What Is ISO in Photography?

ISO, also known as ASA for film photography, has been a foundational term since the inception of photography. Sitting alongside shutter speed and aperture as the three pillars of photography, the ASA film speed number or ISO settings determine the sensitivity and the amount of light captured in a single image. While its meaning and methods of use have changed slightly from the transition from analogue to digital photography, understanding how to use ISO is still an essential skill for any photographer, professional or not. To help you understand what ISO is, we’ll break down its function, alongside our ISO settings guide for shooting in the dark or low light conditions.

Before we begin, let's break down the difference between ASA Film Speed and ISO Settings. Standing for American Standards Association, ASA is an arbitrary rating of film speed that determines the sensitivity of the film to light. It’s basically a measurement for how good a particular roll of film is at gathering light. The higher the number, the higher the sensitivity.

ISO, on the other hand, stands for the International Organization for Standardization. These guys set international standards for all different kinds of measurements. For DSLR cameras, the ISO Setting refers to the camera’s sensitivity to light. The major difference between the two is that when shooting film, the ASA is fixed to your chosen film. Nowadays with digital cameras, ISO can be electronically adjusted to brighten or darken your image.

Understanding what ISO settings you can use in different conditions will help you take properly exposed photographs that don’t only look great, but require less editing after the fact. While every digital camera is different, here is a general guideline for when to use specific ISO settings.

Keep in mind that the higher the ISO, the more noise you will gain. Although several modern day DSLRs and Mirrorless cameras can go beyond ISO 3200, your image can vary drastically at higher ISO settings.

Understanding how ISO works in conjunction with shutter speed and aperture, collectively known as the exposure triangle, is a crucial component in making creative choices as a photographer. If you want to use a faster shutter speed to shoot sports or action, you will know to use a slightly higher ISO setting to make this possible. 

Alternatively, if you want to open up the aperture of your lens to capture portraits with beautiful bokeh, try limiting the light with a lower ISO setting. Although certain ISO settings are more suitable for a particular condition, they must be combined with Shutter Speed and Aperture settings to properly expose your photos.

Shooting indoors requires a wider aperture and a slower shutter speed to increase the available light, which can result in blurry photos. Increase your ISO to avoid blurry images. Another great way to avoid camera shake is to mount your camera to a tripod if you’re unable to use a higher ISO setting or want to keep your image as noise-free as possible.

Under direct sunlight, images can often be overexposed if your camera settings aren’t adjusted properly. Using a faster shutter speed alongside a low ISO setting like 100 will help you to avoid blown out images. Alternatively, you may need to use a Neutral Density lens filter to reduce the amount of light entering your lens and use slower shutter speeds without over-exposing your image.

Increased sensitivity to light may allow photographers to take photos after the sun goes down, it’s not without its drawbacks. Similar to how a bigger net can catch more fish, a higher ISO settings result in your camera capturing more light, however, with that comes digital noise, which can negatively affect the picture’s overall quality. Fortunately, this can be avoided with more premium cameras like full-frame Mirrorless or DSLR cameras, due to the increased size and quality of the sensor. However, if you’re not able to splurge on more expensive photography gear, a good rule of thumb is to gradually increase your ISO overtime until you find a sweet spot. It’s also best to avoid extremely high ISO settings unless you’re unable to adjust your shutter speed and aperture. 

Those new to photography should start out at around ISO 400 as this is considered an excellent general-purpose setting for most conditions by professional photographers.