or if
RAW
images are not post-processed using the same value.
This variation of 150K is relatively small, compared with the
fixed white balance values on the camera that ranges between
2000K and 10000K.
Intra-frame Brightness Variation Assessment - Aperture Influence
Intra-frame brightness variation stemming from vignetting
and other off-axis optical effects, was more pronounced for
images with a wider aperture (lower
f
-stop number), as theory
would suggest. As seen in Figure 5, irrespective of the
f
-stop
used, or the degree of onboard vignetting compensation used,
all images exhibit some degree of vignetting in the approxi-
mately on the image periphery. Striking the appropriate
balance between selecting an
f
-stop that allows for proper im-
age exposure when paired with a quick
shutter speed, while
maintaining a tolerable level of image vignetting, will depend
on to what degree brightness variation in the corners of the
frame may be detrimental in the intended application. When
implementing
RSI
with same
DSLR
camera, frame centers are
nearly identical for both images of the bi-temporal pair, such
that any vignetting effects will be manifested similarly and
impacts of brightness follow-off on change detection should
be minimal. For almost all consumer digital cameras, vignett-
ing mitigation is usually a component of the digital image
development process.
(a)
(b)
Figure 4. (a) Dynamic range portrayed by minimum and standard deviation of intensity digital number values for the 16-
bit container as a function of
EV
offset settings; and (b) Dynamic range portrayed by minimum and standard deviation of
intensity digital number values for the 8-bit container as a function of
EV
offset settings.
Figure 5.
f
/2 to 16 (19 different apertures), captured with
no vignetting reduction applied at
EV
0; plotted by
DN
of
intensity and distance from the image corner.
Figure 6.
EVs
0 and −
⅓
imaged using
f
2 with no, low, normal
and high vignetting correction, plotted by
DN
of intensity and
distance from the image corner.
154
March 2018
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
