How to Understand ISO on Your Digital Camera

Are you the type of photographer who shoots in Manual mode? Or do you go to the other extreme and use one of your camera’s fully automatic exposure modes, such as Program? If you tend to go the fully automatic route then it’s quite possible you’ve never paid much attention to your camera’s exposure settings – ISO, aperture and shutter speed.

Aperture, you probably already know, controls the depth of field. Shutter speed affects the way moving subjects are recorded by the camera. But what about ISO? ISO is a remarkable setting in that it enables you to take photos in any scene from bright sunlight to candle light. It’s thanks to ISO that your digital camera is so versatile.

What is ISO?

In simple terms, ISO is a measure of the sensitivity of the camera’s sensor to light. The lowest ISO setting of most digital cameras is 50, 100 or 200. At this setting, the camera’s sensor is least sensitive to light. At higher settings, like 3200 or 6400, the sensor is more sensitive to light.

Note: yes it’s more complex than that – this is the simple explanation for those who are new to this setting so they can understand it better.

Where does ISO come from?

The letters ISO stand for International Standards Organization (more correctly known as the International Organization for Standards). The International Organization for Standards lays out the criteria that camera manufacturers use to calibrate the ISO settings on their cameras.

Having a universal standard is important when photographers use light meters. For example, if a studio photographer sets up the lights and uses a flash meter to work out that the required exposure is, say, f/11 at ISO 100, then it’s important to know that these settings work for any camera.

In practice, there are often variations in the accuracy of ISO settings between different camera models. But for the most part, these are minor and nothing to worry about.

How to use ISO

ISO is part of the exposure triangle. It works with shutter speed and aperture to (hopefully!) give you a good exposure for the ambient light level of your scene. One of the benefits of digital cameras is that ISO is a variable that you can change from shot to shot if necessary.

The advantage is that you can use your digital camera in just about any lighting situation. When light levels are low, you have the option of raising the ISO, in addition to using a larger aperture or a longer shutter speed, to help you obtain a good exposure.

But you need to be aware that raising the ISO has a side effect – it increases the amount of noise in your photos, especially in the darkest tones. This is not the problem that it was 10 years ago as modern sensors are very capable (amazingly so) at high ISO settings. But you do need to be aware of it.

I used a low ISO of 50 for this landscape photo to help obtain a slow shutter speed (to blur the water) and for optimum image quality.

Avoiding Auto ISO

If you use Program, Aperture Priority, Shutter Priority, or Manual exposure modes on your camera you can set the ISO yourself rather than let the camera decide what it should be. I encourage you to do this as it makes you think about the relationship between ISO and image quality.

Full-frame versus crop sensor cameras

Generally speaking, digital cameras with full-frame sensors create images with less noise at any given ISO setting than crop sensor cameras (that is, the sensors in APS-C and Micro Four-thirds cameras).

But, as ISO performance has increased, the gap between full-frame and crop sensorhas narrowed. Image quality (noise) is not the only reason why you might buy a full-frame camera instead of a crop sensor one, but it’s no longer the major consideration it once was.

For example, the photos taken at high ISO with my newer Fujifilm X-T1 camera (APS-C sensor) easily match the quality from my older EOS 5D Mark II full-frame camera. The high ISO performance of modern crop sensor cameras is more than good enough for most photographers.