Digital Imaging

By

Philip

Lawson

Digital Imaging•The Basics - Scanning and Printing•The Master Image•Working with Layers•Working with Channels•Black and White

Digital Imaging 1•The Basics - Scanning and Printing•The Master Image•Working with Layers•Working with Channels•Black and White

The Basics

Colour

ResolutionLuminosity

Light• In the real world,

Luminosities range from:

No Light

Infinitely Bright

Light meter• The light meter can

only measure a middling range of Luminosities: Bright EV 17

1 EV = x 2

Dark EV 5

Film Emulsion• The Film Emulsion can

record only a narrower range of Luminosities7 Evs to 9 EVs

Max

Min

High Key

1

Film Emulsion• But it is our choice

which range to expose on

Max

Min

Low Key

2

Film Emulsion• If the film stock has a dynamic

range of 7 EVs it has a tonal range of 27 = 128 levels

• If the film stock has a dynamic range of 8 EVs it has a tonal range of 28 = 256 levels

• If the film stock has a dynamic range of 9 EVs it has a tonal range of 29 = 512 levels

3

Film Scanners• The Film Scanner

converts these into Numbers in the range 0 -255

255

128

0

• It might not be able to record the full range of Film Luminosities

Print Scanners• The Print Scanner

suffers in that it– Is scanning a print that

already has a reduced luminosity range

– Is scanning the print by reflection

255

128

0

The Monitor• The monitor, being a

light source is capable of a large dynamic range

255

128

0

1

The Monitor• This is adjustable via:

– Brightness– Contrast– Gamma

all of which must be calibrated, set and never again adjusted for consistent results.

2

The Print• The Print, being a reflective

medium is only as bright as the base paper and the light source

• And as dark as the minimum reflectance

Luminous Range• Strive to retain the highest

range of luminosities at EACH conversion.

Luminous Range• Transparent Film has a higher latent DMAX

than Print, so use a FILM SCANNER, min DMAX = 3.6

• Use the best image sources• IF possible scan to 16-bit greyscale and

retain for a.l.a.p.

• Ensure scanner settingsdeliver a histogram showing even distribution over the full range.

Caution• Care when modifying the image

– BRIGHTNESS– CONTRAST

• A pixel cannot be brighter than 255– Burn out

• A picture cannot possess a greater dynamic range than 256

Colour• In the real world, Colour

is absolute and of infinite variation

• Each Primary R, G & B, may be thought of as a Colour Luminosity and follows similar degradation

No Light

Infinitely Bright

Colour Gamut• The Gamut of visible

colours is described by the LAB space in the Chromaticity diagram

Film Emulsion• Film Emulsions record

colour differently (CYM)– Manufacturer/Stock– Reciprocity Failure

Max

Min

Film Gamut• The Gamut of the

Film Emulsion is described by the EKTA space in the Chromaticity diagram

Film Scanners• The Film Scanner

converts Red, Green and Blue into Numbers in the range 0 - 255

255

128

0

255

128

0

255

128

0

The Monitor• The monitor, being a

light source is capable of a large dynamic range

1255

128

0

255

128

0

255

128

0

The Monitor• Provided that you’ve got

it set right!

2255

128

0

255

128

0

255

128

0

The Monitor• 256 Colours or “High Colour (16-bit)”

will give you a Posterised representation

3

• Must use “High Colour (24-bit)”

Monitor (RGB) Gamut• The Gamut of the

Monitor is described by the RGB space in the Chromaticity diagram

Note the mismatch

The Monitor• However the colour that

you get is influenced by:

4

• Your Phosphors and selected colour temperature

The Monitor• However the colour that

you get is influenced by:

5

• Your Gammasettings

The Monitor• However the colour that

you get is influenced by:

6

• Your Profiles

The Monitor 7• The colour that you

SEE is influenced by:

• Any funny desktop coloursor artificial light

The Print• The Print, being a reflective

medium has Cyan, Yellow and Magenta translucent inks applied

The Programperforms the RGB -> CYMKTranslation

Print (CYMK) Gamut• The Gamut of the

Printer is described by the CMYK space in the Chromaticity diagram

Although this is very dependant upon the light under which it is viewed

Colour Range• Whilst modifying colour

balance, ensure that you do not create an OUT OF GAMUT colour

1The Printer• The Printer makes numerous

Necessary and Unnecessary Corrections

• Colour Space

2The Printer• The Printer makes numerous

Necessary and Unnecessary Corrections

• Transferand

• Bleed

3The Printer• The Printer makes numerous

Necessary and Unnecessary Corrections

• Mode

4The Printer• The Printer makes numerous

Necessary and Unnecessary Corrections

• Quality• Media• Halftoning• Adjustment

The Printer• Basic Advice• Print at highest Quality (1440 dpi)• Select the Correct Media Type• Use “Error Diffusion” Halftoning• Use “Photo-realistic”Use “Photo-realistic” Colour

Adjustment

• Not “Automatic”!!

5

Characterisation• Colour Management requires each

device to be Characterised (by an ICC Profile)

MonitorICC

Profile

PrinterICC

Profile

ScannerICC

Profile

Kodak CDICC

Profile

Profiles• Each Device is Characterised by a

PROFILE that in turns needs Calibration

CalibrationCalibration requires • Consistent Conditions• A Known Calibration

Source

• Your Gammasettings for the Monitor

Profiles• Photoshop manages your Profiles

Colour Range• Use a scanner that Optimises the

dynamic range of each colour independently.

• Colour Casts can easily be eliminated• Take care when modifying the image

– BRIGHTNESS– CONTRAST

• Ink is not permanent. Protect from extreme light sources and Save data files.

Resolution

• In the real world, the In the real world, the resolution that we resolution that we achieve is governed achieve is governed by:by:– FocusFocus– Resolving power of Resolving power of

lenslens– Camera ShakeCamera Shake– Heat HazeHeat Haze

Film Emulsion• The Film Emulsion is limited by the

clumping of the grains.• Typical Resolving power is <50

lpmm

35mm

24

mm

Max = 1750 lines

Film Scanners• Typical Film Scanners can detect

2700 d.p.i.

• This is sharper than the finest level of detail but not enough to see the individual grains.

Max = 4000 pixels

35mm

24

mm

Print Scanners• Typical Print Scanners can detect

600 d.p.i.

• On a 5” x 7” Printthis would produce4200 pixels and a38Mbyte file.

7”

5”

Max = 4200 pixels

The Monitor• The monitor resolution can be a

number of prescribed settings LESS than the number of Phosphor dots on the screen.

• Choose the highestas this gives youmore desktopspace for all thosepalettes

1

The Monitor• Always Zoom In or Out and

Pan around.• Learn to use the quick keys:• Ctl-0• Z followed by

drag a rectangle• Ctl + drag a rectangle

in the Navigator• Space Bar

2

The Monitor• NEVER change the Image Size down• This throws vital information away!

3

The Print

• A typical, photo-quality Ink Jet Printer is A typical, photo-quality Ink Jet Printer is capable of printing 1440 d.p.i. capable of printing 1440 d.p.i.

• An A3 or 16” x 12” printAn A3 or 16” x 12” printwould have 23,000 dotswould have 23,000 dotsdown the long sidedown the long side

The Print• An A3 or 16” x 12” print would have:

23,000 dots down the long side, 17,280 down the short side

and therefore

397 Million dots397 Million dots

But…….

The Printer• Most Ink Jet technology cannot print

dots of – Differing sizes.– Differing Densities.

• Therefore the 255 intensities of C,Y and M inks must be simulated by a 16 x 16 grid of dots

• This DITHERINGDITHERING is still so small it is invisible.

The Printer

Green = 6 Green = 250

The Printer• This means that the actual colour

resolution of the 1440 dpi printer is only 90 patches per inch, or about 1/100th of an inch.

• Newer Ink Jet technology is capable of variable dot size (4 only)

• Dye Sublimation is capable of variable dot density – better!

The Printer – 4 sizes

Green = 6 Green = 250Green = 128

This means that the actual colour resolution of the 1440 dpi printer is only 360 patches per inch.

The Printer – 4 densities

Better – less granularity beyond Green = 16.

Green = 6 Green = 250Green = 128

The Printer• The Printer software (driver) will

make the best job of this (and must do the final dithering as only it “knows” how best to do this in order to minimise pattern effects )

• To increase the Image Size in Photoshop using RESAMPLING RESAMPLING might make a better job of it but the file sizes will cause the computerto overheat!!

The Printer• This discrepancy between the file size

4000 x 2700 pixels = 32Mb• and the print size

23,000 x 17,280 = 397 Million dots.

(which would need 1.2Gbytes)• must be created• This is called INTERPOLATIONINTERPOLATION• If you must, increase the Image Size to

360 d.p.i. (Best Setting for Dot Matrix Printers)

Conclusions• Scan the image to the

highest resolution you can• Use a slide/negative scanner

if possible

Conclusions• Always control the dynamic

range of each colour.• Never over-saturate.

Conclusions• Never throw information

(colour or resolution away) unless CROPPING

• Always save your work– The original Scan, cleaned and

corrected– Any critical mid-points– The end result, printable version– A small JPEG version, thumbnail, for

the index files.

• The cheapest form of storage is CD-R (Recordable CD)

Conclusions

End of Basics

Photographs courtesy of Janet Edwards

The End

Photographs courtesy of Janet Edwards