ISO Explained – Chapter 8
While starting to write this chapter I assumed this would be the most easiest of the chapters to write in this series of my Free Basics of Photography Course. I was wrong, I am still unclear whether I was clear enough to make you understand about the concept of ISO. But still here we go.
In this chapter you’ll learn about the important concept of ISO of a typical camera
So, what is ISO? A simple answer this question would be the “a measure of sensitivity of the camera sensor towards light”.
To understand the answer we have to go a little bit into history. During the film camera days different films used an emulsion which was sensitive to light. This emulsion was responsible to collect the light. The sensitivity of this emulsion towards light gave the exposure in different lighting conditions. ISO or the International Standards Organization provided a method to standardized this sensitivity of the emulsion. For instance a 100 ISO film was less sensitive to light as compared to say a 200 ISO film. This number 100 ISO/200 ISO was called the film speed.
Fast forward to the digital cameras ISO speeds were borrowed from the film days to keep things simple for photographers. ISO now is a method to provide a measure of sensitivity of the camera sensor towards light. Change in sensitivity of the sensor towards light affects the exposure setting. Higher the ISO the more sensitive is the camera sensor to light. So at 100 ISO the sensor will less sensitive to light as compared to at an ISO of 200.
How ISO Work is related to the exposure calculation?
ISO in any camera is provided as a rating in the camera. Typical ISO rating in a camera are as follows:
50 . . 100 . . 200 . . 400 . . 800 . . 1600 . . 3200 . . 6400 . . 12800 . . 25600
These increments are full stop increments in the rating. If you dont understand what a full stop of light is click here to understand the concept. In the mid to higher end cameras you can have 1/3rd stops increment. The third stops would be as follows:
50 64 80 100 125 160 200 250 320 400 500 640 800 1000 1250 1600 … an so on.
Changing the ISO acts in the same was as the change in aperture or shutter speed. So when the ISO is doubled say from 100 to 200 you’ll need to reduce the shutter speed a full stop to get the same exposure as it was at 100 ISO. So if you were getting a proper exposure at a shutter speed of 1/250 then doubling the ISO from 100 to 200 will allow you to increase the shutter speed to 1/500. Similarly, instead of changing the shutter speed you can close the aperture by one stop to get the same exposure.
The other aspect of increasing the ISO is that higher ISO’s increases the noise in the image.So as a photographer you should be aware that what is the ISO at which you can make an image with an acceptable noise.
Understanding of the ISO can impact your photography to a large extent. Besides providing a helping hand in low light conditions ISO provides the flexibility of choosing Aperture and Shutter Speed setting. And you are already aware from the respective previous chapters that how you can use aperture and shutter speed creatively.