Understanding ISO in Photography: All you need to know

ISO in photography is one of the three elements of Exposure Triangle that are responsible for correctly exposing a photo. ISO stands for International Standards Organization, and it is an industry standard scale to measure sensitivity to light.

In this blog post, I am going to talk about what is ISO in photography and how you can control it to click photos with desired results. In case you want to learn more about shutter speed and aperture, read Understanding Shutter Speed and Understanding Aperture.

What is ISO in Photography?

ISO is the level of sensitivity of the image sensor inside your camera. Your camera allows you to choose ISO value ranging from 50 to 12,800 (or even higher) depending on the light conditions. ISO can affect your photo in two ways, it allows you to control the exposure and amount of grains/noise.

The lower the ISO number, the less sensitive the image sensor is to light. Thus, more light is required by the image sensor in order to properly expose the photo. On the other hand, the higher ISO number increases the sensitivity of the image sensor, therefore less light is required to properly expose the photo.

As the sensor is less sensitive when you are using lower ISO number, the possibility of grains/noise appearing in your photos is very less. As you increase the ISO number and make the sensor more sensitive to the light, the grains/noise starts appearing in your photos.

How is ISO measured in Photography?

Usually, the ISO number in a modern day camera starts from 50 or 100 and as you increase the value by rotating the wheel on your camera, the number doubles or halves with each click of the wheel. Suppose the base ISO of your camera is 100, as you move the wheel towards the right (or any other key that increases the ISO number) the ISO value increases to 200. Similarly, if you move the wheel towards left it decreases to 50.

With each step towards increasing the ISO value, the sensitivity of the sensor doubles and the exposure increases by 1-stop. With each step towards decreasing the ISO value, the sensitivity of the sensor halves and the exposure reduces by 1-stop. This is the reason the ISO sequence is such: 50, 100, 200, 400, 800, 1600 and so on.

This means that ISO 800 is two times sensitive to light than ISO 400, four times sensitive than ISO 200 and eight times sensitive than ISO 100. Similarly, ISO 800 would be two times less sensitive to light than ISO 1600, four times less sensitive than ISO 3200 eight times less sensitive than ISO 6400 and so on.

Example: Suppose the settings on your camera are as following: 1/60 sec, f/1.8 and ISO 100 and you are still getting the photo as 2-stops underexposed. As you are clicking handheld you can not reduce the shutter speed as it will introduce shape and you also can not open the aperture more as your f/1.8 lens does not allow so.

In order to increase the exposure by 2-stops, you will have to increase the ISO by 2-stops so you will have to change the ISO from 100 to 400. As you change the ISO from 100 to 400 the sensor will become more sensitive to light thus increasing the exposure by 2-stops to give you a correctly exposed photo.

what is iso in photography — Clicked at ISO 1600 using Canon 5D Mark iii

Understanding relation between ISO and Shutter Speed

Shutter Speed is the length of time for which the shutter is open to expose the image sensor. The shutter speed is denoted in seconds or fractions of a second, e.g. 2 sec, 10 sec, 1/50 sec, 1/250 sec, etc.

The ISO value and shutter speed are directly proportionate. This means that if you increase the ISO value, you will have to increase the shutter speed as well. Similarly, if you reduce the ISO value, you will have to reduce the shutter speed as well (assuming the aperture value is constant).

Suppose you are shooting at f/2.8, 1/500 sec and ISO 200, as you increase the ISO to 400 the sensor becomes two times more sensitive to light and thus the exposure increase by 1-stop. To compensate, you will have to increase the shutter speed by 1-stop from 1/500 second to 1/1000 second to correctly expose the photo.

Similarly, if you reduce the ISO value from 200 to 100, the sensor becomes two times less sensitive to light and thus the exposure decreases by 1-stop. To compensate, you will have to slow down the shutter speed by 1-stop from 1/500 second to 1/250 second to correctly expose the photo.

To help you understand this better, let’s assume that throughout the numbers listed below the aperture value is constant.

At ISO 100 you get a shutter speed of 1/50 sec

At ISO 200 you will get a shutter speed of 1/100 sec

At ISO 400 you will get a shutter speed of 1/200 sec

At ISO 800 you will get a shutter speed of 1/400 sec, and so on.

If you look at the example above, as we increase the ISO by 1-stop the shutter speed also has to be increased by 1-stop in order to get the correct exposure. This means that if you want to shoot a fast-moving subject, you can do so by increasing the ISO value. Similarly, if you want to click a longer exposure you can do so by reducing the ISO value.

Understanding relation between ISO and Aperture

Aperture is the opening of the lens’s diaphragm made up of multiple blades which allow the light to enter the camera. The amount of light that enters through the lens depends upon the opening of the aperture. The wider the aperture opening (small F number), the more will be the amount of light that enters the camera. Similarly, the narrower the aperture (high F-number), the less would be the amount of light that enters the camera.

The ISO value and aperture value are directly proportionate. This means that if you increase the ISO value, you will have to increase the aperture number as well in order to properly expose the photo. Similarly, if you reduce the ISO value, you will have to reduce the aperture value as well (assuming the shutter speed is constant).

Suppose you are shooting at f/2.8, 1/500 sec and ISO 200, as you increase the ISO to 400 the sensor becomes two times more sensitive to light and thus the exposure increase by 1-stop. To compensate, you will have to increase the aperture number by 1-stop from f/2.8 to f/4 in order to correctly expose the photo.

If you reduce the ISO value from 200 to 100, the sensor becomes two times less sensitive to light and thus the exposure decreases by 1-stop. To compensate, you will have to open the aperture opening by 1-stop from f/2.8 to f/2 to correctly expose the photo.

To help you understand this better, let’s assume that throughout the numbers listed below the shutter speed is constant.

At ISO 100 you get an aperture value of f/2.8

At ISO 200 you will get an aperture value of f/4

At ISO 400 you will get an aperture value of f/5.6

At ISO 800 you will get an aperture value of f/8, and so on.

As we increase the ISO by 1-stop the aperture value also has to be increased by 1-stop in order to get the correct exposure. So if you want to shoot a scene with deep depth of field (less blur effect), you can do so by increasing the ISO value. Similarly, if you want to click a scene with shallow depth of field (more blur effect) you can do so by reducing the ISO value.

Controlling ISO using Automatic ISO (in manual mode)

By navigating through your camera settings you can set ISO value selection to automatic, thus allowing the camera to choose the required ISO value as per the exposure. This allows you to set the aperture and shutter speed (while using the manual mode) and the ISO value is selected by the camera as per the light entering the camera.

So if you are sure about the shutter speed that you want to use and the aperture value that would give you the desired depth of field, all you have to do is click photos without worrying about the ISO value. Though if the light condition is not that good, selection of high ISO will introduce grains in your photos. To counter that you can set the ISO limit in the settings to a number that you are sure would not destroy the photo.

When to use low ISO value?

When you are photographing a scene or a subject and you are carrying external light along, try using the minimum ISO value possible on your camera. As you have the control over the light, you can adjust the intensity of light to set the desired shutter speed and aperture.
When you are shooting long exposures using a tripod, use a low ISO value as the tripod can hold your camera still for any duration of time.
If you wish to click a photo with deep depth of field, increase the aperture value instead of ISO value and keep the ISO low.

If you can think of any other situation when you can use low ISO value, comment below.

When to use high ISO value?

If you intentionally want to get the vintage grainy look in your photos, then you can increase the ISO value to introduce grains.
When you are shooting in low light conditions without a tripod and you are using the minimum possible aperture number. You can increase the ISO value in order to boost the exposure and maintain the shutter speed which does not introduce shake in photos.
In situations when you are using fast shutter speed in order to freeze a motion of a fast-moving subject and getting an underexposed photo, you can increase the ISO value to increase the sensitivity of the sensor and correctly expose the photo.

If you can think of any other situation when you can use high ISO value, comment below.