A great video that explains the relationship between Aperture, ISO and Shutter speed. Once you understand this relationship its much easier to dial in the look you want and get great photos! A little long winded but stay with it as it is very informative.
As nice description below
All digital cameras when faced with dark areas try to artificially add brightness. Some do it better than others. To see what I mean take your iPhone into a dark room and it will get all grainy.
In a DSLR you increase your brightness by using a couple of settings but raising the ISO is the most common way. ISO’s above 800 are considered fairly high in the Video World.
I recently upgraded from the T2i to the 70D. I wanted to see if there was any significant improvement in noise levels at high ISO’s. I did a quick non-scientific test of both cameras of ISO’s 800, 1250, 1600, 2500 and 3200. I would never use the T2i over 1600 but with the 70D I might just be able to. Take a look at the video and see what you think.
You will want to view the video in full 1080P and on an iPad retina or desktop computer. It will be hard to see any difference on an iPhone.