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Digital Cameras


T
he stream of technical data that is routinely dispensed in promotional literature and by salespersons in photographic stores may appear daunting. This article introduces the prospective buyer to some of the hardware and software terminology that manufacturers and retailers use to describe the specification and performance of imaging products.

 
Introduction
At the heart of all digital cameras (digital compact and digital SLR) is an image sensor that employs solid state electronic processes to record a scene. In nearly all mainstream cameras the image sensor is a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) two - dimensional focal plane array
that is typically silicon based, though more exotic materials are used.
Today, CMOS image sensors that
have added functionality and benefit from large scale manufacture are achieving the performance levels of image sensors founded on the optimised CCD technology that has dominated the camera marketplace.
The camera lens focuses an image of the scene (primarily reflected light) onto an image sensor comprising a mosaic pattern of light - sensitive picture elements or pixels. The image sensor effectively segments the image of the scene and generates data for the pixel spatial location, called the pixel 'address' and the response.
The response is proportional to the image related electronic charge that is released from silicon (material) bonds and accumulates at the pixel 'address' for the duration
of the exposure, under the control of an electronic and/or mechanical shutter.
Electronic charge is accessed using registers (systems of interline or frame transfer are typical) and reduced to digital data that is passed to an electronic processor (image processing engine) and then saved in a compressed and/or uncompressed file format on a removable memory card.
To preview/review images an integral colour LCD viewing screen that can also display image data and control menus is provided. The description is greatly simplified, though digital and film cameras share many components and features, the digital camera offers much more. Once downloaded onto a computer,
electronic images
may be edited and manipulated, emailed, exhibited
online, exported to a database
or spreadsheet, ..., and integrated into multimedia applications.


Image Sensors

The scale of excellence most used by manufacturers and retailers to distinguish their digital cameras is the pixel count. The effective pixel count is the number of pixels that contribute to each image (some edge pixels are unused). At the time of this writing, the effective pixel count of low - end (point & shoot) cameras is typically
8 MegaPixels, of mid - range (creative compact, SLR style, entry level DSLR) cameras is typically 8 - 12 MegaPixels and of high - end (for the professional photographer market) cameras is typically 14+ MegaPixels. Based simply on the pixel density, 
point & shoot cameras surpass professional cameras, though image quality is not the same. Even though the pixel count continues to rise, understand that MegaPixel count is not the whole story (1 MegaPixel = 1 million pixels).
The effective pixel count may be specified differently, using an array notation,
for example, my Nikon D80 has 3872 2592 pixels, that means an image width of 3872 pixels and an image height of 2592 pixels. Most cameras support image capture at lower resolutions, for example, the 6 MegaPixel camera may shoot at 1.5 MegaPixel resolution (this is useful for Powerpoint presentations or web publishing) to save storage space on the memory card. The baseline criteria are not set in stone, 
camera specifications change as a result of demand driven technological innovation.  
To create photo - quality prints a resolution of 300 ppi (pixels per inch) is the rule, however, this is printer/paper dependent, 200+ ppi is often acceptable.
While the visual differences in doubling the pixel resolution are modest, doubling the pixel resolution does provide scope to recompose (crop and enlarge). Notice that the displayed image is dependent on the absolute pixel count, when displayed at 100%
a 1024 x 768 pixel image fills a monitor screen set to 1024 x 768 resolution.


The spatial resolution or sharpness of an image (sharpness is a subjective quality) is determined by the pixel size (typically 2 - 8 μm, rectangular or square) and the pixel count, 8, 12, 14+, ... MegaPixels, you can resize/downsample to capture an image at lower resolution. Of course, for the same pixel count and fill - factor
(ratio of pixel (photosensitive) area to pixel area), larger/smaller image sensors have larger/smaller pixels. For any image sensor format, decreasing the pixel size enables higher spatial sampling (higher resolution image capture). Naturally, there are
trade - offs, though smaller pixels can record more faithfully the fine spatial detail (the change in brightness with distance) in a scene, they are less sensitive to light, (compared to larger pixels, lower SNR at the same ISO). 
As the pixel size is decreased, the degrading effects of diffraction and photon noise (fundamental limits) on image quality become more pronounced. Furthermore, increasing the MegaPixel count necessitates a larger file size, that lengthens the upload/download time.


The aspect ratio is the ratio of the format width to the format height and determines the relative dimensions of an image (photographic print). These dimensions relate
to the image sensor active area (that includes the pixels that contribute to each image). A square has an aspect ratio of 1:1 or (1.00), the 35 mm
Full Frame format has an aspect ratio of 36:24 or (1.50), most compact camera formats have an aspect ratio of 4:3 or (1.33). My Nikon D80 DX format camera produces an image with
23.6:15.8 or (1.50) aspect ratio. Of course, you can resize a digital image (generate an image with any aspect ratio) using image - editing software.
Traditional computer monitors have a 4:3 or (1.33) aspect ratio. Widescreen computer monitors have different (1.60, 1.85, ...) aspect ratios. HDTV has a 16:9 or (1.78) aspect ratio.


Some of the image sensors used in consumer and prosumer digital cameras are listed. FLM is the focal length multiplier (see Depth of Field). Notice the relative shapes and sizes (approximate) of the image sensor formats. 
                               

RELATIVE SIZE OF IMAGE SENSORS
 
       Format  Aspect Ratio   Diagonal (mm)  FLM
               1/3"            1.33                   6    7.2
            1/2.5"            1.33                   7.2    6.0
               1/2"            1.33                   8    5.4
            1/1.7"            1.33                   9.5    4.6
               2/3"            1.33                 11    3.9
                  1"            1.33                 16    2.7
               4/3"            1.33                 21.6    2.0
Canon APS - C            1.50                 26.7    1.6
Nikon DX            1.50                 28.4    1.5
Canon APS - H            1.50                 34.5    1.3
    Full Frame            1.50                 43.3    1.0

The field of view, (FOV), or picture angle is the acceptance angle of light from the scene and is determined by the focal length of the lens and the dimensions of the field stop. In the absence of a physical stop, the active area of the image sensor (the linear dimensions) decides the extent of the scene that can be reproduced.        

Field of view may be decomposed into horizontal (width) and vertical (height) components. Using simple trigonometry, the field of view is given by
 
                           


where
SensorDimension is the horizontal or the vertical dimension of the image sensor. At close focus distances, the denominator is replaced by 2FocalLength(M+1), where M is the magnification.

The image sensors used for digital cameras are different in size (1/3", 2/3", ...) and type, there is no format standard (unlike the standards for film).

Given a 20 mm lens, the corresponding fields of view (relative to the diagonal) for 1/3" and 2/3" image sensor formats are 17.1 and 30.8.
To demonstrate the field of view change with focal length, consider a 28 - 300 mm telezoom mounted on a full frame DSLR body (image format, 36 x 24 mm).
At the 28 mm limit the horizontal FOV is 2tan-1(36/56) = 65.5 and the vertical FOV is 2tan-1(24/56) = 46.4
.
At the 300 mm limit the horizontal FOV is 2tan-1(36/600) = 6.9 and the vertical FOV is 2tan-1(24/600) = 4.6.
 

  • Short focal length lenses (focal length shorter than the image format diagonal) give a wide FOV that can capture the broad extent of a scene. An extended depth of field means that most foreground and background features are
    in - focus. Short focal length lenses appear to expand perspective and may distort near subjects. Most cameras are supplied with a 28 mm equivalent lens for wide angle capture. For sub full frame cameras, 12 and 15 mm ultra wide angle lenses are available.
     
  • A standard lens has a focal length of approximately the image format diagonal and gives a FOV similar to the central FOV of the human eye. For the 35 mm format the focal length of a standard lens is 50 mm.
     
  • Long focal length 'telephoto' lenses (focal length longer than the image format diagonal) give a narrow FOV that can isolate and (magnify) the scene detail.
    A restricted depth of field means that most foreground and background features are not in - focus. Long focal length lenses appear to compress perspective. Perspective is determined by viewpoint not focal length.

     

Camera Sensitivity
The ISO equivalent speed is a measure of the (camera) image sensor sensitivity to light. Like traditional film speed that is assigned a scale, ISO 25 (slow) to ISO 3200 (fast), at low ISO equivalent ratings the image sensor is less sensitive to light, at high ISO equivalent ratings the image sensor is more sensitive to light.
The ISO range 50 to 400 is typical for compact cameras (with smaller image sensors), the ISO range 200 to 1600 is typical for DSLR cameras (for some cameras the ISO range is extended both lower and higher). My Nikon D80 has an ISO range 100 to 1600 (high options, 2000, 2500 and 3200). The freedom to select an appropriate ISO for each image capture is a feature of digital cameras, however, noise (like grain in film) can become objectionable at high ISO ratings.

The
noise performance of digital cameras is a subject much debated. Sources of noise may be classified as sensor noise or non - sensor noise. Sensor noise includes dark current (temperature dependent) noise, read noise associated with the signal processing electronics and spatial (pattern noise variants) noise due to pixel to pixel non - uniformities associated with the manufacturing process. The most significant non - sensor noise is photon (shot) noise that determines the fundamental limit on noise performance. Photon noise is present at all times and best understood as a natural variation in the arrival (and the subsequent detection) rate of photons (light) from the scene. This is a property of nature, not a property of the camera.
In digital images, the above sources of noise are discernable as luminance and colour variations (speckles). The significant noise contributions (three noise regimes) are illustrated using the generic photon transfer curve.
                                


The most fundamental measure of digital camera performance is the sensitivity or

signal to noise ratio (SNR) that is dependent on the total noise (the sum of all sources of noise). The sensitivity (SNR = 1) is limited by the total noise. For high quality reproduction, the spatial resolution and signal to noise ratio of digital images must be at least 'as good as' film images.

Photon noise and the full well capacity (FWC) limit the maximum SNR that can be achieved, the FWC is the maximum charge (measured in electrons) that each pixel can store before saturation and is related to the pixel size. For most quality cameras, compacts and DSLRs, the noise performance is limited by photon statistics (photon noise is proportional to the square root of the average number of photons). At pixel level, photons are converted to electrons, SNRelectrons = √η x SNRphotons, and
SNRphotons = √N, where the conversion factor η is the quantum efficiency
(η < 100%) and N is the average number of photons. On condition that noise is not visible, electronic gain to increase the ISO rating (like pushing film) may not significantly degrade the image quality. The maximum ISO rating (highest sensitivity) is determined by the noise performance and the lowest SNR that is practical. By careful design of circuitry and optimisation, one or two sources of noise dominate the noise performance.
Even so, compact cameras and DSLR cameras do not routinely offer the same image quality for the same pixel count. In general, compact cameras have smaller image sensors and smaller pixels. Typically, smaller pixels have smaller FWCs, have
lower SNRs
and a reduced dynamic range. The term dynamic range (the ratio of the FWC to the noise floor, characterises the capability of a digital camera to record the darkest and lightest (black to white) regions of a scene.
At high light levels, photon noise is the limiting factor, at low light levels and for
long exposures, read noise and dark current noise are the limiting factors.
Typically, the SNR of high - end DSLR cameras (FWC 50,000+e @ ISO 100) is
more than 3x the SNR of low - end compact cameras (FWC 5,000+e @ ISO 100). High fill - factor, low cross - talk image sensor designs using novel pixel structures and microlenses can enhance the sensitivity. Adding the charge from neighbouring pixels, binning, can increase the signal to noise ratio, but lowers the resolution.
However, the strengths and weaknesses of digital cameras are only revealed by spatial frequency domain analysis of a range of parameters.
Today, some digital cameras achieve the imaging and performance standards
(similar noise and resolution figures) of film cameras (understand that image quality is subjective). Nonetheless, be aware of unrealistic performance claims (especially for compact cameras employing small format image sensors) that appear to challenge basic physical principles.

For low light photography, both indoors and outdoors, higher sensitivity guarantees faster shutter speeds (to freeze motion for action and sports photography) to reduce the loss of definition due to camera and subject movement. Furthermore, higher sensitivity may circumvent the need for flash illumination, so that images appear more natural. At higher ISO ratings, because fewer photons are collected for a correct exposure (and electronic gain introduces noise), the corresponding SNRs are lower. Look for cameras that provide a noise reduction setting at higher ISO ratings (long exposures in low light conditions), where (at large print sizes) digital noise can become unacceptable. To minimise luminance and colour noise, noise reduction software is available as a stand - alone package or compatible plug - in for the popular image - editing applications.
 

Colour
In most digital cameras the image sensor is fabricated using silicon microelectronics technology. The image sensor is responsive to the broadband scene luminance and cannot distinguish colour. The spectral response of silicon extends from about
0.35 - 1.10 μm (1 μm = 1/1000 mm), beyond the region of the electromagnetic spectrum that stimulates the human visual system (0.4 - 0.7 μm). 
 

Most cameras include a short pass filter (hot mirror) to remove the near - infrared (NIR) component of the scene luminance. Use a TV remote control to check the infrared response of your camera. On condition that you can access the long wavelength response (about 0.7 - 1.1 μm), infrared photography is practicable.
Remember, this is reflected solar energy, not self - emitted thermal energy.
To prepare your camera for infrared photography attach an IR filter (Hoya R72, Wratten 88A, ...) that is opaque to visible light. Use the camera self - timer and a robust tripod, long exposures are typical. An alternative approach is to modify the camera, the IR short pass filter is replaced by an IR long pass filter, and the autofocus is fine - tuned to compensate for the focal shift. Several companies can provide this specialist service.
The NIR reflectance of familiar subjects is variable, foliage has a high reflectance, water has a low reflectance. To finish, create a monochrome image of contrasting tones (even colour images) using an image - editing software package. A complete treatment of infrared photography is given in a forthcoming article.
 

Any perceivable colour is a mixture of the primary colours Red, Green and Blue suitably combined. For the reproduction of colour most digital cameras use a Red, Green and Blue primary colour filter that is registered (in a repeating sequence on the face of the image sensor) with each pixel. Each pixel is responsive to only one colour. The Bayer colour filter is configured as a 2 x 2 array of one Red, one Blue and two Green filters (the human visual system is most sensitive to yellow - green light) to capture 25% of the red light, 25% of the blue light and 50% of the green light. The mosaiced image data from an 8 MegaPixel camera consists of 2 MegaPixels filtered red, 2 MegaPixels filtered blue and 4 MegaPixels filtered green.

Given that one primary colour is recorded at each pixel address, theoretically,
the resolution and sensitivity are reduced by factors of 4 and 3 (compared to the panchromatic (B&W) image), however, spectral interpolation (to recover the omitted colour channels, the demosaicing algorithm combines the RGB responses from neighbouring pixels) is used to reconstruct the full colour image. There are many demosaicing techniques using non - adaptive or adaptive, hardware - software based interpolation. Interpolation is a mathematical technique used to estimate unknown data values from known data values, within the range of the known data values.
The most common implementations of the demosaicing algorithm use bilinear or cubic B - spline interpolation. Of course, there are trade - offs between colour fidelity, sharpness and image defects. RAW capture allows you to post process (on your PC) using more sophisticated demosaicing algorithms that introduce less errors.
Alternative colour filter configurations are the CYGM (Cyan, Yellow, Green, Magenta), the RGBE (Red, Green, Blue, Emerald) and the RGBW (Red, Green, Blue, White).

In a discrete sampling process, erroneous data (aliased data) may appear unless the signal is band - limited, consistent with the Nyquist - Shannon theorem. For cameras that use a colour filter array, moiré patterns and colour artefacts are sometimes evident. Most digital cameras use an optical low - pass (anti - aliasing) filter (based on birefringent crystal polarisation or phase delay) placed on top of the image sensor to (slightly) blur the image and remove the high frequency content, thereby exchanging sharpness for reduced aliasing. Some cameras use an optical filter and image processing software. Most image - editing software packages provide an
anti - aliasing tool that can reduce the most distracting artefacts.
 
There are many optical techniques for colour separation, most consumer and professional (compact and DSLR) cameras employ some variant of the Bayer colour filter (lateral colour filter). The pioneering non optical capture technology developed by Foveon is based on solid state electronic processes. To produce full colour
non - interpolated imagery, the
Foveon X3 CMOS image sensor captures three
colour channels (100% of the light) at each pixel address using Red, Green & Blue photosensitive layers embedded at the absorption depth of the primary colours, similar to a film emulsion (vertical colour filter). Furthermore, the demosaicing process and the separate anti - aliasing filter are unnecessary. The Sigma SD1 DSLR uses an APS - C X3 image sensor.

Most cameras convert the response to light (scene luminance) from analogue form to digital form, typically 8 - bit data that corresponds to a tonal range of 256 (28) levels on a scale from 0 (Black) to 255 (White). For photographic quality full colour images the three colour channels are coded as 8 - bit data (bit (colour) depth of 8)
to produce composite 24 - bit colour, about 16.7 million hues, more than the human eye can distinguish. Some high - end cameras use 48 - bit or 36 - bit colour palettes, images are pre - processed and downsampled to 24 - bit data.

The image histogram is a graphical representation of the distribution of tonal values (number of pixels of a given brightness) that can be used to optimise the camera settings and for post production. Learn how to interpret the image histogram
(all DSLRs display a histogram, some compacts display a live histogram and some
high - end cameras display separate R G B histograms).
For an image that has no major highlights (image highlights flash on the LCD screen) or shadows the distribution of tonal values is centred (mainly mid - tones). For a high - key image, the distribution is displaced to the right (toward lighter tones), for a low - key image, the distribution is displaced to the left (toward darker tones). All scenes are different, all image histograms are different, there is no 'ideal' image histogram.
Use exposure compensation, typically 1/3 stop increments, (−ve) to darken an image that is clipped on the right, (+ve) to lighten an image that is clipped on the left. Modern cameras can analyse the scene dynamic range (the difference between the lightest and darkest regions of an image) to optimise the distribution of tonal values. Of course, histogram manipulation is possible using image - editing software.
 

White Balance
The spectral power distribution of a light source that determines the visual appearance can be described by a colour temperature that is measured on the Kelvin temperature scale. To accurately record coloured and white objects as perceived by the human visual system (under all lighting conditions), the camera has to perform a white balance that adjusts the image sensor RGB reponse.
The process ensures colour correctness and that objectionable colour casts are not introduced. Digital cameras provide an automatic white balance (AWB) and presets for the common lighting conditions, Cloudy, Shade, Direct Sunlight, Fluorescent, Incandescent (Tungsten) and Flash (names vary). The white balance presets are selected using a button and dial, or multi - selector, typically an icon is displayed on the LCD panel.
To manage unique lighting conditions (mixed lighting, ...) use the manual (custom) white balance setting to record a white balance under the same lighting conditions. Useful features on some advanced cameras are white balance bracketing (allowing you to capture a sequence of images with colour temperatures slightly lower and higher than the preset) and fine tuning. There is an air of mystery associated with white balance (the concepts are not intuitive), trial and error is the best approach to gain an understanding. The colour temperature for several artificial and natural light sources are listed (see Colour Temperature).


Digital and Optical Zoom
Most cameras have both digital and optical zooms. The digital zoom provides an apparent increase in focal length by means of software to perform an upsampling operation. Lens elements do not move. Using an image interpolation algorithm (similar to the image enlargement software on your PC), the central part of the image is magnified to produce a cropped image at full resolution but of degraded quality. Though in - camera processing is applied before JPEG compression, typically, upsampling artefacts and more subtle image defects are visible. To preserve image quality, some cameras feature an 'intelligent' zoom function that obviates interpolation. The digital zoom specification should be a secondary consideration.
The optical zoom lens, either built - in (telescoping) or interchangeable, has a variable focal length that changes the field of view and the true magnification as the lens is zoomed. The optical zoom specification should be a primary consideration. The focal length of a super zoom may change from 28 mm (wide angle) to 300 mm (narrow angle) plus. Optical zooms are ideal for general purpose photography and useful for scene composition from a set location, zoom ratios (the ratio of longest to shortest focal length) of 10:1 and more are commonplace. To achieve high zoom ratios there must be design compromises, do not expect the optimal performance 
for the entire focal range. For basic (Point & Shoot) cameras a 3x optical zoom is typical, (for example, 38 - 114 mm equivalent). Digital and optical zooms can be operated alone or in combination. A 3x digital zoom coupled with a 6x optical zoom has an 18x zoom range. The focal length and field of view of a prime (non - zoom) lens are fixed. As a general rule a prime lens provides superior performance.

Though 'older' interchangeable lenses were mass - produced for 35 mm (film) SLR cameras, most can be used effectively with digital bodies. Even so, some lenses have been re - engineered for use with digital and film cameras. Today, lenses that are designed exclusively for digital SLRs (the APS - C image sensor format includes Canon EF - S, Nikon DX, Pentax DA, ...) have matched image circles and optimised coatings to reduce vignetting (shading at the corners of an image) and flare and enhance sharpness. It is not only DSLRs that benefit, many compact cameras can accept a telephoto and wide angle adapter to extend their focal range and versatility.

Image stabilised lenses (on some cameras, the image sensor has 'built - in' stabilisation) employ anti - vibration mechanisms (move lens elements so the image at the focal plane is stationary) to compensate for camera movement (shake).
The practical benefit is up to 2 - 3 stops compensation (slower shutter speed) for image blur (allowing the lens to be closed for greater depth of field). Bear in mind, image stabilisation compensates for the image blur due to camera movement,
not subject movement.

 
Flash Modes
The integral electronic flash unit has several modes of operation that provide some control of the ambient lighting. The standard modes are auto flash, fill - in flash and red - eye reduction. High - end cameras offer more advanced flash features.
No flash
is the default setting.

  • Auto flash: the camera on - board computer estimates the available light and need for the flash.
     
  • Fill - in flash: used for a back or sidelit subject, to even the illumination (reduce contrast), remove unappealing shadows from foreground subjects and create catchlights.
     
  • Red - eye reduction: a pre - flash that causes the eye pupil to close
    (the pupils are dilated in low light) so light that is reflected from the
    blood vessels/pigment in the retinal layers of the eye (primarily the choroid) is reduced. This feature is useful for animal (green eye) and people photography.
     
  • High - end cameras may offer dedicated flash modes, front and rear curtain sync and slow sync for more creative photography. Using multiple flash you can create a time sequence of images.

The flash guide number, GN (f/# x distance) is a measure of the power output
at a particular ISO setting (usually ISO 100) and is used to determine the working distance for a correct exposure at a given aperture. The more powerful the flash (higher guide number), the greater the working distance, remember, for a point source the illuminance is governed by the inverse square law. The flash hot shoe (synch terminal) is used to connect an external flash unit (the integral flash is disabled) for added illumination. You can use gelatin filters to correct the colour temperature of the flash unit light. Off - axis flash units can reduce many undesirable optical effects such as background shadow and red - eye. To remove red - eye, use 
in - camera software (some cameras can correct images) or image - editing software.


Image File Format
Image files can be saved to compressed or uncompressed formats. Compression routines are described as lossy or lossless, dependent on the image data that is discarded.
The standard file formats for digital cameras are: 

  • JPEG (Joint Photographic Experts Group), is a compressed lossy universal  format that is ideal for continuous - tone images. The JPEG format (the original standard) is used by nearly all digital cameras and is identified by the (.jpg file extension). The JPEG image compression standard uses discrete cosine transform coding.
    Always post process your JPEG images in an uncompressed format.
    JPEG2000
    benefits from higher compression ratios, flexibility, selectivity and superior image quality and is identified by the (.jpg2 file extension).
    The JPEG2000 image compression standard uses advanced wavelet transform coding.
     
  • RAW is unprocessed data from the image sensor. My Nikon D80 uses a 12 - bit
    image processing engine (compressed NEF).
    For RAW capture, the image data and camera settings are separated for versatile post production. Most cameras can record RAW and JPEG files (RAW + JPEG) simultaneously. On your PC, convert RAW files to JPEG, TIFF or PNG (Portable Network Graphics) files.
     
  • TIFF (Tagged Image File Format) is an uncompressed format that is suitable for desktop publishing applications and archiving digital images (the industry standard). Though write times to the memory card are long, LZW lossless
    (no quality loss) compression reduces the file size. The TIFF:JPEG High Quality
    file size ratio is about 3:1. Many cameras do not support the TIFF format.
     

The table compares JPEG and RAW file formats.
               

                 JPEG                     RAW
Compatibility The files are compatible with all standard image - editing software and internet applications. The files are not compatible with most standard image - editing software.
There is no standard format for RAW image files, common proprietary formats are,  Canon (CRW), Nikon (NEF), SONY (ARW).  
RAW converter software is used to read and edit files. Some third - party image - editing software can read RAW files.
Editing Options
Using image - editing software limited enhancement options are available post capture.
 
Image data and camera settings are separate. Using image - editing software unlimited enhancement options are available post capture, colour (brightness, hue and saturation), sharpening and white balance. Highly versatile, most camera settings can be changed.
File Size The small file size allows a large number of images to be stored on the memory card. Fast write times. The compression ratio can be adjusted to trade image quality for file size. Most cameras offer image quality and size options. The large file size, smaller than TIFF, allows a small number of images to be stored on the memory card. Slow write times.
Image Quality Some loss of image quality.
JPEG - FINE is ideal for home - use. Multiple saves result in a progressive loss of image quality. To retain image quality, process images in the TIFF format.
No in - camera image processing. No loss of image quality.
 


Digital Storage Media

Digital cameras store (and exchange) captured image data on a removable memory card. Once the image data has been downloaded to your computer or peripheral storage device (portable hard drive) clear the memory card for reuse.
The popular memory cards are
Compact Flash, Memory Stick, MultiMedia, Secure Digital, Smart Media and xD - Picture. Some cameras provide slots that accept two (different) memory cards. The PC Card (PCMCIA card) is a memory card and adaptor that is inserted into the PCMCIA bay on your computer, (on most laptops). Less fashionable is the small (CD - R) disc. The storage capacity of memory cards varies greatly, some professional cards can store up to 32 GB of data (at the time of this writing). Though high capacity memory cards (indispensable for high resolution cameras) are useful, never forget, your memory card can become corrupted and data can be lost. Today, there is image retrieval software to recover damaged images.


                                            

Compact Flash A larger memory card available in Type 1 and Type 11 variants. The card is rugged and versatile. The Type 11 card has the same dimensions as the IBM Microdrive (miniature hard drive). The preferred card for many professional SLRs. At the time of this writing, the capacity is up to 16GB.
Memory Stick

(Duo, PRO Duo)

A small memory card developed by SONY and compatible with other SONY audio visual products. Standard - size adaptor. Integral erasure prevention switch. At the time of this writing, the capacity is up to 8GB.
Multi Media A smaller memory card, smaller than Smart Media. Integral erasure prevention switch. Slower speed than secure digital. At the time of this writing, the capacity is up to 1GB.
Secure Digital A small memory card. Many compacts use the SD card. SDHC variant. Integral erasure prevention switch and security functions. Fast speed. Used in audio visual products. At the time of this writing, the capacity is up to 8GB.
Smart Media A large memory card but smaller/slimmer than a CompactFlash card.
Gold connections on one edge. At the time of this writing, the capacity is up to 2GB.
xD - Picture Fuji/Olympus co - developed. A very small memory card, approx postage stamp size. Fast speed. At the time of this writing, the capacity is up to 2GB.

 

Below is a ballpark guide to the number of images (JPEG Fine - Large) that memory cards of different capacities can accommodate (the image quality and image size that you select determine the file size).                              

                        

   Pixel Count
        
(MP)
  512 MB      1 GB    2 GB
                  4         256          512      1024
                  6          160          320        640
                  8         128          256        512
                10         102          204        408
                12           80          160        320


The card speed 20x, 40x ... is a measure of the rate of data transfer (20x = 3MBps) to (remove data from the buffer) or from the memory card. Unless you require to capture many images in rapid succession, speed is not a critical factor.

For images saved to a JPEG format, the camera may offer image quality (fine - basic) and image size (large - small) options, dependent on the in - camera processing. Selecting the fine/large option for all your photography has a downside, larger file sizes that place more demand on the memory card capacity. For web and email applications, select the basic/small option to maximise storage. The table lists the image quality options (and the data compression applied) for Large, Medium and Small image sizes on my Nikon D80.

                                                

     Compression Ratio
   Fine
1:4
 Normal
1:8
  Basic
  1:16


To archive your digital image collection, use CD - Rs (the storage capacity
is 700 MB) rather than DVDs, should the media malfunction, the loss is less painful. The life of a CD - R is several tens of years and for archival quality media, even longer.


Connectivity
The standard connections and ports used to transfer image data to a computer hard drive or peripheral device are FireWire (other names, IEEE 1394, iLink), SCSI (Small Computer System Interface, pronounced 'scuzzi') and USB (Universal Serial Bus).
The speed of data transfer is measured in units of megabytes per second (MBps).

                               

FireWire A fast serial interface that comes in several variants. Data transfer of up to 50 MBps for Firewire 400 and 100 MBps for FireWire 800. Used on high - end cameras and camcorders. User - friendly.
Hot swappable.
SCSI An older interface with four speed options. Data transfer speeds vary from 5 MBps for the original standard, to 40 MBps for the current (Ultra 2) standard. Not so user - friendly.
USB 1.1 The original standard interface. For the earlier standard, data transfer is up to 1.5 MBps. Hot swappable.
USB 2.0 The latest USB variant is faster and backward compatible. For the fastest standard, data transfer is up to 60 MBps.
User - friendly. Hot swappable.


To transfer (download) images to your computer hard drive use the (proprietary) software supplied with the camera (most operating systems can recognise cameras) or connect a dedicated or multiple format card reader to your computer by means of Firewire or USB 2.0 cables. A card reader places no demand on the camera battery and the download time is typically much faster. Once the transfer is complete use the camera to clear and reformat the memory card. You can also use a memory card adapter that slots into the PCMCIA bay on your computer.

Wireless communication between compatible devices is available for cameras that
are equipped with Bluetooth, Wi - Fi or Infrared (IrDA) - transceiver/software to send and receive radio or infrared signals.
 

Camera Software
In addition to the (in - camera) algorithms that control image capture, image transfer, printing, ..., image processing software is included to enhance and optimise images. While automatic optimisation is normally adequate, (subtle and more obvious) changes are possible by means of options that are accessed using embedded menus:

  • Colour mode (sRGB, Adobe RGB)
     
  • Contrast
     
  • Hue 
     
  • Saturation (normal - enhanced), the hue 'undiluted' by white
     
  • Sharpening (normal - high) 
     

Most digital cameras are packaged with proprietary image - editing and file browser/organiser software (file management becomes a major issue as your image library develops), that uses keywords/tags to access and catalogue images.
For cameras that save to a proprietary RAW file format, RAW converter software
(often extremely basic, when supplied free with the camera) is available from the manufacturer that allows you to apply different camera settings (colour, sharpening, white balance, ...). For my Nikon D80, there is Nikon Capture NX. Though some RAW files can be read by the popular image - editing applications, plug - ins (add - on software used to extend functionality) may be necessary. There are many suppliers of compatible plug - ins for the popular image - editing applications.
Digital cameras store EXIF METADATA (EXIF - Exchangeable Image File Format is
the standard format), embedded data that provides a record of the camera settings (Aperture, Date , Exposure, Format, ISO, White Balance, ...) for each image.
For newcomers with an enthusiasm to develop photographic skills, METADATA is a useful learning tool, retrieve the metadata to see the effect of different camera settings on your images.

Images can be printed directly, (no PC required), using digital cameras and printers (inkjet, dye - sublimation ...) that are PictBridge compatible. PictBridge is the standard for direct printing, look for the logo


Batteries
The major claims on the battery capacity are previewing (live composing) and reviewing images on the LCD screen, using the integral electronic flash unit and using the autofocus: super zoom lenses in particular have a sizeable power demand. For digital cameras that use CMOS based components and devices, the power profile is typically low and the battery life is extended. On most cameras the battery condition is indicated by an icon on the LCD panel.

For most digital cameras the power resource is a rechargeable Li - ion (Lithium Ion) or NiMH (Nickel Metal Hydride) battery, both are durable and can support several hundred (some 1000+) image captures. Alternatives are the Li (Lithium) battery
and the NiCd (Nickel Cadmium) battery that is based on older technology.
My Nikon D80 uses a rechargeable Li - ion battery that has a fast recycling time and for backup power, CR2 lithium batteries.
For photographic holidays, carry a spare proprietary battery or bolt - on battery pack. Non - proprietary AA and AAA NiMH batteries (minimal memory effect) are available off - the - shelf, are environmentally friendly (unlike NiCd that is toxic) and compared to alkaline batteries, have superior endurance. Bear in mind, for different battery types (Alkaline, NiMH, ...), the voltage stability throughout the discharge cycle may not be the same.

The higher rated batteries, (mAh (Milliamp Hour) is a measure of the capacity) are longer lasting. A 1000 mAh battery can deliver 10 mA for 100 hours (or 100 mA for 10 hours). The unit of capacity is mAh, (I x t), where I = current, t = time. Since energy E = V x (I x t), for a given voltage, V, E is proportional to mAh. Sometimes
the capacity is quoted in Wh (Watt Hour), to convert, Wh = V x mAh /
1000.
Your domestic electricity supply is billed in kWh (Kilowatt Hour).

Batteries should be changed in sets and never mixed. During extended periods of photographic inactivity, remove the batteries from your camera. Store batteries in a cool dry environment (water can corrode) and do not expose to direct sunlight.
Other useful accessories are a battery charger (with 12V DC car (or boat) adapter) and power inverter.

For users of digital cameras, the efficient management of battery power is a major issue.
 

Camera Features
In general, the low - end (point & shoot) cameras are compact, lightweight, inexpensive and offer basic features, the mid - range cameras (creative compact,
SLR style and entry level DSLR) and high - end DSLR cameras are less compact, heavier, increasingly more expensive and offer advanced features.
 

Whether you are a newcomer to digital (and want to infuse new life into
your photography) or thinking of upgrading your digital camera, before you purchase, ask two fundamental questions: 

  • do I want the camera to produce large prints, then, the resolution
    (pixel count)
    may influence your buying decision, recall the 300 ppi rule, (the professional print standard). To create photo - quality 8 x 10 inch prints a 7.2 MegaPixel (2400 x 3000) camera is recommended.
    However, this is printer/paper dependent, 200+ ppi is often acceptable.
     
  • do I want the camera for general or specialised (macro, sport, wildlife, ...) photography.                             

The answers should enable you to decide on the specific features that best suit your requirements.

General features to look for:

  • The camera should have a near instantaneous startup, so that unforeseen photo - opportunities are not missed.
     
  • The camera auto focus should operate quickly (no autofocus lag) and silently, even in low light conditions. Most cameras have multi point autofocus and some feature predictive autofocus that tracks a subject in motion.
     
  • There should be no shutter lag, so that passing photo - opportunities are not missed.
     
  • The camera should provide programmed scene/shooting modes (from day one you can use the camera).
                                            
  • The camera should provide some creative photographic controls.
     
  • The camera should have an easy to view (anti - glare, wide viewing angle), high resolution LCD screen. On some cameras the LCD can tilt to check a composition from an inaccessible viewpoint (low, high - angle shots, ...).
     
  • For cameras that use interchangeable lenses, there should be a range of compatible lenses (forward and backward compatibility) and upgrades.
     
  • Handle the camera, test the electronic or optical viewfinder, convince yourself that on - screen menus are easily navigated and all the camera controls are user - friendly.
     
  • Look for innovative features that may be useful at once or maybe later,
    (motion picture recording, noise reduction, high sensitivity, built - in image stabilisation, dust removal technology, face detection technology,
    DSLR live view technology, ...).

     
  • Above all, capture several test images, both indoor and outdoor,
    that you can review on the colour LCD screen (or print).

     
All images and text imajtrek