Digital Photography: Understanding ISO
Author: Andrew Goodall

ISO in digital photography is not complex, yet many new
photographers have a hard time learning and understanding
this fundamental aspect of photography.

I suspect this is because of the way it is being taught.
You see, ISO started out as a property of film, and it was
much easier to visualise it in terms of the old technology.
So that's where I want to start my explanation, before
bringing you into the 21st century with ISO today.

ISO actually started out as ASA, which stands for American
Standards Association. Decades ago, a commercial film
manufacturer came up with a set of numbers to define the
sensitivity of different types of film. That set of numbers
was accepted by the American Standards Association, so all
American manufacturers could use the same system. Later,
the American standard was adopted by the International
Standards Organization, so ASA became ISO.

What does all that mean? Well, it means that the letters
ISO didn't really stand for anything except for the name of
an organization.

What is important is what ISO referred to, which was the
sensitivity of the film. Some films were made to react
faster to light, and some to react slower. Slower films had
a smaller ISO number, like 25, 64, 100. Faster films had a
higher number, like 200, 400, 800.

A slow film needed a relatively high level of light to
create a well-exposed photo. That meant that to take a
photo in darker conditions, you would need to use a fairly
wide aperture and/or a fairly slow shutter speed to get a
result. On the other hand, a faster film reacted to light a
lot more quickly, so it needed much less exposure to light
to take a photo.

Fast film sounds pretty good, doesn't it? A chance to take
a photo in any conditions without a tripod, and to freeze
moving subjects with very fast shutter speeds. So why
didn't everyone just use fast films all the time?

The answer is that the advantages of fast films came with a
trade-off; loss of image quality. The grains of emulsion on
a fast film were larger, so a photo taken on a film with
ISO 400 or 800 had a rougher, 'grainier' look. This may not
have been a problem in a small print, but became quite
apparent with big enlargements. Consequently, most
professional photographers preferred to use slower films of
100 or 64 ISO for most of their work.

So is this just a lesson in ancient history? After all, you
have a digital camera, so what does all this have to do
with you. Well, the remarkable thing is that despite the
massive shift in technology, for all practical purposes ISO
has not changed at all.

You should be able to set the ISO on your camera to suit
your needs. Just like in the days of film, if you set your
ISO to a low number like 100, you will need more light to
create a correct exposure. That means that you may need to
keep a tripod handy for cloudy days, and in certain
low-light situations you may not always get the aperture
and shutter speed settings you want. If you set your ISO to
400 or 800, your camera will become much more sensitive to
light; you will be able to shoot in exactly the same
conditions without a tripod, and with greater flexibility
to choose the aperture and shutter speeds you want.

But here is the amazing part. Higher ISO settings still
come with the same trade-off that once existed with film.
With the increased sensitivity to light, you will also have
to accept a grainier image. I don't know if it is
pixellation, or digital noise, or a combination of both,
but it is generally understood that for all their
advantages, high ISO photos come with a reduction of image
quality that becomes more obvious the more you enlarge the
image.

So there you have a quick introduction to what ISO is all
about. Perhaps I am just showing my age, but I find this
subject easier to explain in old-technology terms. For many
people it is easier to visualise when related to something
solid like film, rather than something that happens on a
computer chip. Anyway, I hope this helps you if you have
had trouble understanding what ISO is all about.


About the Author:

ISO, aperture, shutter speed, depth of field...all the
essentials of good photography seem simple when they are
explained in terms you can understand. Visit
http://www.naturesimage.com.au and check out Andrew
Goodall's ebook "Photography in Plain English" to discover
your own talent for photography. While you are there,
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