Essentially, (and originally) since the age of digital film ISO became known synonymously as film speed. We say “ISO” because the system used to measure the film’s sensitivity to light was developed by the “International Organization for Standardization.”
To understand this, you must know what film speed is.
Simply put, film speed is the measure of film’s sensitivity to light. In the digital era, we essentially tell our computer (camera) how sensitive we want it to be.(i.e. we adjust the ISO)
When you have film that is relatively insensitive (ISO100), more exposure to light is needed to produce the same image density as a more sensitive film (ISO800). This would be why insensitive film is called “slow” film. Think about it. If you change the exposure/shutter speed to stay open longer, it takes longer to capture the image.
In correspondance, highly sensitive films are called “fast films”. However, the reduction of exposure corresponding to use of higher sensitivities generally lead to reduced image quality. (Graininess, and noise). Basically the higher the filmspeed (higher ISO) the worse the photo quality.
So, What is ISO again?
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 that I borrowed from here –
the one on the left is taken at 100 ISO and the one of the right at 3200 ISO
According to DigitalPhotographySchool.com, 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 you should ask yourself 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, you want the most crisp image possible, and especially if you are willing to use a tripod on a still subject you will be safe using a pretty low ISO rating.
However if it’s dark, you don’t mind grain or even want grain, and if you aren’t using a tripod or the subject is moving, or both, use a higher ISO, so you can shoot with a faster shutter speed enable to expose the shot as good as possible. However be aware that your shots will be a bit “noisier”.
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.
Most people never take their cameras off of Auto ISO–if your one of those people that’s OK. But now at least when you purchase your next camera, you’ll know why the camera with ISO up to 3200 is going to be more expensive than ISO up to 1000! 🙂