This is part of a series of articles dealing with the fundamental technical aspects of using a DSLR or mirrorless camera.

In previous articles, we discussed the basics of Shutter Speed and Aperture in photography. In this article, we’ll talk about ISO, the third peg of the Exposure Triangle.

ISO is what’s commonly known as film speed – a relic from the days of film photography – which measured photographic film’s sensitivity to light. A higher film speed would mean the photographer would be able to shoot in lower light conditions with a faster shutter speed and/or smaller aperture.

In digital photography, ISO more or less does the same thing. A higher ISO means the photographer can shoot using faster shutter speeds and/or smaller apertures in lower light conditions.

But does a higher ISO really make the camera sensor more sensitive to light?

One of the main misconceptions about ISO.

An ISO of 100 for most modern digital SLR and mirrorless cameras is the base, or native, ISO. This is the measure of how sensitive the camera is to light.

Setting the ISO higher than this base simply means your image is starting at a higher base brightness (higher gain) relative to the base ISO. That’s it.

It’s equivalent to opening an image in an image editing program such as Photoshop and adjusting the image brightness upward. Only instead of you doing it – when setting the ISO upward, the camera does it when it processes the image file after taking a photo.

So no, a higher ISO doesn’t make your camera sensor more sensitive to light in the same way that higher speed film is more sensitive to light. It’s quite literally a virtual replication of this.

Trade offs: higher ISO = more image noise

An image taken at a high ISO, just as an image taken with higher speed film results in grain, is susceptible to digital image noise when not properly exposed. Image noise can be seen in the image above.

Digital image files are made up of pixels, individual squares of colour that the camera processes and creates when you take a photograph. The process of pressing the camera shutter button and processing the resulting image all happens within a split second.

When brightening an image, whether in an image editing program or on camera by increasing the ISO, the image editing program or your camera needs to “guess” what colour individual pixels (generally the darker colours) would be.

The higher the brightness means that the colour of more pixels will be “guessed”. This will result in more noticeable spots of the image being visibly off colour, or brighter, relative to the rest of the image.

This distortion is noise.

To avoid noise at higher ISO, you still need to set the Aperture and Shutter Speed correctly in order to get a correctly exposed image.

ISO usage situations

Below is a list of common ISO settings and situations they may be applied:

• 100 or base ISO – 200: use in bright, sunny conditions.
• 200 – 400: used in situations with slightly less light, such as an overcast day or outdoors in the shade.
• 400 – 800: used indoors where its well lit well or with a flash. Outdoors during dusk or dawn.
• 800 – 1600: used low light indoors or at night without a flash.
• 1600 – 3200: Used in extremely low light situations when not using a flash.

We’ve now covered ISO, along with Aperture and Shutter Speed. More articles covering other aspects of photography will be published in the coming days and weeks.