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IV. SHARPNESS EVALUATION 29
Motion Blur
Camera or Subject
Movement After
Focus Lock
Diffraction
bou
nd to look softer than it would at greater viewing distances or lower magnification.
Ther
efore, if printed output, especially at 8.5" x 11" or less, is the final objective, it’s
be
st not to rely completely on monitors to judge image quality. To prove this point, we
enc
ourage you to compare your printed images to the on-screen versions.
When high-r
esolution images are enlarged to 100% magnification on screen, image
defects of any kind are accentuated. One such defect that is often mistaken for
inaccurate focusing when viewed at smaller magnifications is motion blur. This can be
caused by subject movement and/or camera movement, and is especially likely to occur
at relatively slow shutter speeds. Remedies include shooting with a tripod to eliminate
camera movement, using an Image Stabilizer lens during hand-held photography to
minimize the effects of camera movement, and using either high shutter speeds or
electronic flash to freeze subject movement. Secondary “ghost” images in flash photos
can be minimized or eliminated by using faster sync speeds and/or lower ISO speeds or
smaller apertures to reduce the effect of ambient light.
The closer the subject, the more critical accurate focusing becomes. One potential
problem to avoid is camera or subject movement after focus lock. This can happen more
easily than one might think, especially during hand-held photography when the camera
is set for One-Shot AF. Even slight camera movement or subject movement after focus
lock can result in soft images, particularly when shooting at wide apertures with narrow
depth of field.
Diffraction is an optical phenomenon in which light waves pass around the edges of an
object and enter the shadowed area of that object
†
. In photography, diffraction flare
often occurs when images are shot at small apertures with short focal length lenses. The
visual effect is a softening of fine detail that cannot be corrected in post-processing. If
you are concerned about this optical phenomenon, we recommend avoiding apertures
smaller than f/16 with wide-angle lenses and wide-angle zooms.
†
EF
L
en
s
Work III