What is ISO?

ISO is one of the three pillars of exposure in photography. The other two being shutter speed and aperture. As with shutter speed and aperture, ISO is another fundamental aspect of photography that really needs to be understood to get the very best out of your photographic opportunities in the situations you face.

In very simple terms, ISO is the camera sensor's sensitivity to available light. A lower ISO number relates to less sensitivity to available light. This means you'll need more light to hit the camera sensor than at higher ISO numbers, to achieve the same exposure value or brightness. The benefit of lower ISO numbers are hugely cleaner image files! You'll find next to no noise in a photo shot a a low ISO. Another benefit is the need to drag the shutter speed. At first this may seem counter-intuitive, but increasing the shutter speed can add blur to a photo to imply motion. Think of silky smooth waters... Photomappers edition 4 shows you how to achieve this. Click here to read it.

A higher ISO number relates to the camera sensor being more sensitive to available light. Higher ISO numbers will allow you to shoot handheld without a flash in darker locations and smaller apertures. The downside to higher light sensitivity is the digital noise that comes with it. Noise/grain literally looks like grains of sand all over the photo. 

  • The lowest ISO/base ISO of most cameras is around ISO 100.
  • Moving up to ISO 200 makes the camera twice as sensitive to light.
  • Doubling the ISO to 400 will again, make the sensor twice as sensitive to light.
  • Doubling ISO 400 to ISO 800 doubles the light sensitivity a further time.
  • And so on...

Based on this, we can see that ISO 100 requires twice as much light as ISO 200. You with me? However, ISO 100 to ISO 400 is 4 times as much light. If we keep it going, ISO 100 to ISO 800 is 8 times the amount of light, and ISO 100 to ISO 1600 is 16 times the amount of light.

Let's do a brief example with an exposure scenario;

ISO 100 - 1 sec

ISO 200 - 1/2 of a sec

ISO 400 - 1/4 of a sec

ISO 800 - 1/8th of a sec

ISO 1600 - 1/16th of a sec

ISO 3200 - 1/30th of a sec

ISO 6400 - 1/60th of a sec

As you can see from this example, the ISO has a huge impact on exposure times. We've gone from an exposure lasting 1 full second, all the way down to 1/60th of a second, and that can be shot handheld! That's sixty four times less than at ISO 100! Admittedly we will have introduced some noise with, but it's not the end of the world. Imagine if we had started with a faster shutter speed in the first place of 1/125th of a second... At ISO 6400, we could have a shutter speed of 1/8000th of a second! This is sure to eliminate camera shake!

ISO Noise

Below are a few examples of the noise impact of increasing the camera's ISO. The cropped photos are portions of the full sized photos. Take a closer look and see for yourselves.

When to increase ISO?

The main time to increase ISO is when there isn't enough light for your camera to capture a photo sufficiently. Meaning that the shutter speed falls below the acceptable limit to hand hold and shoot. As a rule of thumb, make sure your shutter speed is above your lens focal length. If you're shooting with an 85mm lens and your shutter speed is 1/20th, you should think about increasing your ISO. 

Generally indoor lighting conditions suck for our cameras so higher ISOs need to be used from the very start. I will personally set my ISO to 800, take a test shoot and adjust accordingly. 


While it would seem that shooting at the lowest ISO your camera can possibly shoot at is the best option, keep in mind that a noisy photo that captures the moment is better than a clean, noiseless photo that missed the moment.