ISO Numbers

It is challenging to take good pictures without a good understanding of how ISO works and what it does. Camera ISO is one of the three pillars of photography (the other two being Aperture and Shutter Speed) and every photographer should thoroughly understand it, to get the most out of their equipment. 

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.

The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera. 

The component within your camera that can change sensitivity is called “image sensor” or simply “sensor”. 

Darker situations, indoor shots or sports shots called for the higher speed films. The same number now refers to the the sensors on digital cameras, which are measured in the same numbers. 

The lower the number, the less sensitive, the higher the number, the more sensitive the camera's sensor will be. 

ISO is one of three determining factors of the exposure of a photo, along with aperture and shutter speed. 

These two affect the lens and exposure time respectively, with the ISO affecting the sensor (or film). 

To be more specific, the ISO determines how well exposed a photo will be by changing the sensitivity.

1) What is ISO?

In very basic terms, ISO is the level of sensitivity of your camera to available light. The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera. The component within your camera that can change sensitivity is called “image sensor” or simply “sensor”. It is the most important (and most expensive) part of a camera and it is responsible for gathering light and transforming it into an image. With increased sensitivity, your camera sensor can capture images in low-light environments without having to use a flash. But higher sensitivity comes at an expense – it adds grain or “noise” to the pictures.

Take a look at the following picture:

he difference is clear – the image on the right hand side at ISO 3200 has a lot more noise in it, than the one on the left at ISO 200.

Every camera has something called “Base ISO”, which is typically the lowest ISO number of the sensor that can produce the highest image quality, without adding noise to the picture. 

Typically, ISO numbers start from 100-200 (Base ISO) and increment in value in geometric progression (power of two). So, the ISO sequence is: 100, 200, 400, 800, 1600, 3200, 6400 and etc. The important thing to understand, is that each step between the numbers effectively doubles the sensitivity of the sensor. 

So, ISO 200 is twice more sensitive than ISO 100, while ISO 400 is twice more sensitive than ISO 200. This makes ISO 400 four times more sensitive to light than ISO 100, and ISO 1600 sixteen times more sensitive to light than ISO 100, so on and so forth. 

What does it mean when a sensor is sixteen times more sensitive to light? It means that it needs sixteen times less time to capture an image!

ISO Speed Example:
ISO 100 – 1 second
ISO 200 – 1/2 of a second
ISO 400 – 1/4 of a second
ISO 800 – 1/8 of a second
ISO 1600 – 1/15 of a second
ISO 3200 – 1/30 of a second

In the above ISO Speed Example, if your camera sensor needed exactly 1 second to capture a scene at ISO 100, simply by switching to ISO 3200, you can capture the same scene at 1/30th of a second! That can mean a world of difference in photography, since it can help to avoid camera shake or motion blur.

When to Use Low ISO

You should always try to stick to the lowest ISO (base ISO) of your camera, which is typically ISO 100 or 200, whenever possible. When there is plenty of light, you should use the lowest ISO to retain the most amount of detail and to have the highest image quality. 

There are some cases where you might want to use low ISO in dim or dark environments – for example, if you have your camera mounted on a tripod or sitting on a flat, non-moving surface. In that case, bear in mind that your camera will most likely need more time to capture the scene and anything that is moving is probably going to look like a ghost: 

When to Increase ISO

You should increase the ISO when there is not enough light for the camera to be able to quickly capture an image. 

Anytime you shoot indoors without a flash,  set your ISO to a higher number to be able to capture the moment without introducing blur to the image. 

Another case where you might want to increase ISO is when you need to get ultra-fast shots, like the bird picture I posted above. Before increasing the ISO though, you should first decide if it is OK for you to introduce some noise to the image. 

Remember, the bigger the ISO number, the more noise you will see in your images.

On many of the newer cameras, there is a setting for “Auto ISO“, which works great in low-light environments. The beauty of this setting, is that you can set maximum ISO to a certain limit, so when ISO is automatically increased based on the amount of ambient light, it does not cross the set limit. So, if you want to limit the amount of grain in your pictures, you typically set maximum ISO to something like 800 or 1600 on most entry-level cameras and push that number even higher on higher-end full-frame cameras.