Rig Shots 12:
Common problems, dust flare, vignetting
Dust, flare and vignetting are common
and simple problems which can often be avoided. Spending a few extra
minutes before taking any photo's can save a lot of time in
post. Sure you can fix it later in Photoshop but why fix it several
times in post when you can fix it once when shooting.
Sensor dust appears as small black or
dark spots on the image. These spots stay in the same place on every
image until the dust moves (due to air turbulence from the shutter
opening and closing and mirror flipping up and down) or the sensor is
cleaned. Sensor dust is a significant problem with
rig shots because the blurred background acts to highlight any sensor
dust, just like a clear blue sky would. These dust spots typically have
to be cloned out in post production so it's a good idea to minimise dust
spots because this minimises the work needed to finish an image.
It's a good idea to clean your cameras sensor on a regular basis. Keep
your camera clean, it's just common sense. You might also find that some
cameras are more likely to show dust than others. If you have several
different cameras to choose from then it might be worth comparing them.
Some cameras are simply dust magnets because of their design or
technology whilst others rarely show significant dust spots in use.
The aperture used on the lens, and the lens itself, can have an effect
on the likelihood of dust appearing. As you stop down the aperture on a
lens the likelihood of dust appearing increases so shooting at F5.6-8,
for example, will show far less dust than shooting at F16-32. Of course
you may be forced to stop the lens down further than preferable simply
because you need greater depth of field so it's not always possible to
minimise dust spots by using a wider aperture.
Another consequence of dust is flare
from dusty filters or even lens surfaces. Flare can take many forms but
it is essentially a reduction in contrast due to light bouncing around
the lens where it shouldn't be. Dust on filters or lens surfaces can
leave bright spots on the image similar to sensor dust spots but where
dust on the sensor leaves dark spots, dust on filters or lens surfaces
leaves bright spots. In this case flare is caused by bright light
striking the dust on the lens and filter surfaces and actually appearing
in the image. This would never be a problem with normal or telephoto
lenses but the extreme wide angle lenses used for rig shots allows these
sunlit dust spots to be partially in focus, hence the bright spots
appearing in the image. Make sure your lenses and filters are
clean and dust free so as to minimise flare when shooting towards bright
light sources such as the sun or street lights at night. Use lens hoods
whenever possible, day or night.
The white spots seen at the top
left corner are caused by flare from strong side lighting striking
dust on filter surfaces.
Stacking Polarising and ND filters on wide
angle lenses is bound to cause vignetting at certain focal lengths. For
example, a Canon 10-22 EF-s lens with B+W slim Circ-Pol filter stacked with an ND
filter will results in vignetting if shot any wider than about 12mm. Other
lenses exhibit vignetting at different focal lengths and with different
filters so it pays to know the limitations of your lens/filters and to
check for vignetting before getting stuck into shooting.