http://www.amateurphotographer.co.uk 43
hit consumer camera lenses in the
mid 1990s, we used the Reciprocal
Rule to work out how low a shutter
speed we could use without
incurring camera shake. That rule
states that your shutter should be at
least as fast as the inverse of the focal
length you’re using. So, a 200mm
lens would need a 1/200sec shutter
speed or faster. The longer the lens,
the more camera shake is magnified
and therefore the faster a shutter
speed you need to offset it.
Of course, this was always a fairly
arbitrary rule as it also depended on
outside factors such as your handling
skills, the weather, and how many
coffees you’ve had.
IS is rated in stops, so if you want a
rough idea of how slow you can
expect to shoot when it’s turned on,
just take the Reciprocal Rule and
factor in the claimed IS benefit of
the camera or lens. So, for that
200mm lens you’d be looking at
attainable shake-free images around
1/13sec for a four-stop IS system, or
just over a second for a class-leading
8-stop IS system as on Canon’s new
EOS R6. Remember you need to
apply any focal length crop factor to
those calculations though, so a
200mm lens on a 1.5x crop sensor
would be equivalent to a 300mm
lens for the purposes of calculation.
Relate this to shorter lenses and it
can appear that IS is less of a
requirement. But if you’re working in
low light, then it’s well worth keeping
it switched on when handholding
the camera, because it will improve
results at marginal shutter speeds
where you could conceivably hold
the camera still anyway. It’s also
worth remembering that IS can be
particularly helpful on very high-
resolution sensors which naturally
show camera shake more acutely.Which is the best type of
image stabilisation?
Although manufacturers all have
their own names for image
stabilisation, the technology used to
compensate for camera shake is
broadly split into optical and
mechanical solutions.
Optical IS is lens-based and uses
gyro sensors to detect movement
which is then compensated for by
shifting an element or group of
elements within the light path.
Mechanical IS (which is also called
sensor-shift, in-body IS or IBIS)
moves the sensor in the camera
body to compensate for any
movement that it detects. In both
cases, the greater the camera
movement, the more the element or
sensor needs to shift to compensate,
so there are physical limitations to
the amount of stabilisation that
can be achieved.Image
stabilisation can
keep static
subjects sharper
at slow shutter
speeds, but it
can’t do anything
about subject
movement. This
can be a problem,
but also a creative
opportunity
Nikon D810, 24-70mm,
1/6sec at f/16, ISO 400