Aperture, Shutter speed, and ISO form the basic building blocks in photography. If you want to be a good photographer, then you should be able to control these 3 parameters according to the available lighting. Understanding ISO sensitivity is essential to get the correct image exposure. I will explain everything about ISO in detail in this guide.

Let’s see, what ISO sensitivity is all about and what values need to be set to get the perfect shot.

What is ISO?

It stands for International Organization for Standardization. ISO is related to the camera sensor, and ISO sensitivity is a measure of image sensors sensitivity to light.

The sensor is an important part of the camera and is responsible for capturing the light falling onto it and transforming it into an image. Lower the ISO value; lesser will be the sensor’s sensitivity to light and vice versa.

The commonly used ISO values are 50, 100, 200, 400, and 800. Nowadays, there are cameras which support ISO values up to 1024000.

When to Use High ISO?

High ISO values are generally used in two conditions.

Low light conditions

If you are shooting at night, you won’t have sufficient light to capture the details in the image. In such situations, you can go for a high ISO setting. If you are doing astrophotography like capturing milkyways or star trails, then you need to use high ISO values. If you are photographing nocturnal animals or birds, high ISO settings are required.

To Freeze Motion

Also, a high ISO can help to freeze motion. For example, wildlife and sports photographers use high ISO values as they use a higher shutter speed value to freeze the action. When you choose a higher shutter speed, you will have to go for a high ISO value to get the correct image exposure.

An ISO value of 1600 was used for photographing waterfalls at night. The low light condition as well as the fast motion of the water made me select a  high ISO value to freeze the movement of the water.

ISO in the range of 800 and above is considered a high ISO value.

When to use Low ISO?

You can use low ISO values in normal lighting conditions, like outdoor shooting in daytime or indoor shooting with adequate lighting. If you are using flash as the fill light then you can go for low ISO values.

Now the question will be what are the low ISO values? Any value of ISO between 50 and 800 (both inclusive) is considered as a low ISO value. For good image quality, it is always better to select a lower ISO value.

In the above image, an ISO value of 100 was used. Low ISO value was used since the lighting was good.

ISO Vs Noise

ISO and noise are closely related. The higher the value of ISO, higher the level of noise in the image. This noise will be visible in the form of filmy grains.

An important factor that affects the amount of noise is the pixel size of the camera sensor. The larger the size of the pixel, the lesser the noise. The camera pixel size will be larger in a full-frame camera than in a crop camera.

It is the reason why the noise response of a full-frame camera is better than a crop body camera. So, when you select the same high ISO level, say 1600, you can notice very less noise in the image captured using the full-frame camera than the crop camera.

The above picture was taken at night at a high ISO of 1600

Picture shown below is a 100% crop of the above image.

You can see the effect of higher ISO of 1600 visible in the form of noisy grains in the picture. so, in order to check the noise level, you will have to zoom the image to 100%.

Now, the question is; Why to go for a high ISO value if that increase the level of noise? In certain conditions like bad lighting, sports photography and bird photography, you will not be able to capture the image perfectly if you are using a lower ISO value. So, go for a higher ISO value, capture the image in RAW format (if you have an SLR camera) and then use some image editing tools to remove some amount of noise from the image. Tools like Topaz DeNoise, Noise Ninja are good at removing Noise.

In most of the DSLRs, there will be an Auto ISO option. If you select this option, then your camera will automatically select an ISO value based on camera metering. But, I will not recommend you to use this automated feature because by doing so you will never learn to set the ISO depending on the lighting situation. 

NOTE: Higher the ISO, Higher the Noise

Now let us see how the change in ISO values affects the quality of the image with the help of some examples. Below given shots are taken using Canon EOS 7D with Canon EF 50mm f/1.8 lens. The ISO sensitivity value is varied from 100 to 6400.

From the above pictures, it is clear that when we increase the ISO sensitivity the amount of noise in the image increase, which is more visible in the 100% crop of the image as shown in the top right corner of the above images. It is not always possible to shoot at a lower ISO value, especially when the lighting condition is not so good.

Use High ISO values in following situations: 

Case 1: Poor Lighting condition.

For example, if you are shooting indoor without much lighting or shooting at dawn or at night, outdoor under shade etc. Night photography demands a higher ISO value if you are shooting handheld without flash and a tripod.


Case 2: Subject in the frame is in motion and the lighting is not good.

Consider the example of a bird in flight in low light. Here the subject is in motion. There is not enough light to set a fast shutter speed to freeze the bird. So, we will have to bump ISO value to get the correct shutter speed.


Case 3: Shooting Handheld with smaller Aperture Opening.

If you are shooting handheld with a higher f-number (smaller aperture number) then you will have to go for a higher ISO value. This is because smaller aperture opening demands a slower shutter speed. Slower shutter speeds are difficult to shoot handheld. So, you will have to increase the ISO value, which in turn will help in increasing the shutter speed. In such situations, you can go for a higher ISO setting.


ISO, Aperture and Shutter speed

ISO, Aperture and Shutter speed are all inter-related. Changing any one of the above values will definitely have an impact on the other. If you increase the ISO value, then you can go for a smaller aperture opening (higher F number) and higher shutter speed.

Consider the situation wherein you have to bring all the details in the scene. This calls for a smaller aperture opening. If you have a tripod, then you don’t have to go for a high ISO value. But, if you don’t have one then you will have to go for a higher ISO value. Also, when you are photographing birds in flight, you will have to set a shutter speed of 1/1000th of a second and above, which demands a higher ISO value.

Suppose, if you capture an image at a shutter speed of ½ seconds with an ISO value of 100, then you can capture the same image with a shutter speed of 1/4th of a second if you increase the ISO to 200. Always try to use an ISO value under 800 for noise reduction. So, there should always be a balance amongst these three parameters. Only then you will be able to capture the perfect shot.

After reading this article on “Understanding ISO Sensitivity” you would have found the answer to the question, what is ISO? Now, I recommend you to Read Understanding Aperture & Understanding Shutter Speed. These 3 are the 3 pillars of photography.

Do Share Your Comments & Reviews!!!