Digital Photo Output [PDF]

Remember the children's game Telephone: whisper something in one person's ear, who repeats it to the nest sand so on. Because of the way people talk and listen, and make assumptions, the message "The weather is fair in Monte Carlo this year," can eventually become "They went to the fair and found Carl'd lost his ear."

Digital Photography Output:

• Images exist only as binary numbers inside a computer until they are output to either
• monitor - where they remain binary.
• hardcopy - where they become analog.
• print.
• transparency.

Output: is the reconstruction of a visible image of pixel values from binary form to visible image form. With Digital photography, the perceived value of the whole system is related to how closely the output resembles a conventional photograph.

Only since 1989 have desktop digital printers had the capability to render photographic output from a personal computer. The first desktop printers (inkjet and laser) could not produce very high quality compared to the darkroom photograph, but have improved exponentially since that time. The best, most photographic, quality has always come from the dye sublimation printer. The drawback for most digital printers is the lack of archival quality of the output. However, even that aspect of digital photography continues to change and improve. Today's acid-free papers and pigment-based inks rival darkroom color photographs for longevity.

High quality output from a digital imaging system can only be achieved when both the input device (scanner or camera) and output device (printer/monitor) are capable of processing high-fidelity images. However, please remember that you

cannot compensate in print for a poor scan!

Compare traditional silver halide print to digital output:

• Silver halide technology is mature compared with digital:
  • silver technology is over 150 years old.
  • digital technology is still in infancy.

• Digital quality is influenced by choice of equipment:
  • Silver technology quality is also dependent on the quality and choice of equipment, but there are flexibilities. One can use old or defective equipment to achieve interpretive, fine art results that may stand for the antithesis of high latest-quality technical equipment. Also, 25-50 year-old cameras can still render excellent results.
  • Old computer equipment is usually only good for doorstops.
• Expense is the issue:
  • if expense is no object, print quality is diverse.
  • some digital prints can be indistinguishable from conventionally produced prints or transparencies.
  • otherwise there are trade offs between cost of equipment and quality of print.

The technical quality of digital photographic equipment is measured by:

• sharpness - spatial and tonal resolution.
• definition - spatial and tonal resolution.
• contrast - tonal resolution or dynamic range.
• color accuracy: complex mathematical representation of color (scanner, software, printer).

Resolution: perhaps most misunderstood and misused term in the digital photography glossary. There are two distinct components that relate directly to traditional silver photography:

• Spatial resolution = grain size.
• Tonal resolution = brightness/contrast (dynamic range).

Spatial Resolution: of output device is easiest to understand - simply describes how many pixels are needed to fill a square inch of the outputted image. Since most output devices have the same spatial resolution in both the horizontal and vertical directions, spatial resolution is usually measured in dots per inch or dpi (alternatively: lines per inch - lpi - for halftones) rather than dots per square inch. In continuous tone images - a dot is the equivalent to a pixel. Therefore, the spatial resolution of a digital printer is similar to that of traditional grain size in a photographic print or transparency. The lower the spatial resolution, the courser the image. At a minimum, output devices should be able to render image pixels at a high enough spatial resolution so that the pixels are not individually distinguishable and therefore seem to disappear into a continuous range of color or gray values.

Tonal Resolution: is a more complex issue:

• digital images must divide up the image into a finite number of discrete tonal levels.
• cannot render tonal variation as a continuous gradation.
• if there is an inadequate number of levels to represent tonal range, there is a noticeable loss of detail, especially in the extreme values.
• despite the fact that digital printers are unable to produce continuous tones in output, they are referred to as such to distinguish them from halftones.

Color Accuracy: How good the output system matches the input system: scanned print to:

• monitor.
• digital print.
• digital color matching system technology (CMS).

Color scanners separate colors of input in to RGB:

• primary additive color components (during the scanning process).
• 24 bit represents 8 bits per RGB color.
• Color Space is a mathematical model used to describe colors in an image.
• Many different color spaces.
• RGB (scanners, monitors).
• CMYK (printers).
• lab (hue, saturation, luminance) represents color similarly to human eye; perception of color; more intuitive for digital photographers to understand and control:
• Hue -color.
• Saturation - purity.
• Luminance - brightness, intensity.

Image is scanned in RGB color space. It is not necessary to convert to CMYK while working because monitor only works with RGB signals.

Output Media: three different classes:

• monitors.
• hardcopy.
• electronic (disks, electronic transfer).

each has unique properties with which to become familiar:

Monitors:

• "soft" proof.
• remains intangible - #s and electrons floating in color space.
• can manipulate indefinitely.
• does not need papers, chemicals or consumables.
• not transportable (except with a laptop).
• can not be distributed other than electronically.

Higher resolution than conventional  analog TV monitor , more expensive. Macintosh monitors are designed to display 72 dpi. Bigger monitors have more pixels available per color image.
Color cards operate 8 bits per color or 24 bits per pixel (over 16 million colors).

Display quality is a function of bandwidth: the amount of information that can be transmitted through a particular electronic system - measured in Khz. More information = better resolution

• monitor screen size.
• maximum screen dpi displayed.
• interlaced or non-interlaced.
• dot pitch (size of dots).

A video display board interfaces a monitor with the computer. Actually defines color resolution of displayed image.

must contain enough memory to fill monitor screen at whatever color resolution the display board supports (24 bpp) - translates directly to max number of pixels or largest portion of the image that can be shown on a monitor.

Note: digital image itself is likely to be much larger than the available frame memory on the display card. Computer software will automatically shrink (subsample) the image to enable lower spatial resolution version of the image to be displayed. Does not effect the ultimate quality of the image or the ability to work on parts of it.

To minimize cost of TV transmission bandwidth, a standard TV display uses a technique called interlacing or writing only half of the screen image at a time, every other line, then returning to write the other half, 60 times a second for 30 frames. A complete image is rewritten 30 times a second. The light-emitting phosphors on the monitor's tube normally have a half-life of 1/20 of a second, so interlace refresh rate is usually acceptable. However, there is some flickering. Usually this is not troublesome because one watches moving pictures on a TV rather than working with a still image or text. The flickering would cause severe eyestrain for working with still images or text.A non-interlaced monitor writes a complete image 60 times a second, without flicker.

Flicker can also produce undesirable image degradation and color shift as the phosphors fade. Monitors for digital imaging are always non-interlaced.

Monitors for digital imaging should be at least 13Ó diagonal (ideally 17-19Ó, have spatial resolution of approximately 72 dpi (for Macintosh) or 75 dpi (for PC),  be non-interlaced, have at most .28mm dot pitch, and about 30 MHz bandwidth.

The first problem with color matching can be seen at this point:

• not all monitors are alike.
• RGB is a relative measure - not absolute (calibration is necessary).
• displays differ due to phosphors use and age.

Further complications arise when transferring a monitor image to paper, especially if it is necessary to convert to CMYK (secondary, subtractive colors used in print media).

Transfer a process from RGB space to CMYK space further complicates the color-matching problem. Like RGB, CMYK is a relative measure depending on variances in:

• printing presses.
• types of inks.
• calibration of devices.
• No single optimal formula for the translation;
• The more different types of input and output devices,
• the more complicated the color matching problem.

Hardcopy Printers:

A vast variety of printers is available on the market. At one time, most inkjet and laser printers were not capable of producing quality of output needed for digital photographers. However, today there are many high-end laser and inkjet printers available for this purpose.

3 different types of output media;

• reflective print.
• film transparency (film writer).
• image setter ( color separation films for printing plates).

2 different types of prints

• continuous tone ( or contone).
• Halftone.

Continuous Tone printers: each pixel of an image is transformed into a similar sized overlapping dot of CMYK color of ink or toner - transferred to paper in different intensities - 1:1 sampling.

Halftone printers: two step fusion of digital and traditional printing technologies to produce 4 color separation films from a digital image setter. Prints are produced on conventional printing papers. Colors are made up by changing the size of CMYK ink dots relative to each other, rather than their intensity (referred to as rosette pattern). Note: as printing companies modernize, image setters and intermediary color separation films are becoming obsolete. Printing plates can be created for each of the four colors directly from the digital file with plate setters.

Digital halftone printers, on the other hand, can print directly to paper with no intermediary film stages (such as color laser and inkjet printers). These printers place dots in complicated, more random alignments called dithering which can control the size and position of the digital halftone dot.

Contone printers:

• thermal dye sublimation.
• film writers.

Digital halftone printers:

• laser.
• Inkjet.
• Electrophotography.

Choosing a printer concerns:

• budget trade-offs with quality.
• output needs - types of media; number of prints.
• print quality.
• print size.
• computer compatibility.
• archival quality.

Print quality:

• printer's spatial and tonal resolution.
• dynamic range.
• color accuracy.
• photographic quality.
• at least 16 bits per color resolution and 720 dpi spatial resolution (higher quality, higher price).

Output media:

• paper, film, disk (CD).
• variety of substrate.

film can provide the most convenient path between  the digital and conventional photography worlds. (35 mm slide or negative output gives widest variety of applications). Once printed on film, the negative or transparency can be taken back into the darkroom to print as a traditional silver print with a wide dynamic range and continuous tone quality.

Cost of printer: can range from $200 to $200,000. Also to be considered are the cost of consumables (paper, film, CDs, film processing). cost of maintenance.

Software and computer compatibility:

• Is the printer driver designed to work with your desktop device?
• Are your printer and driver software compatible with your computer?
• Is the image processing software capable of sending to that printer?

Processing Speed:

• throughput - time it takes to process hardcopy.
• what are your needs?
• a few high-quality prints each day.
• short-term print productions.
• high-volume printing.

Archival quality:

• How fade resistant or permanent is the hardcopy?
• Even film dyes can fade over time.
• Thermal dye sublimation - fades in a few months if exposed to direct UV light of kept stored in plastic.
• Chemical color prints also fade.
• Resistance to aging effects best done in digital form - back ups.
• Substrate resistance to deterioration? Is it acid-free?
• Laser prints last longest.
• Inkjet  inks subject to fading.
• Iris Inkjet output is expensive but high quality. Coatings are used for fade resistance.

Reflective Art Printers:

Photographers are most accustomed to seeing their work on paper or film. Digital imaging technology has increased the options - output or hardcopy other than paper.

Inkjet Printers:

• halftone printer.
• work by shooting different sized dots onto paper surface; four basic printing ink colors - CMYK.
• created through process of dithering rather than halftone screening.
• spots of ink on paper.
• maximum quality level resembles quality of four-color offset printing press reproductions.
• good candidates for producing prepress image proofs.
• recent improvements in inkjet printers makes them now capable of producing truly "photographic quality" prints due to improved dithering process which renders richer tonal range.
• Iris inkjet printer is capable of producing large format prints of remarkable quality on almost any substrate.
• Advantages:
  • low cost (some exceptions, including the Iris).
  • high quality for cost.
  • wide variety of papers.
• Disadvantages:
  • requires very precise mechanisms for controlling flow of ink - maintenance can be high.
  • slow speed.
  • initial cost of the printer is usually low, but consumables can be expensive.

Thermal Dye Sublimation:

• continuous tone printer.
• contains a roll of ribbon that holds the dye pigments used in printing.
• material is in contact with a strip of thousands of small heating elements contained in the printing head.
• as each row of pixels is sent to the printer, each element in the printer is heated proportionately to the intensity of color and value in the corresponding pixel.
• causes the dye pigments to be sublimated into gaseous form. The hotter the print element, the more dye sublimated. The process causes slight spreading and melding of color, which results in thousands of shades of continuous color.
• companies making this printer include: Kodak, DuPont, Nikon, Sony, Tectronics, Fargo.
• capable of producing small (4x5) prints to large (11x17Ó). Cost is proportional.
• can produce 3-color RGB or CMY or four color CMYK. Cost is proportional.
• Advantages:
  • relatively inexpensive to buy the printer.
  • fairly fast
  • print quality approaches photographic quality
  • dynamic range and theoretical spatial resolution similar to that of silver halide prints.
• Disadvantages:
  • requires special expensive consumables.
  • individual print cost is high.
  • maintenance can also be costly.

Electrophotography:

• is the technology behind most standard office photocopiers and laser printers.
• electrostatic charge created on a drum by projecting an optical image onto it.
• charge attracts toner that is subsequently transferred to paper.
• laser printer - laser rather than optical projection.
• color electrophotographic printing is a halftone process - usually (CMYK) dry powder toners (not inks!).
• maximum achievable quality for medium range printers is not as good as that of offset press or inkjet (not as good dynamic range).
• very high quality (and expensive) laser printers will print on a variety of substrate and approach photographic quality.
• most laser printers are optimized for rapid reproduction of color graphics and text.
• Advantages:
  • prints on plain as well as a variety of papers.
  • relatively high speed.
  • relatively low cost per copy or print.
  • relatively long lasting archival quality (especially when printed on acid free papers).
• Disadvantages:
  • printers/copiers usually are costly.
  • must have an expensive printer to achieve photo quality output.

Film Writers:

• continuous tone output (to film that can be printed or used for reproduction).
• produce photographic transparencies that are difficult to tell from a camera-made original.
• modified camera body.
• cathode ray tube (CRT) display screen.
• camera shutter remains open while the RGB electron beams sweep across the face of the tube, exposing the film, line by line.
• capable of 8,000 lines of data for a 35mm slide.
• grain size comparable to ISO 100 or 200.
• Advantages:
  • High quality images on film. Resulting image can be printed on traditional photographic papers.
  • Good way to make slides of digital work.
• Disadvantages
  • Expensive initial cost.
  • Requires additional steps to print.