OK, so filters aren't the issue.

I just checked the exif data on #3 in the first set and exposure compensation is set to 0!

Why it exposed so dark is a bit of a mystery to be honest. and it seems to have been shot with the 35/1.8 which (from memory) usually exposes a bit on the brighter side.
ie. T stop on this lens is better than the kit lenses tend to be.


So far I'm at a bit of a loss.