ISO is an acronym for International Standards Organization. Basically in photography, ISO stands for the sensitivity of film or camera sensors to light. ISO word came in picture from earlier photography, just like aperture and shutter speed. In those days, when digitalisation of photography was yet to happen, ISO value was used to indicate sensitivity of film to light. Different types of films were being used, each one to serve specific purpose. Each film type was produced with specific ISO value. You must have seen photo film rolls with ISO values as ISO-100, 400, 800. More is the ISO value, more is the film sensitive to light.
After digitalisation, films were replaced by digital sensors. But yet to measure the sensitivity of sensor to light, ISO is still used as a standard. One advantage we got in digital era is that we can vary and adjust this sensitivity, i.e. multiple ISO values on single sensor, unlike traditional films with fixed ISO value.
What is ISO?
ISO is related to sensitivity of light, which means more is the ISO value, more is the exposure value. Now one thing we should understand that change in ISO value is not an optical phenomenon. Here camera sensor adjusts the sensitivity digitally. When light is incident on camera sensor, the processed image is enhanced according to specified value than original. Thus higher is the ISO, brighter will be the image.
But to gain this enhancement, sensor has to add pixels digitally, which reflects as “digital noise” or “digital grains“. While capturing images in low light, sometimes you must have seen lots of grains in your image. This noise is due to the high ISO value, as sensor has to add random pixels and colours to brighten the image. Now the question is which ISO value should we use? Answer for this is related to your camera sensor size, light, scene, equipments you are using and mainly your subject or theme.
Effect of ISO on images and its use
ISO value adjust the exposure as well as digital noise. Now if Lower is the value, lower will be digital enhancement by sensor. Thus images captured in low ISO value, like ISO-100, looks much original, smooth and rich coloured. while on high ISO value, images have digital noise, roughness, random colour noise. Thus to avoid digital noise, generally ISO is kept in range of 100-400 or 800. But keep in mind that digital noise is not always bad. It can also be used artistically to create some exceptional images.
Sensor size also effects the ISO value. Larger is the sensor size, more will be the sensitivity of sensor towards light. Thus full frame sensors (35mm sensors) are much affective even at high value like ISO 1600 and can create much smoother images. While cropped sensors are less effective at such high value. High digital noise is visible for cropped sensors at high ISO. Digital noise is worst nightmare for small sensor cameras, like digicams or prosumers. Colour noise and digital noise effects them at most.
How to choose desired ISO value?
While choosing the ISO value, you must consider various factors like light, your position, your theme and equipments. In many scenes, use of artificial light (like flash) is not permitted, for example indoor games or concerts. Then you must increase the ISO as much as you can to get correct exposure. Sometimes you need to capture much frequently, or the subject is in motion, like while you are travelling. At that time, higher ISO value is a fair option. If you want to decrease the ISO value you can use tripods to stabilize your camera, or flash to create artificial light. But always remember that if you changes the ISO value, other two values must also be changed if you wish to keep the exposure same.
It is always better to keep ISO as minimum as possible so that images will be much clear and smoother. But sometimes digital noise is also helpful. In fact, many times photographers add digital noise externally to create some artistic results. So it should be your vision and imagination that should decide ISO value. Creativity always leads towards something magnificent.
So up to this stage, you’ve learned about all the three kings of exposure. Now it is up to you how long you try to defeat them because these rulers are invincible! More you try to learn about them more paths you’ll see in front of you but keep in mind, photography is a journey not a destination. As much you travel, more will be your achievements. Always keep on experimenting. Try to capture images on various shutter speeds, then view the world with different apertures, create something new with different ISO. And once you get your hands on the exposure, I bet you will see a whole new world! Just keep on travelling, and one day you will find a road less travelled, your own path!