Photography Tutorial #2: What is the right exposure?
moving on let's cover four technical terms that will come up a lot.
The first is a
"stop". Stops are a reference to a measured specific amount of light
that is consistent throughout the range of light and in all equipment.
Like a pound or kilogram in weight, but for light.
Speed: This is
how fast the shutter opens, exposes the
sensor to light and closes. The faster it moves, the less light the
sensor receives. Faster shutter speeds stop action because very little
movement happens during the brief moment it's open. Slower shutter
speeds expose the sensor to more
light, but moving objects will blur because they change position while
the shutter is open. As a general rule of thumb I
consider 1/500 the absolute minimum while trying to freeze action. I
try to keep it from 1/800 to 1/2000.
size of the
opening in the lens. It is adjustable just like your shutter speed and
the second means of controlling the amount of light reaching the
sensor. The numbering seems backwards at first, because the smaller the
number, the larger the opening (letting in more light) and the larger
number is of course a smaller opening. Your maximum aperture will vary
depending on the lens, the chosen Aperture is often referred to as
F-stop or F plus Aperture number. For example F5.6.
Once known as
ASA, ISO speed is the digital equivalent to film speed. This
the third way of adjusting a exposure. Lower ISO speeds absorb less
light than higher ISO but retain better detail and less
a rule of thumb keep your ISO as low as possible for the situation. On
stop of ISO is always double, such as from 100 to 200, or 400 to 800.
If a shot is too dark you can either slow down your
by one stop, or
aperture one stop, or
up your ISO by
one stop. Choosing which one to adjust is the key of control in
photography. Adjusting any one of
these three one stop will increase your exposure the same exact amount.
tuning, modern digital cameras can adjust in 1/3, 1/2 or full stops
depending on user preference.
So we have our three methods of adjusting exposure, now
what is the right exposure? Generally speaking, the right
exposure is when the subject (what the picture is about) is exposed
correctly. In my opinion, good whitewater shots are really all about
good whitewater. So I try to expose for the whitewater in
the image. I do this by using the histogram on my camera. The histogram
is how the camera displays the light it just captured as a graph
instead of as an image.
This is the histogram graph from a typical sunny day whitewater
scenario, where there is a wide range of light from the dark
shadows to sun on white water. Let's say this bright sunny day that has
twelve stops of
light in it.
Dynamic range is a fancy way of saying how many stops of light the
sensor (or film) in a camera can record. In 2005 most cameras had eight
or nine stops of dynamic range. As
of 2013 that's up to 13-14, and in general lack of dynamic range is a
lot less of a problem now. Still at some point we will encounter a
range of light than the camera can record. Thus a compromise must be
made. If you leave your camera to it's own
metering, this is how it will most often deal with a large dynamic
range of light:
red box is the amount of light
your camera can capture. As you can see this scene has more range than
the camera is able to capture, and the camera defaults to the middle
the range of light
extends off the left side of the graph, shadows fall to pure
black. If the graph goes past on the right, the brightest spots get
pure white. Both are happening in the above. Below we have an
exposure "exposed for the right" which works well for whitewater. More
shadow detail is lost to pure black, but this looks more natural to the
human eye than the highlights going to pure white, especially in
whitewater where the water will lose all texture if it's
Now the water retains detail because the highlights are not pure white.
we expose for the right, look
how the shadows may be black but the whitewater retains texture,
histogram overlayed on the bottom left.
If the image was exposed for the left, now there is detail in
shadows but none in the bright spots, a very unnatural look.
Every camera has a histogram, turn it on so that it
pops up every time you review an image, or shows in live view if your
camera has an electronic viewfinder. A tip for more advanced users:
Digital cameras take in red, green, and blue. The histogram on most
cameras only reflects the red color channel. To get a truly accurate
histogram turn on the three channel histogram, so you can see if
another color is overexposed.