What is ISO – Dictionary Definition
ISO stands for International Standards Organization. It denotes the industry standard for the sensitivity of emulsion based camera film or digital sensor.
What is ISO – All about Fast or Slow Film
In the golden days of film you could buy fast film, meaning film with a higher ISO. If the film was more sensitive to light, it would record the image faster, meaning the shutter did not have to stay open too long. Likewise, the aperture did not have to open so wide, which creates a shallow depth of field.
Higher ISO = Noise
The same applies to a digital camera’s sensor. At low ISO, the sensor is not that sensitive or fast. A lot of light, or light for a long time, needs to fall on it for a proper exposure to occur. But at low ISO, you get the most detailed images with the least amount of image noise. Slow and steady wins the race here. Fast film/exposures at ISO 800 or higher can expose an image very quickly, but the higher the ISO (sensitivity), the more noise you will see in the image.
The image below left was taken at ISO 6400. The image on the right was taken at ISO 400. There is a lot more noise (white spots) at ISO 6400 than at ISO 400.
The Exposure Triangle
The balance between shutter speed, aperture and ISO is a proportional relationship and can be shown in the following diagram:
These three components collectively decide the total exposure of the picture and hence are referred to as the exposure triangle. In situations where we are faced with dark images and we cannot increase the aperture opening or increase the shutter speed, we are left with only one option – bump up the ISO value. Having said that, increasing the ISO value beyond a certain point will cause noise. Sometimes having noise in the picture is the creative effect a photographer tries to achieve, so in certain conditions they intentionally bump up the ISO.
Conversely, low ISO is the friend of, for example, a landscape photographer who is shooting a still scene and has a camera mounted on a tripod. In these instances, the ISO can be set extremely low (ISO 100 or lower) so the sensor can take its time to expose. The lower the ISO, the less noisy the image will be.
A Final Word about Auto mode
Your camera’s automatic mode might ruin your best effort to take a sharp photo. The problem is that auto mode does not know exactly what you are trying to achieve. Are you trying to take a portrait, do a low light shot on a tripod, or photographing a landscape? Not knowing your intentions, auto mode tries to guess what settings to use, and it often makes the wrong decision.
At some point, as you are beginning to take photography more seriously, you will want to consider taking control over the settings shown in the triangle above.
In the next article I will show you how to make a big stride forward with your photography – moving away from auto mode.