ISO in Photography Explained

ISO in Photography is part of my Photography Basics series on PhotoTraces. You can find the rest of the tutorials here: Photography Basics.

Photography 101 Series: What is ISO in Photography?

Upon purchasing a DSLR for the first time, most people are confused by the array of buttons, dials and settings on the camera. They might have read discussions or articles online that mention terms like ISO, shutter speed and aperture, but have little idea of how these factors interact in their camera to produce an image. In this article, I will provide easy to understand answers to the following questions:

  • What is ISO in photography?
  • How do I use ISO on a digital camera?
  • How does ISO work with other camera settings?

What is ISO in Photography?

To understand what ISO means in the world of digital cameras, it is useful to go back in time to look at film cameras.

Digital cameras may seem quite different to film cameras because of all of the technical bells and whistles they include. But the way a camera mechanically allows the recording of light has not changed much at all in over a century. The biggest change is in the way a modern digital camera records light through the use of a digital sensor chip. By contrast, film cameras record light onto a film emulsion that photographers need to physically load into the camera.

Each roll of film has a film sensitivity rating that is expressed as a number

ISO 100, ISO 200, ISO 400, ISO 800, ISO 1600  and so on

Each number represents the sensitivity of the film to light. Lower ISO numbers mean that the film is not as sensitive to light, and higher numbers mean that the film is more sensitive to light.

For The Love of Grain

The plastic emulsion of camera film is coated in delicate light sensitive layers of silver halide crystals. Film emulsion that is rated as highly sensitive to light has layers of crystals that are coarser and larger. The larger grains allow the emulsion to record light more quickly so that higher shutter speeds can be used. Likewise, film emulsion that is rated as having a lower sensitivity to light has a finer film grain.

If you set your camera to a base ISO of 100, for example, you will find that any photos you make have more saturated colors and less noticeable noise in the image. As you increase the ISO setting, you will notice that the noise in your photos becomes more pronounced, with less detail and less saturated colors.

The idea of film sensitivity expressed as an ISO number has been directly translated to our modern digital cameras.

The base ISO of a digital sensor represents the normal operating voltage of the sensor chip. As you increase the ISO number through settings changes, you increase the voltage to the digital sensor. This makes the sensor more sensitive to light, but also increases the level of noise in the recorded photo.

For example, by changing from ISO 100 to ISO 200 we double the sensitivity of the sensor.

It also means one of two things:

  • we increase the exposure by a factor of 2, letting the sensor to record twice the amount of light.

The relationship between ISO, Shutter Speed and Exposure

ISO Shutter Speed Exsposure
ISO 100 (Base) 1 second Base Exposure
ISO 200 1 second Base Exposure x 2
ISO 400 1 second Base Exposure x 4
ISO 800 1 second Base Exposure x 8
ISO 1600 1 second Base Exposure x 16
ISO 3200 1 second Base Exposure x 32
  • or, we can record the same amount of light (equal exposure) 2 times faster.

The relationship between ISO, Shutter Speed and Exposure

ISO Shutter Speed Exsposure
ISO 100 (Base) 1 second Base Exposure
ISO 200 1/2 second Base Exposure
ISO 400 1/4 second Base Exposure
ISO 800 1/8 second Base Exposure
ISO 1600 1/16 second Base Exposure
ISO 3200 1/32 second Base Exposure

At the same time, the increase of the amount of noise in our photographs is not proportional.

Of course, it is also worth noting that noise and grain can be a positive rather than a negative in certain kinds of photography.

How to Use ISO Effectively On Your Digital Camera

If you take out your digital camera and switch it on, you will notice that the LCD screen displays the current ISO setting. On many cameras, it is possible to change the ISO number through the simultaneous operation of a function button and a command dial. You will notice that you can quickly scroll through the ISO settings in this way.

What does all of this mean for your photography in practical terms? Let’s look at a few real world examples of ISO in use.

Photographing a Beautiful Sunset

Let’s imagine that you are admiring a gorgeous sunset at the beach, you have your camera with you set to Aperture Priority and you are shooting hand-held. You’d like to record the scene, but the sun is almost below the horizon, it getting darker every minute and the shutter speed is becoming dangerously slow. This is when ISO settings come to the rescue.

By increasing your ISO value to ISO 3200, for example, you also increase the sensitivity of the sensor by a factor of five (ISO 100-200-400-800-1600-3200).  It means that by changing ISO from 100 to 3200 you will need 5 times less time to achieve the equal exposure. This also means that a higher shutter speed can be used and you can still capture an accurate exposure of the sunset scene.

What is ISO In Photography - Boosting ISO when it is dark
For example, before shooting a winter landscape I had to boost the ISO to the value of 400. It was getting darker and I was shooting hand-held without a tripod.

Photographing a Landscape on a Sunny Day

Let’s imagine that you are out on a bright and sunny day in the countryside. You stop on the side of the road and see beautiful rolling hills adorned with flowers in front of you. You have your camera with you, and it is set to Aperture Priority once more. Because there is plenty of sunlight available to light the scene, you understand intuitively that you can use an ISO setting of 200 on your DSLR camera and take advantage of a shutter speed setting that is fast enough to combat the erratic breeze that is blowing at your back. The abundance of sunlight also means that you can use a narrower aperture of f8 to ensure that everything in the scene is sharp and in focus.

What is ISO In Photography - Low ISO in a Broad Daylight
Since I was shooting in a broad daylight I did not have to worry about ISO. I kept it at 100.

Different Approaches to the Same Scene

Photography is all about recording light. The sensitivity of a digital sensor to the light is a key element in the photographic process. The examples above illustrate two different approaches of dealing with various light conditions. It is important to remember that one can approach the same scene in different ways, depending on what you want to achieve and what tools you have available to you.

For example, if the photographer had a solid tripod during that gorgeous sunset, he or she could actually have used a much lower ISO setting. By stabilizing the camera on the tripod rather than hand-holding it, it would be possible to use the base ISO setting of 100 rather than 3200.

What is ISO in Photography - Long Exposure with Low ISO Settings
Since I was using a tripod here and I wanted to achieve long exposure effect in the water, I kept ISO at 100 which allowed me to get a 4 seconds Shutter Speed.

A dark scene and low ISO would result in a slower shutter speed so that enough light could be recorded for an accurate exposure. But if the camera had been stabilized on a tripod, there would be no camera shake and vibration to deal with. The result would have been a photo with less noise, more saturated color, and greater levels of detail.

Setting ISO to Suit Your Photographic Vision

By practicing photography on a regular basis, one develops a more sophisticated understanding of the dynamic relationship between ISO, aperture and shutter speed, and how these variables combine to produce a photo.


Once you understand that the ISO in photography is an expression of the sensitivity of your camera’s digital sensor to light, you also start on the path to understanding how changing the ISO setting produces changes in both shutter speed and aperture settings.

By developing an understanding of these dynamic relationships, you will also be able to more accurately match the vision you have of your final photo with the scene in front of you and the light conditions.

  • Thank you for this simple and easy to understand explanation. It’s still kind of illogical in my head that the smaller the f-number, the wider the opening, and also why a wide opening means a blurry background. In my head, it should be the other way round. But as long as I can remember that it’s the opposite of what I think it should be, I guess I’ll be fine. But if you could come up with a rule of thumbs that makes it more logical, that would be great.

    • you can use the following mind trick: f22 means 22 arbitrary units of depth of field and sharpness and f2 means only 2 units 🙂

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