ISO Settings in Digital Photography

What is ISO?

In traditional (film) photography

ISO (or ASA) was the indication of how sensitive a film was to light.
It was measured in numbers
(you’ve probably seen them on films – 100,200, 400, 800 etc).
The lower the number the lower the sensitivity of the film and the
finer the grain in the shots you’re taking.

In Digital Photography

ISO measures the sensitivity of the image sensor.
The same principles apply as in film photography – the lower the number
the less sensitive your camera is to light and the finer the grain.
Higher ISO settings are generally used in darker situations to get
faster shutter speeds (for example an indoor sports event when you want
to freeze the action in lower light) –
however the cost is noisier shots.
I’ll illustrate this below with two elargements of shots that I just
took – the one on the left is taken at 100 ISO and the one of the right
at 3200 ISO (click to enlarge to see the full effect).

Iso-1

(you can see larger sized images of both shots here
for the 100 ISO
and here for the 3200 ISO)

100 ISO is generally accepted as ‘normal’ and will give you lovely
crisp shots (little noise/grain).

Most people tend to keep their digital cameras in ‘Auto Mode’ where the
camera selects the appropriate ISO setting depending upon the
conditions you’re shooting in (it will try to keep it as low as
possible) but most cameras also give you the opportunity to select your
own ISO also.

When you do override your camera and choose a specific ISO you’ll
notice that it impacts the aperture and shutter speed needed for a well
exposed shot. For example – if you bumped your ISO up from 100 to 400
you’ll notice that you can shoot at higher shutter speeds and/or
smaller apertures.

When choosing the ISO setting I generally ask myself the following four
questions:

  1. Light – Is the subject well lit?
  2. Grain – Do I want a grainy shot or one without
    noise?
  3. Tripod – Am I using a tripod?
  4. Moving Subject – Is my subject moving or
    stationary?

If there is plenty of light, I want little grain, I’m using a tripod
and my subject is stationary I will generally use a pretty low ISO
rating.

However if it’s dark, I purposely want grain, I don’t have a tripod
and/or my subject is moving I might consider increasing the ISO as it
will enable me to shoot with a faster shutter speed and still expose
the shot well.
Of course the trade off of this increase in ISO will be noisier shots.
Situations where you might need to push ISO to higher settings include:

  • Indoor Sports Events – where your subject is
    moving fast yet you may have limited light available.
  • Concerts – also low in light and often
    ‘no-flash’ zones
  • Art Galleries, Churches etc- many galleries have
    rules against using a flash and of course being indoors are not well
    lit.
  • Birthday Parties – blowing out the candles in a
    dark room can give you a nice moody shot which would be ruined by a
    bright flash. Increasing the ISO can help capture the scene.

ISO is an important aspect of digital photography
to have an understanding of if you want to gain more control of your
digital camera. Experiment with different settings and how they impact
your images today.

Source :  Digital Photography school -
by Darren Rowse