What is ISO? Understanding ISO for beginners

ISO is one of the three basic pillars of photography along with the aperture and shutter speed.  It is meant to make our images brighter or darker and to freeze the image objects in motion. In cameras, ISO is provided in number range, normally starting from 100 and varies to 6400 or higher. Today cameras can keep up with a lot higher ISO.

ISO, the term associated or comes from the abbreviation “International Standard organization”. Which is an organization that standardizes thousand of products and definitions all over the world but camera ISO has nothing to relate with this organization name. actually, two film standards ASA and DIN standardized by the International Standard organization, later became a standard for digital camera photography, so the combined ASA and DIN standards are now known as ISO standards both for film and digital camera photography.

How it works:

ISO number determines the sensitivity of camera sensors towards the light. A higher ISO number represents greater sensitivity towards light resulting in a brighter image. This feature allows taking brighter images in darker situations when it is dark outside and the camera needs more light to brighten up the image.  ISO is used with a fine combination of aperture and shutter speed.

Every ISO number (also referred to as ISO speed) is double of its previous one which means double in brightness to the previous number. For example, if you are capturing an indoor or night event, and ISO speed is set to 800 that means it gives a double brighter image than ISO 400 which too gives a double brighter image than ISO 200.

Also, a higher ISO number is used to freeze the moving objects in a frame when there is a constant motion detected.

Relation with image noise:

Sticking to base ISO (Minimum ISO sensitivity at 100) will result in more precise and better quality. But as far we increase the ISO sensitivity to higher, the image will be brighter but cost with the appearance of graininess and blotchy colours of noise in the image. So, a higher ISO sensitivity is avoided where the quality of an image is concerned that decreases with an increase of ISO number.  This is why the higher ISO is constrained unless the conditions required to use it.

Noise balance and Auto ISO:

From a recent year, an option of auto ISO is provided in digital cameras that decides the ISO speed automatically by sensing the outside light conditions also concerning shutter speed and aperture. For example, an auto ISO provides an ease to photographers to auto-adjust, like if the shutter speed is too low, so for better results ISO will be automatically set to high. Normally Auto ISO setting is preferred where there is a constant change in outside light so the photographers need not adjust ISO speed again and again.

Here is some generic guidance to set ISO based on outside light:

  • Daylight: ISO 100 – 200
  • Indoors at day/Shadow: ISO 200 – 400
  • Flash Indoors: ISO 400 – 800
  • Darker Indoors: ISO 800 – 1600
  • Indoors at Night: ISO 1600 – 3200
  • Extra Low Light: ISO 3200 or 6400+


Self-questioner while using ISO:

  • light conditions?

Parameterized as above.

  • Effectiveness with grain or noise?

Can the image quality compromise on noise?

  • Tripod?

With or without tripod effects the shutter speed and so the ISO.

  • Motion?

Does moving objects need to freeze? If yes then you have to balance with Shutter speed and compromise on the ISO.

What is ISO? Understanding ISO for beginners

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