Lesson 3 – ISO

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 enlargements of shots – the one on the left is taken at 100 ISO and the one of the right at 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:

Light – Is the subject well lit?
Grain – Do I want a grainy shot or one without noise?
Tripod – Am I using a tripod?
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.

• 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 is 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.

6 Questions to Ask to Help Choose the Right ISO

Of course when photographing low light scenes there may be no other alternative so I’d suggest asking yourself some of the following questions when choosing what to set ISO at:

• Am I hand holding the camera? – when using a tripod you might be able to use a slower shutter speed which would allow you to lower your ISO.
• Is my subject moving? – if your subject is perfectly still (like when shooting a still life) and where you’re using a tripod you’ll be able to slow your shutter speed and lower ISO.
• Do I need a big Depth of Field? – If you don’t need a large depth of field you might be able to increase your aperture which allows more light into the camera and will allow you to lower ISO.
• Can I use some Artificial Light? – using a flash or even switching on a light can help to get more light into your camera – allowing you to decrease your ISO setting.
• Can I get away with Grain? – sometimes a photo can actually look better with grain. Some photographers love the mood and atmosphere that a little noise can add and will bump up their ISO in the hope of getting it.
• How big will the image be enlarged? – the reason that noise is not able to be seen on your camera’s LCD is that it is very small. As a result the pixels in the picture are small also and the grain is unable to be seen. It is only when you enlarge the picture on a larger screen or in printing that it becomes noticeable. If you’re only ever going to use the shot in a small size you can probably get away with a higher ISO.

Keep in mind that it’s only when you shoot in a manual or semi-manual mode that you need to change ISO. When you’re in auto mode or one of your camera’s preset modes it will select the lowest one that it can for you.

A Word Of Warning on Changing ISO

Also keep in mind that if you change ISO that you will need to get in the habit of checking what setting is selected at the start of every photo shoot.

Many photographers have been disappointed at the end of a shoot to find that they’d forgotten to check what ISO setting they’d left their camera on in their last shoot. There’s nothing worse than thinking you’re shooting at an ISO of 100 only to find you forgot to switch it back from 1600.

To help with this always check your ISO setting before starting to shoot – but also try to always switch it back after a shoot. 

Well, that’s it for the ISO. Next lesson – the Aperture.