Photography and Imaging


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This guide was last revised 25 August 2010

One of the most common forms of digital content creation is through use of a scanner or camera to take digital images. Digital photography is a rapidly maturing field, but has some importance differences from film-based photography. One of them is that pre-print images are usually viewed through a computer monitor. These differences can greatly affect hardware choices, workflow and formats in creating a usable digital image. This section provides an overview of digital still image technologies and some of the considerations that may affect your hardware and workflow choices.

Film versus Pixels

Film and digital cameras differ significantly in the way they capture images. Print film is made of polyester coated in a light sensitive silver-halide emulsion. The size of the silver halide salts determines the light sensitivity and resolution (or grain) of the film, with superfine film allowing more fine detail but less light sensitivity. Colour film has at least three layers of emulsion, containing red, green and blue dyes. Exposure to light through a lens creates a negative image that is then used for printing (slide film is different in that it produces a positive image designed for direct viewing through a light projector). Different size film formats also affect the resolution of the image, with medium and large format film generally providing more detail than standard 35mm film.

A digital camera has an image sensor made up of millions of light sensitive pixels (picture elements), with the size and quality of the sensor affecting detail, colour and noise levels, while the number of pixels determines the native resolution. A chip within the camera processes the information captured by the sensor to create an image recorded to a storage device, such as a flash memory card. Higher-end digital cameras will allow the image to be stored unprocessed (raw) for later processing in software.

Film grain (left) and pixels compared

Grain And Pixels

In both film and digital photography, the function and quality of the lens used generally has more impact on the clarity of the final image than the grain of the film or the number of pixels. A poor quality lens will not allow enough detail to reach the film or sensor for it to be visible in the final image. This makes the lens optics potentially the most important consideration when selecting a camera to use for accurate digitisation or detailed digital photography.

Digital image compression and reduction

Image file compression is a technology adopted from the computer industry. Image files such as an uncompressed TIFF or a raw digital camera image are generally large, which is a limitation when storage is expensive. It is also a limitation when files are being transferred over a network such as email or the internet. Compression is useful as a means to temporarily reduce the size of an image file for purposes of storage or transfer, and is generally lossless and reversible. A well-known example of lossless compression is the Windows Zip file compression, where a file can be restored to its identical state and size after unzipping, and no information is lost or altered. For an image, consecutive pixel values containing no information may be recalculated (e.g. “0000000” can be represented as “7x0”) and restored when decompressed. For image files, TIFF with LZW compression, PNG and JPEG2000 (not the same as JPEG) formats all use lossless compression.

In contrast to image compression, lossy reduction removes information in a file in order to make it smaller. Image formats such as GIF and JPEG for instance, were originally designed for easy viewing on computer screens and transfer over networks. Lossy reduction averages, or interpolates, the difference between two or more bits of information, and discards those bits for one of average value. The discarded bits are lost, and can never be restored to the original, even if an attempt is made to resize the file. This is compounded whenever a lossy file is edited or saved, as each time more bits are averaged and discarded.

Until quite recently, storage capacity for digital cameras and computer hard drives was a significant factor in determining the quality of image coming from a digital camera or scanner. Digital camera manufacturers almost all used lossy image reduction to create smaller JPEG image files to reduce file storage costs. Scanners, while having capacity to capture lossless TIFF images, still often have the lossy reduced JPEG image as their default setting. With storage media now incredibly cheap compared to just a few years ago, file size is much less of a consideration.

Example of detail loss through 'lossy' reduction

Lossy Jpeg

In simple terms, lossless compression is fully reversible, while lossy reduction is irreversible. Lossy reduction should be avoided where high image quality and accurate reproduction is desired, such as the creation of reference or archival images, images that may be edited, retouched or cropped, or images that will have derivative copies made of them. Raw digital camera files and raw, uncompressed or lossless compressed scanner files will always offer the best output for image quality, accuracy and editing.

Hardware and calibration

In relation to content creation and usage, arguably the biggest difference between a non-digital and a digital object is that the digital object requires a machine to view or use it (the same can be said of electronic content such as magnetic tape). As such the hardware used to create and view digital content becomes particularly important, as it can greatly affect our perception of what we see or hear.

Colour calibration

Anyone who has seen a bank of flat screen televisions in an electronics store will have experienced the variations in image between different screens, including those with the same make and model. Those same variations occur between digital cameras, scanners and computer monitors, and will directly affect what different users see as well as what might be output to a printer. Calibration and image management techniques are used to help overcome these differences.

Understanding the basic differences between the way colour works with light and with print is essential if digital images are likely to ever be made into prints or used in a printed resource.

Cameras, scanners and monitors all create colour with light. Red, green and blue are the primary colours for these devices, and when overlaid produce white. With print, colour is created with pigments, the primary colours being cyan, magenta and yellow (black is added as a separate ink to improve contrast). Overlaying these colours create darker shades. Viewing digital images on a monitor or projector transmits coloured light to your eyes, while viewing printed materials involves reflecting light off the colours. The number of colours able to be transmitted by a monitor is significantly greater than the colours possible with pigment inks, which means that a printed digital image is likely to be less dynamic, or colourful, in its colours than the same image seen on a monitor.

One means of managing these differences is to assign a ‘colour space’ to work in (such as ProPhoto, Adobe RGB or sRGB), which identifies the colour origins of the image and allows it to be translated for different environments. Calibrating and profiling different cameras, scanners and monitors to aid accurate representations of images is also a simple but important part of image workflow to ensure that what you see on your screen can be seen by others whether on screen or in print. Colour or greyscale charts or targets included with a scanned image can also assist with recreating the original colour tones of an image when being printed.

Digital Cameras

There is a vast number of guides and advice available in bookshops and on the internet around choosing and using digital cameras for image capture. Perhaps the most useful thing to be aware of is to choose a camera that is fit for purpose.

Point and shoot digital cameras seldom have the kind of sensor or lens quality of Digital SLR (Single Lens Reflex) cameras, which is most important for image detail, even if the pixel count is the same or higher. Choice of lens will affect factors like image distortion, which is particularly noticeable at image corners when taking photographs close up or at full zoom.

Many cheaper digital cameras have difficulty capturing clear images under low light or indoors, while almost all indoor images can be improved with some attention to lighting and use of a stand alone camera flash instead of a built-in flash. Using a camera mount or tripod will improve the sharpness of images and allow longer exposures.

Only a few high end point and shoot cameras currently support raw image saving, while all Digital SLRs do. Saving images in raw format commits you to processing each image in software, but can make it easier to preserve the original settings an image was shot under. Ensuring camera time and date settings are correct will also help with later filing, while some cameras now are GPS enabled, which is useful identifying information if your photography is location-specific.

Scanners

All digital scanners are effectively digital cameras, and are subject to the same kinds of features and limitations. The sensor quality, native resolution and lens quality all determine how capable a scanner is in reproducing an image – a cheap scanner will have a lower image quality in the same way a cheap digital camera does. As most scanners are controlled by a computer, the software settings used are also a key factor. In contrast to cameras, scanner lighting is always direct and artificial, making the quality and evenness of the lighting also important to image quality.

Scanning film

While flat bed scanners with film adaptors have improved greatly in the last couple of years in the quality of their optics, sensors and mounts, dedicated film and slide scanners will more readily provide a better and faster result from scanning film. Film scanners tend to have a larger sensor that can capture more image detail, while their dedicated lens makes a properly focused image a more likely result. Film scanners however are more expensive than flat bed scanners, so the volumes involved over time may need to be traded off against image accuracy. An important factor in choosing a scanner is knowing the quality of the film being scanned (superfine grain film and slide film will offer more detail to be captured if shot with a good lens) and what will be done with the scanned output (e.g. small prints or viewing on screen versus high quality prints for exhibition).

Scanning reflective materials

Drum scanners are still widely considered the highest quality for scanning reflective materials such as photographs and text. However drum scanners are becoming less favoured due to their continued high cost and relative improvements in flat-bed and dedicated book and document scanning technology.

Many reflective materials are not as demanding for scanning as film due to their lower information density, however other factors such as fragility and large size of the materials may pose practical difficulties. Guides on how to prevent damage to original materials should be consulted to ensure the hardware being used is suitable. An appropriately mounted camera may be more suitable for some tasks.

Image adjustments

Most scanner software have settings that allow the user to manually adjust exposure and black and white points in the scanner hardware. These are important settings to check, as detail in image highlights and shadows can be obliterated by the default settings of a scanner, result in a poor quality image. Other settings such as colour correction, dust reduction and sharpening should be used sparingly. Images taken from printed documents may need a de-screening filter in order to produce a viewable image.

Pixel Resolution

In the early days of digital cameras and scanners, pixel resolution was a major feature used by manufacturers to sell their equipment. The biggest claims were usually made using interpolated resolution (or digital zoom) rather than optical resolution. It is evident today however that increasing pixel resolution does little to improve image information and detail beyond a certain point, usually one limited by sensor and lens quality.

Megapixel ratings are the most common means of describing digital camera resolution. Megapixels are a spatial resolution – that is they are calculated by multiplying the horizontal and vertical numbers of pixels covering a digital sensor. Film cameras, being analogue, do not have an equivalent resolution as such, but various estimates place the density of information able to be captured on 35mm film at a maximum of about 25 megapixels, and more realistically at about 10 megapixels. A good Digital SLR at a lower megapixel rating can however beat an average film camera by having a better lens or good image processing.

Due to a legacy in the printing industry, DPI (dots per inch) is the most common term for describing scanner resolution. However PPI (pixels per inch) is a more accurate term, given sensors and images are made of pixels not dots. Fortunately the measurement used for both DPI and PPI are the same as most scanner software still refers to DPI.

There are two types of pixel resolutions used in relation to scanners – one is the resolution the scanner hardware itself is sampling at (e.g. 300ppi, 600ppi, 1200ppi, 2400ppi), and the second is the number of pixels on longest dimension of the resulting image. Most scanner software allows you to see both. Practice is divided over which setting takes precedence, but both are relevant.

Best practice often recommends that a scanner be set at its native resolution – being its highest optical resolution – or at a setting equally divides into it to minimise interpolation (see table below for settings for common scanner optical resolutions). Scanners should not be set above the optical resolution even if the software allows it, as all those settings will be interpolated.

6400ppi optical scanner

4800ppi optical scanner

4000ppi optical scanner

3600ppi optical scanner

2400ppi optical scanner

6400

4800

4000

3600

2400

3200

2400

2000

1800

1200

1600

1600

1000

1200

800

1280

1200

800

900

600

400

960

500

720

480

320

800

400

600

400

 

600

 

450

300

 

480

 

400

 
 

400

 

360

 
 

320

     
 

300

     

Sampling settings that minimise scanner interpolation

The desired size of the output image can be determined by calculating the required pixels, and the scanner set to the closest higher sampling resolution that will achieve it. For photographic images we recommend a minimum setting based on a photographic print output of 300ppi – approximately the number of pixels required to create a smooth detailed image for a person viewing at a distance of 8-10 inches or 200-250 mm (note the dpi or lpi - lines per inch - for printer output is not relevant to this calculation). This requires you to do a little maths:

300ppi x length (desired print output on longest side)
length (original image longest side excluding borders)

Note that it is important to use a ruler to measure your image size – for example, 35mm film typically creates an image 36mm x 24mm, or 1.417” x 0.945”. Using the formula above we can arrive at a setting for creating an 8”x10” print off a 35mm film (for those not familiar with inches, the conversion from metric is 1” (inch) = 25.4mm):

In this example, a film scanner with a 4000ppi optical resolution should be set to its highest resolution of 4000ppi. A 4800ppi scanner with a slide adaptor would be set to 2400ppi.

A recommended best practice setting will use 400ppi in the formula above. This will produce the equivalent to the archival standard used by the U.S. National Archives and Records Administration Technical Guidelines.

Monitor resolution and the fallacy of monitor DPI

LCD and CRT computer monitors also use pixels in their resolution, and like cameras and scanners, they have native resolutions that provide the sharpest image. CRT monitors are generally no longer commercially manufactured outside of specialist high-end markets such as medical equipment, so if you are scanning or photographing for the screen, LCD resolutions are what you should be targeting. Unfortunately at present there are a wide range of screen resolutions in use. Some of the most common ones are shown in the table below, with the 16:10 aspect being the fastest growing in desktop and notebook hardware.

Desktop/Notebook widescreen (16:10 aspect)

2560x1600

1920x1200

1440x900

1280x800

Desktop standard (5:4 aspect)

1280x1024

Netbook (16:9 aspect)

1024x600

Smart phone, photoframe (5:3 aspect)

800x480

Mobile (4:3 aspect)

480x320

320x240

Common LCD screen settings

One mistaken practice when creating digital images to be viewed on screen is to refer to the 72 or 96 DPI measure that is used by the operating software to scale icons and fonts. This measure has nothing to do with the resolution of your digital image file (or printer dots) and should be ignored in any image calculations, whether made for the screen or not.

Resolution and image density

Digital copying of analogue content almost always involves some compromise in terms of loss of information, and scanning of print film and slides is no exception. Unless badly damaged, scanning a negative can provide a far superior image to that obtainable from a print, due to the limits of picture information possible on paper. The greater density of information on film negatives and slides also means a much higher resolution scan is required than for paper to reveal detail than for scanning a print. Knowing something about the film stock being scanned can also greatly assist determining the appropriate scanner settings to use.

What value is a digital original?

When copying is at the very basis of most digital technology today, and when hundreds of identical copies can be made with little effort, and thousands of photographs taken at little cost, what value does a digital original have?

Most image workflows used by professional photographers and by archivists make an early distinction between the working image designed for editing or access and the original master image it was made from.

The master image is never edited, and serves as the film equivalent of a negative – a reference version that future copies can be made from. Indeed, raw digital image files from digital cameras are often referred to as digital negatives for this very reason, as they contain all the original information captured. Reformatting these images, unless done with care, can risk losing image detail and descriptive information that can never be recovered. Master files are also often larger and more difficult to work with than derivative files sized and formatted for editing.

As a matter of good practice, a working copy of an original file should be made as early as possible, and renamed using a different naming convention from the original. Originals should ideally be filed and backed up separately from copies. The assumption here is that identical digital photographs can never be retaken and that the effort or cost required in rescanning images or film is greater than that required to make digital copies and back up originals.

Standards for photographs and images

Digital image scanning

In order to create digital objects that are accurate copies of an original and able to be re-purposed, good practice is to create a digital master, akin to a negative. Lower resolution copies for specific purposes can then be made from the master.

Scanning black and white images in full colour allows tinting, discolouration and any markings on the image to be more clearly visible, while improving the dynamic range of greys available for revealing detail.

 

Minimum (safe)

Best practice

Bit depth

24-bit RGB (8-bit per channel) capture

48-bit RGB (16-bit per channel) capture

Capture Format

Uncompressed TIFF

Uncompressed TIFF or JPEG2000

Colour space

sRGB

Adobe 1998 (colour)

Capture Resolution

300ppi x output length

original length

400ppi x output length

original length

Digital photography

The output from a digital camera is dependent on its capabilities. Generally wherever possible the camera’s highest settings should be used to capture as much detail and colour information as possible. If a camera does not support raw image output, the highest detail setting for JPEG is the safest alternative for creating an image that can be re-purposed.

 

Minimum (safe)

Best practice

Bit depth

24-bit RGB (8-bit per channel) capture

48-bit RGB (16-bit per channel) capture

Capture Format

Full resolution JPEG (Fine or Superfine setting)

Camera RAW or

Adobe DNG

Colour space

sRGB

ProPhoto

Capture Resolution

Minimum of 6 megapixels

Minimum of 10 megapixels


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