Understanding Exposure

ISO is the measurement of how sensitive a camera is to light. A higher ISO is used in darker settings to make the camera more responsive to light, while lower settings are used in light conditions. The lower the ISO the better as high ISO numbers lead to a grainier image.

Below is an experiment conducted, one inside and one outside, using varying ISO levels on my Canon 550D.

IMAGE 1: Outside - ISO 100 - shows a clear image free from "digital noise"

IMAGE 2: Outside - ISO 200 - not quite as clear as ISO100, but still an acceptably clear image

IMAGE 3: Outside - ISO 400 - when magnified, image shows signs of graininess

IMAGE 4: Inside - ISO 800 - similar to images 1 - 2 as inside was slightly darker than outside

IMAGE 5: Inside - ISO 1600 - more digital noise beginning to show

IMAGE 6: Inside - ISO 3200 - more noise again. Beginning to see loss of detail when magnified

IMAGE 7: Inside - ISO 6400 - much more loss of detail and evidence of noise than previous images

Findings:
Images shot at a lower ISO proved to be clearer with more detail available on magnification. Image 7 showed graininess and digital noise and altogether a harder image to work with post-production. 

Images 1-2 and image 4 were relatively similar having adapted to the darker scene indoors and captured a clear image while compensating for lack of light by using an ISO of 800.