In the previous article on Understanding the Exposure Triangle, we discussed the importance of light in photography and about controlling the amount of light needed to expose the digital sensor in your camera to produce a better-exposed image.

You learned how ISO, along with the aperture and shutter speed controls how much light enters the camera to make your image. These three things work together to control lightness or darkness of a photograph

Today we’re going a bit deeper and talk specifically about Understanding the ISO Sensitivity.

Stop of light

Before we go deeper into ISO sensitivity, let’s clear out one thing – “Stop of light”.

In photography, a stop of light refers to doubling the light recorded by the camera sensor.

For the correct exposure of our image, a certain quantity of light is needed. Adding a stop of light will double the exposure and so brighten our underexposed image. And in the same way, decreasing a stop will darken our overexposed image.

So now you will be knowing how you can add or take away a stop of light and make the image properly exposed.

What is ISO Sensitivity?

You can think of ISO as the sensitivity of the digital sensor (although it is a lot more complicated than that). Digital cameras convert light that hits the image sensor into electrical signals. Those signals are used to process the image.

Higher values of ISO mean the sensor does not need to collect as much light to make a correct exposure. Low ISO values mean that the sensor will have to gather more light to make the exposure.

Here is the ISO scale. Like shutter speed, this scale is easy to understand. Doubling the ISO equates to a one-stop increase in exposure. Reducing the ISO to half leads to a reduction of the exposure by one stop.

Lower the ISO number, the more light you need to get a properly exposed image. When you need more light, you can use a slower shutter speed. So you use a low ISO range of 100 to 200 when you have plenty of bright light (like sunlight) or if you are mounting your camera on a tripod. If you need a faster shutter speed for something or you are shooting in low light, you need to use a higher ISO.

When you double your ISO setting (let’s say from 100 to 200) your camera needs only half as much light for the same exposure. If you had a shutter speed of 1/125 at 100 ISO, increasing your ISO sensitivity to 200 and changing your shutter speed to 1/250 will get the same exposure (provided you don’t change the aperture).

Depending on your camera, the lowest value is 50, 100 or 200. And for the higher one again it depends on your camera and can go from 25600 to 100000 and above.

Suggested ISO settings for various conditions:

  • ISO 100: Slow speed – Use for daylight, bright sun, beach, snow
  • ISO 200: Slow speed – Best for overcast days and in the shade
  • ISO 400: Fast speed – Good for sports photography and rainy days
  • ISO 800 and above: High speed – Used in low light, evenings, sunsets, inside with a flash

Why we need high ISO?

Often, when working in lower light, you will find yourself at a point where you are using the widest possible aperture and the slowest shutter speed you need to stop the action but still your image is getting underexposed. At this point, your only choice is to increase the ISO. The lens cannot physically open itself any wider then it’s technical specifications and we can’t sacrificing sharpness by reducing the shutter speed below one limit. Having a grainy image that shows a well-defined subject is always better than a smoother image with a subject lost in the blur.


As always, there’s a drawback of everything: increased ISOs result in increased noise and less detail.

Using a higher ISO will also cause noise in your image. Generally speaking, visible grain is usually undesirable and degrades the quality of an image. Most photographers strive to take images with the least amount of noise possible.

The size of the pixels in the camera’s digital sensor is the main factor of digital noise. A DSLR will produce better images than a point-and-shoot because the size of the pixels on a DSLR sensor is larger. And same applies to the cropped sensor to the full-frame sensor. Large pixels produce less noise than smaller pixels. Digital SLR cameras perform better at higher ISOs than compact cameras.

Fortunately, noise reduction processing built into digital cameras continues to improve allowing photographers to shoot at higher ISO’s with less noise than ever.


So today we learned about the Light stop and discussed the ISO sensitivity. Let’s keep more advanced technical stuff about ISO for next E-course and conclude this article here. If you have anything to ask or discuss feel free to share them in email or through a message on any social media.