What exactly is ISO?

Whenever I sit down with a new student, we spend some time together exploring what they know and what they don’t know.

There are a series of questions I ask which help me determine the extent of their knowledge as well as their experience and what camera and gear they use. Experience is wildly variable; some come as complete beginners, others as long-serving amateurs, others still, as semi-professionals, earning a modest income in their spare time at weekends.

Whilst a number will have a clear understanding of certain basics of photography, such as the function of the camera’s aperture and shutter, their real grasp of what ISO is all about, often remains vague.

There’s usually a comment about ISO making the picture brighter or some reference to the fact that the higher the ISO the grainier the picture gets… but few seem to genuinely understand ISO.

What is ISO?

A few years back I had to present a series of talks about the three key elements of the exposure triangle (aperture, shutter speed and ISO). To be honest, I felt pretty confident about the first two.  I could talk to ages about how they work and show lots of images to illustrate their effect. 

But ISO seemed less tangible, it was a bit of a mystery. I knew what it did, but it’s origins were a bit vague, lost in time, stuck somewhere between the my experience of film speed and the complexities of digital camera technology.

What the hell is ISO anyway, and what does ‘ISO’ stand for? 

The initials come from the Geneva based organisation called the International Oganization for Standardization. 

ISO sets global standards for just about everything on earth, covering almost every industry: from technology, to food safety, to agriculture and healthcare.  

ISO 9001 logo


You may have come across ISO 9001, which sets standards for how businesses are run.


And of course, there is an ISO standard for the camera industry too: ISO 12232:2006. 

ISO websiteThis is the little fella that looks after our digital sensors and when you adjust the ISO setting on your camera, you are adjusting the sensor’s sensitivity in accordance with ISO’s guidelines. 

This is great, because whether you own a Canon or a Fuji, you can be sure that your camera’s manufacturer has abided by the same set of rules. ISO on one camera, will work the same way as ISO on any other.

So having discovered what the ISO initials stand for, what does it actually do? Let’s look at the microphone analogy

The Microphone Analogy

Imaging you’re on stage, singing into a microphone. 

Bowie and microphoneOut in the audience there is a sound engineer sitting at a mixing desk, controlling the volume of your microphone. You sing too loud or too close and he lower’s the volume, you sing too quiet, and he pushes the volume up.

Now, assume you stood back some distance from the microphone.

If you stand too far away, the microphone won’t pick you up (usually stage microphones are designed to be held close to the mouth), so the sound engineer now has his work cut out. 

He pushes the volume control higher and higher in hope of picking up your voice. 

In the real world, we’d start to hear feedback, that God-awful screeching sound that does our ears in when microphones and volumes start interacting. But let’s assume that technology has beaten that and on this occasion there is no feedback.

Eventually, if the volume control is able to go high enough, the audience will begin to hear your voice. But that is not all they will hear. In the background, behind the voice, they will also hear the audible hiss of background noise. Digital noise caused by the over amplification of the microphone’s signal.

Hiss, lots of hiss.

Turning up your camera’s ISO is a bit like turning up the volume control of this microphone. If you turn it up too far, you will begin to notice noise too… but not audible noise. The noise from high ISO is digital noise in the form of a grainy-looking image. It can look very much like the grainy images we used to get back in the days of film, but it is caused by the over-amplification of the signal coming off the camera’s sensor.

So while the camera’s aperture and shutter speed control the light coming into the camera by natural means, the ISO artificially amplifies (or brightens) the picture. 

That, for me, is why I personally do all I can to get the correct exposure by working with the aperture and shutter speed first… and ISO last. If I can get the best exposure without affecting picture quality I will. 

Other photographers may have a different approach.

I know one street photographer who shoots everything at ISO 6400. He loves the gritty look he gets from all that ISO noise – and his pictures are spectacularly distinctive, very reminiscent of Ilford’s Delta 3200 black and white film stock. 

In the end, remember that photography is an art form and there are few (if any) fixed, intransigent rules. If you like clean images, keep your ISO settings low. If you like your photos to have that urban look with bags of gritty atmosphere – crank up your ISO dial as much as you like.

Experiment – always be willing to experiment.

Kevin Ahronson 

Learn Photography

If you’re looking for photography workshops, photography courses or photography lessons, check out our website at hampshirephotoschool.com or call Tuesday – Friday (11am-5pm) 01252 643143

Kevin Ahronson

Kevin is a full-time professional photographer and has been teaching photography since 2009. He founded the Hampshire School of Photography where he runs photography workshops and gives one to one mentoring to photographers at all levels, from complete beginners through to those who want to turn professional

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