What do variable
ND filters do?
They control the intensity
of light getting into the
camera and affecting the
exposure. A single or
straight ND filter has a
fixed level of light control,
but a variable ND can be
changed to allow more
or less light to enter, to
suit the conditions you’re
shooting in and the effect
you want to create.How do they work?
They are two separate
polarising filters, with the
first (the inner filter) fixed
in position when you
screw onto the front of
your lens. The second filter
can then be rotated by
hand: as they come
further into alignment, the
two layers block out more
and more light. However,
they don’t have the same
effect on your shot as
using an individual single-
layer polarising filer.Do they fit on any lens?
Just like any circular filter
that screws directly onto
the lens, you have to get
a variable ND that is
designed to fit the lens
you want to use it on. All
lenses have a filter size.
It will usually be written on
the lens itself so that you
can choose the correct
filter for your needs.How much light do
variable NDs block out?
Typically a variable ND
filter will block out up
to eight stops of light,
in one-stop increments.What sort of subjects
would I use them for?
They are especially useful
for slowing down the
shutter speed, to
introduce blurred motion
to subjects such as water
or clouds. They can also be
used to help achieve a fast
aperture, such as f/1.4 or
f/2.8 in bright conditions.Tech check
Variable ND filters
Control the amount of light hitting
your camera’s image sensorhttp://www.digitalcameraworld.com MARCH 2020 DIGITAL CAMERA^97
Fast focus
Q
How can I focus on
a subject moving
directly towards meat speed, and keep it sharp?
Colleen Morris?
A
When you are trying to keep
focus on a fast subject coming
directly towards you, you are
really working your camera and
lens to the maximum, so you need to give
it all the help you can. If you are trying to
do this on the default AF mode of your
camera you can forget about getting it
right, other than an occasional bit of luck!
You need to set the AF mode of your
camera away from its default setting,
which is intended for more static subjects,
and onto the AF mode designed to be
used with a moving subject. For Nikon
users, this is AF-C. Most other makes have
a name utilising the world ‘continuous’- although Canon calls it AI-Servo,
which stands for Artificial Intelligence!
Since the subject is moving in your
direction, you may need to fire several
shots to capture it at is optimum point,
so set your camera so it fires off a
sequence of shots one under another.
Now point the camera at your subject; as
it comes towards you, keep your AF point
(or points) on the subject and half-depress
the shutter button so the camera tries to
keep its focus. It might not get it right every
time, but shooting this way will increase
your chances of getting several pinsharp
shots. You’ll need to couple this technique
with a fast shutter speed, or you’ll get
movement blur ruining your photo instead!
Above: The intensity of light in this desert portrait meant that a variable ND
was needed, to control the amount of light hitting the sensor.