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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