Aperture, Shutter speed and International Organisation for Standardization (ISO) form the basic building blocks in photography. So, if you want to be a good photographer then you should be able to control these 3 features in accordance to the available lighting.

As a prerequisite for learning about ISO, I recommend you to have a look at topic “Understanding Aperture”.

Now, let’s see what ISO sensitivity is all about and what values to be
set for a perfect shot?


What is ISO?

ISO is related to the sensor in your camera and ISO sensitivity is a measure of image sensors sensitivity to light. Sensor is an important part of your camera and is responsible for capturing the light falling on to it and transforming it to an image. Lower the ISO value, lesser will be the sensor’s sensitivity to light and vice versa. The usual values of ISO are 50, 100, 200, 400 and 800. Nowadays, there are cameras which support ISO values up to 256000.


When to use High ISO?

Higher ISO values are used if the lighting condition is poor. For
example, if you are shooting at night then you won’t be having sufficient light
to capture the details in the image. In such situations you can go for a higher
ISO setting. Also, higher value of ISO is used to grab image of objects in
motion. For example, bird photographers and sport photographers use higher
value of ISOs as they use a higher shutter speed value to freeze the motion.
Any value of ISO above 800 is considered as a High ISO value.

In the above image an ISO value of 1600 was used. This is because this shot was taken at night (not good lighting) handheld. This situation demanded for a high ISO value.


When to use Low ISO?

Low ISO values are used if the lighting conditions are normal, like outdoor shooting in daytime or indoor shooting with adequate lighting. Now the question will be what are the low ISO values? Any value of ISO between 50 and 400 (both inclusive) is considered as a low ISO values. For better image
quality, it is always better to select a lower ISO value.

In the above image an ISO value of 100 was used. Here the shot was taken during the daytime, in which case the lighting was good. So, lower ISO is needed.


ISO and Noise

ISO and noise are closely related. Higher the value of ISO, higher will be the level of noise in the image. Noise will be visible in the form of filmy grains. An important factor which affects the amount of noise is the size of the pixels used in the sensor. Larger the size of the pixel, lesser will be the noise. So, full frame sensors will have lesser noise at higher ISO settings when compared to a half frame sensor having the same ISO setting.

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. Noise Ninja is one such commonly used Photoshop plug-in to remove noise from the images.

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 for you depending on the lighting. 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 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.


Higher ISO values are mainly used in 3 situations:


Case 1: Poorlighting condition.

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


Case 2: Object in the frame is in motion and the lighting is not good.
For example, to capture birds in flight, moving vehicles, action sports, cultural events etc in low light, a higher value of ISO is used as they need quicker
shutter speed to copy the object in motion.


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 for 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.


Any value of ISO above 800 is considered as a High ISO value.




ISO, Aperture & 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, which 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 for 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. If possible, always restrict the ISO value to set below 800 (noise can be reduced in the final image). So, there should always be a balance amongst these three parameters. Only then you will be able to capture the perfect shot.