Dive Pacific Iss 171 Oct- Nov 2019

surface or vertically up. Use ISO
-100, shutter speed 1/500 s.
Sensors (ISO) and pixels
A camera sensor is an electronic
microchip that consists of photodiodes
(pixels) that absorb light
and transform it to a flow of
digital data which the camera
then records on a memory card.
Resolution, or the ability to show
fine detail, defines the total
number of photodiodes or pixels of
a sensor.
The number of pixels on the
sensor of a modern camera can
reach tens of millions and keeps
on increasing. Not too long ago
a camera with a 6 million pixel
sensor was considered top notch;
now a 24 or 36 million pixel camera
surprises no one.
First place in the category of
“Wide Angle” White Sea Cup 2006
Russia
Nikon D70 10,5 mm F2.8
(f3,5; 1/80 ñ; ISO400)
The physical dimensions of
individual pixels are currently
0.005-0.006 mm. The larger
the pixel, the larger the area it
occupies and the amount of light
it absorbs, hence higher sensitivity
and better signal-to-noise ratio
(colour noise). The pixel size also
determines another important
characteristic: photographic tolerance
or dynamic range. This is
the ability of a sensor to properly
render the brightness of a subject
being photographed. The wider the
dynamic range, the smoother the
transition from bright to dark. For
example, the sun photographed
through the water surface by a
camera with a sensor that has a
narrow dynamic range will look
like a ‘hole’ with ragged edges.
Sensors of modern cameras use
different technologies to capture
light. We will not get into that.
Use cameras with the best sensors
you can afford. A general rule of
thumb is: the larger the physical
size of the sensor, the higher the
quality of images. So a lot of underwater
photographers today prefer
cameras with sensors at maximum
size, or full-frame sensors. The size
of the sensor is the same as the
size of a 35 mm film: 24x36 mm.
*Digital cameras with a similar
size sensor are called full-frame or
1.0xcrop sensor cameras (FX). All
other digital cameras have smaller
or cropped sensors (DX). A crop
factor is a value by which the sensor
is smaller than the 24x36 mm full
frame. For example, the crop factor of
Nikon SLR camera sensors is 1.5x. In
other words, the sensor is 1.5 times
smaller than the full frame; for Canon
it is 1.6x. The sensor of Olympus
cameras is exactly twice. Generally,
compact camera sensors are even
smaller.
Why do we have to know that? The
thing is that the majority of interchangeable
lenses that were designed
and made for analogue cameras are
still manufactured and successfully
used with digital cameras. But there
are some nuances: lenses designed
for film or digital full-frame cameras
when used on a crop-sensor camera
lose part of their angle of coverage.
Because the crop sensor is smaller,
part of the image is left out of the
frame as if a 24 mm lens were used
instead of a 16 mm one. That’s why
special lenses were developed for
cropped sensor cameras, and the
diameter of the image they produce is
equal to the diagonal of the cropped
sensor. For example, the angle of
view of the Nikon 16 mm lens is 180
degrees on a full-frame camera, but
the same angle of view for a croppedsensor
camera can be obtained only
with the help of a 10.5 mm lens.
Conclusion: specially designed lenses
should be used with crop cameras in
order to obtain the maximum angle
of view. The only exception is macro
lenses (lenses that shoot with a 1:1
or even higher magnification). When
used on cropped-sensor cameras,
macro lenses for full-frame cameras
will give you an additional magnification,
which will be beneficial.
So what sensor should we go for?
Naturally, the larger the physical
size of the sensor, the better
quality images we will produce:
greater detail, better dynamic
range, and less noise. But is
their superiority over smaller
sensors that obvious? Not at all!
Manufacturers of cropped-sensor
cameras are constantly working
to improve the quality of their
products and the capabilities of
cropped sensors are more than
enough to achieve most of the
photographic objectives of amateur
photographers.
Sensitivity (ISO)
The sensitivity of a sensor (or
film) shows how much light is
needed for your exposure. ‘High’
sensitivity requires less light to
expose an image and can be used
in relatively low lighting conditions,
for example, at dawn, late in
the afternoon or at deeper depths.
‘Low’ sensitivity requires more
light and therefore is used in bright
conditions.
International Standards
Organization (ISO) uses a simple
and easily understood system to
66 Dive New Zealand | Dive Pacific