Sharp as a Tack:

1
Title
Sharp as a Tack On the photographic
reproduction of fine detail
Charlie Rettner
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Test Charts

• Shoot the chart and look for the highest
  resolution
• pattern where detail is visible
• Problems
• Involves judgment and perception
• Needs well defined contrast

Resolution and contrast are inseparably bound. A
discussion of resolution without taking contrast
into account is meaningless
http//www.jimdoty.com/Tips/Equipment/USAF_Test/US
AF_Chart/usaf_chart.html
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Modulation Transfer Function
The modulation transfer function (MTF) is defined
as the modulation, Mi, of the image divided by
the modulation of the stimulus ( the object),
Mo, as shown in the following equation.
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MTF Frequency Dependence
By frequency we mean spatial frequency, typically
line pairs per mm
MTF0.8
MTF0.15
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MTF Rolloff

• Spatial frequency response of an imaging system
  or a component
• Contrast at a given spatial frequency relative
  to low frequencies

http//www.normankoren.com/Tutorials/MTF.html
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MTFs Sharpness and Resolution
Perceived image sharpness is closely related to
the spatial frequency where MTF is 50 (0.5)
where contrast has dropped by half. The
resolution (r) is roughly the point where the MTF
is less than 10 (0.1)
Then, to a good approximation
r1 lens, r2 film, r3 vibration, etc
rsens60 lp/mm
r20 lp/mm
rlens30 lp/mm -gt
rlens40 lp/mm -gt
r24 lp/mm
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Factors that contribute to resolution
This qualitatively intuitive result allows us to
quantify the relative contributions of
photographic capture

• Lens system
• Sensor
• Shake
• Printer

Unfortunately these elements do not have a fixed
relationship to each other
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Canon Lenses
1k each
http//www.luminous-landscape.com/reviews/lenses/c
anon_lenses.shtml
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Lens MTF Charts

• Black lines refer to maximum aperture
• Blue lines refer to f8
• Thick lines refer to 10 linepairs/mm
• Thin lines refer to 30 linepairs/mm
• Solid lines refer to S curves
• Dotted lines refer to M curves

EF 50mm f/1.4 USM
The closer the S and M curves are the better the
bokeh An MTF of 0.8 or more at 10 lines/mm
indicates a superior lens An MTF of 0.6 or more
at 10 lines/mm indicates a satisfactory image
http//consumer.usa.canon.com
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Camera Sensors
Bayer pattern sensors
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Nyquist Frequency
The digital sensor samples image at discrete
points. Thus it can only reproduce waveforms that
are bigger than the pixel spacing
In fact the Nyquist frequency is half the sensor
frequency. The 10D has about 135 pixels/mm so it
can resolve 63 lp/mm
Subject Lens Image Sensor
As the spatial frequency of the image gets near
the sensor frequency, horrible things happen, due
to aliasing. Digital cameras therefore have an
anti-aliasing filter to BLUR the image! This
effect reduces the MTF(50) to about 45 lp/mm
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Film vs. Digital
http//clarkvision.com/imagedetail/film.vs.digital
.summary1.html
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Camera Shake
Motion of the camera while the shutter is a major
cause of Blurry images
Rule of thumb t1/focal length of lens e.g. for
f100mm use 1/125s But for 1.6 crop factor,
should be 1/250 or so.
These numbers refer to average sharpness. If
you are looking for images at the sharpness limit

• 1/(5f)
• Tripod
• Flash

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Image Stabilization
Also, so-called anti-shake technology
These all give about 2 to 3 stops. 1/250s -gt 1/60s
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Printing
40 lp/m on a sensor/film becomes 5 lp/mm on a
10-inch print 125 lp/inch. In fact 250 dpi is
usually enough. 300 dpi is excellent (if its
real!)

• The limit of human visual acuity is about 175
  lp/in
• DOF circle of confusion is about 60 lp/mm or
  120 dpi

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Printing note
Resizing the image to a higher resolution
generally does very little to improve printer
sharpness you are limited by the information in
the original file. Resizing with sharpening might
help a little, but don't expect much. Remember,
the image editor resizes the image
transparently to the user when it sends it to
the printer. You never need to worry about the
exact correspondence between the ppi of your
print and the dpi "resolution" of the printer.
http//www.normankoren.com/Tutorials/MTF3.html
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Putting it together

• In conclusion,
• Tack Sharp requires
• MTF of a 35mm imaging system of gt30 lp/mm
• The 10D sensor is 45-60 lp/mm
• Tripod, flash, or 1/(5f)
• Printing at gt250 dpi
• 100 dpi monitors cant do it!

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Sharpening
All about fooling the eye

• Unsharp mask
• Sharp control
• Focus Magic

http//www.normankoren.com/Tutorials/MTF2.html
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The Unsharp Mask

• Simply overlay a blurred copy and make new print

Original Mask
Result
http//www.unm.edu/keithw/astroPhotography/imageS
harpening.html
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The Unsharp Mask
Amount of contrast Radius of sharpening Threshol
d at which effect is applied
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The Unsharp Mask
Amount controls how much contrast is added to
light-dark transitions in a photo. As it happens,
light-dark transitions are almost always edges
increasing contrast along these transitions
enhances the detail and makes the picture appear
sharper. The higher the Amount value, the greater
the contrast increase. Radius determines how
many pixels out from an edge will be affected by
the edge contrast adjustment. The higher the
Radius value, the greater the width of the
affected area. Setting the radius too high can
result in a halo. Threshold dictates how
different in tone adjacent pixels have to be
before they're considered a light-dark transition
to be sharpened. The lower the Threshold value,
the greater the number of pixels that will be
sharpened throughout the photo.
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USM Guidelines

• In-camera blurring
• Radius value of 0.3 to 0.5
• Threshold in the 0-2 range
• Amount between 200-500.

• For printing
• Radius value of 0.8 to 1.5
• Threshold less than 10
• Amount variable

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Other Sharpening Options
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Examples
Bottom line USM is usually good enough
http//www.earthboundlight.com/phototips/smart-sha
rpen-cs2-and-more.html
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Other Software

• Focus Magic
• Handles motion blur as well (50)
• Sharpcontrol
• Controls halo artifacts (free)
• Intellisharpen II plugin
• Photoshop plugin
• Controls halos (25)