two-fold, firstly the files will be much smaller than any resultant TIFF file that is created from them and also they are in the most original form (closest to capture) possible and it is always good to know that your archive files are as close to source as possible. On the downside, as each camera will capture its data in a slightly different way, there can be no standardisation of this format and the files are therefore completely proprietary, needing some proprietary software to convert them to any standard or even readable image format. Adobe has developed the Digital Negative (DNG) format, which offers the advantages of the RAW format without the dependence on proprietary software created by camera manufacturers. Images are converted from the camera's native RAW format into the DNG using a standalone DNG converter.

5. Choosing a camera

Of course, at some point, you still have to work out what camera is going to be the right choice for your required use. With such a wide range of functionality available at an equally wide range of prices, it is obvious that there won't be just one camera that can fulfil every need, and the first and most important task when choosing a camera is always going to be to research exactly what you want the camera to do. Once you have done that, you can consider the strengths and compromises of each type of capture technology and then make your choice from the digital cameras that fulfil your needs at a cost that fits your budget.

See also Using a Budget Digital Camera for Teaching, Learning and Research and The Digital SLR Camera for Teaching, Learning and Digitisation.

Last updated: 18 May 2009
Published in: Digitising analogue media | Creating new digital media
Tags: business & community engagement | cameras | photography | slr