A simple explanation of the daunting ISO setting. It's actually
very simple, and can be extremely handy when shooting in low-light conditions.
Your camera's ISO setting controls how sensitive its sensor is to
the light that reaches it. It is one of the crucial factors which goes into
determining a photograph's exposure as well as the overall image quality.
A higher ISO setting makes your sensor more sensitive to light,
meaning that you can take photos in darker conditions without the need to use a
flash or tripod. However, a high ISO also creates more noise, reducing the
image quality.
Most digital cameras can automatically adjust their ISO setting to
give an acceptably sharp image when hand-holding your camera. However, you will
often need more control than this, which is why it's important to have a good
understanding of camera ISO and how it affects your shots.
Where Does ISO Speed Come From?
ISO speed actually dates back to film cameras, where it is used to
describe how sensitive a particular film is to light. Of course digital cameras
don't use film, but they do use a sensor which works in a remarkably similar
way. Because people were used to using ISO with their film cameras, the term
was adopted in digital cameras too.
Digital cameras have a big advantage over film cameras when it
comes to ISO - with film cameras, the ISO speed is a property of the film
itself. This means that if you want to use a different ISO setting, you have to
physically change the film. With digital, you can adjust the ISO setting at the
touch of a button because it is controlled electronically.
ISO Values Explained
A typical digital camera will have ISO values of 100, 200, 400,
800, and 1600 as a minimum. The higher the number, the more sensitive the
camera is to light. These values are relative to one another, so ISO200 is
twice as sensitive as ISO100, and ISO800 is four times as sensitive as ISO200,
and so on.
ISO and Exposure Time
Along with aperture diameter and shutter speed, ISO speed is
another factor in determining the required exposure time for any given scene.
Every time you double the ISO setting, you halve the required exposure time,
and vice versa.
Let's say you're taking a photo of a church interior and have your
ISO speed set at 100. In order to expose the scene correctly you might need an
exposure time of 1/15th of a second - far too long to shoot hand-held without
ending up with a blurry scene.
This is where ISO comes to the rescue. You could simply change your
ISO setting from 100 to 400, making the sensor four times as sensitive. This
means that you need an exposure time which is only a quarter of the previous
value, in this case 1/60th of a second. This is fast enough to give us a
sharply focused scene without having to resort to using flash or a tripod.
ISO and Digital Noise
Sadly, increasing your camera's ISO speed comes at a cost - it also
increases the amount of digital noise present in your photo. Noise is the
digital equivalent of film grain, and it shows up as lots of tiny coloured
dots, which are particularly noticeable in the darker areas of your photo.
If you do find that your photo has a lot of digital noise, you can
use a program such as Neat Image to reduce it.
What ISO Setting Should I Use?
You should always aim to use the lowest ISO setting possible,
because this will give you the best image quality. However, your first priority
should be getting a fast enough shutter speed, because a blurry photo is a lot
more distracting than one with a bit of digital noise.
Using a wider aperture or a tripod may be a better option than
increasing ISO because both will help you to shoot in low-light conditions, but
without deteriorating image quality. However, ISO speed is a setting that can
rescue your photos in situations where neither of these is a realistic option.
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