I've noticed that the footage looks quite grainy even when using ISOs of 100, 160 and 200. Now I can't pinpoint what is causing it. I've tested both with and without ML, doesn't look like it's that. A few ideas that I thought could be causing it - lack of lighting, a fault in camera is causing it or my monitor is boosting the gain.

Here are a few stills I saved from Premiere Pro.
ML ISO 640 - 50mm at 1.8
All sizes | 550D Still - Auditions for "The Interview" | Flickr - Photo Sharing! (http://www.flickr.com/photos/matthomasvideo/5551424754/sizes/l/)
ML ISO 160 - 50mm at 1.8
All sizes | Quick 550D Test Still | Flickr - Photo Sharing! (http://www.flickr.com/photos/matthomasvideo/5550867677/sizes/l/)
No ML ISO 100 - 50mm at 1.8
All sizes | 550D Still 100 ISO Test | Flickr - Photo Sharing! (http://www.flickr.com/photos/matthomasvideo/5551601344/sizes/l/in/photostream/)
No ML ISO 200 - 50mm 1.8
All sizes | 550D Still 200 ISO Test | Flickr - Photo Sharing! (http://www.flickr.com/photos/matthomasvideo/5551601738/sizes/l/)

Thanks for reading,

Natural amount of grain for the settings and amount of light? Is it because of the Flat picture style that I used?

Matt.

It seems that all you did between these shots is to change the ISO setting.

It doesn't work like that.

It's the right exposure you are after and not the ISO rating.

If you need to compare ISO performance you need to nail the exposure with the values (shutter speed/aperture) adjusted to fit the ISO setting.

i.e If the ambient light measures for example 1/50th @ f4 @ ISO400 to test performance @ ISO1600 you will need to adjust your exposure and use 1/50th @ f8 @ ISO1600, to test ISO performance @ ISO100 you will need 1/50th @ f2 and so on.

Noise depends on the amount of light that hits the sensor and if you don't change that (physicaly by opening up your lens or using a slower shutter speed) the results are going to be the same.
Noise, in a three stop underexposed picture, shot @ ISO100 is equal to a properly exposed picture, shot @ ISO800

So even if I wanted something underexposed and lighted the scene so only half a persons face was fully lit so that there's a lot of shadow on the other side of his face, even if I use a low iso there will still be noise in the underexposed area of his face? Or a unlit black background with only the subject in the foreground lit, same situation?

Thanks for explaining.

In a high contrast scene were your shaddows are 4 or 5 stops darker than your highlights, yes you will be seeing noise in the shaddows (if you can even see anything in the shaddows with that much of a brightness difference) even at ISO100.

Right, thanks, it's good to learn something new and I'm glad it's not something wrong with my camera.