Steven is right, but has maybe not explained it in the best way. (Keith, if you actually read those articles on equivalence you so dislike, maybe you'd understand
)
I think the clearest way of looking at the change from full-frame to M4/3 is this:
- field-of-view if halved, effectively the same as a doubling of focal length in terms of framing (2x crop factor).
- Depth-of-field increases by the equivalent of two stops, ie, f/5.6 on M4/3 delivers same DoF as f/2.8 on FF calculated as 2x crop factor (when the subject is framed the same from the same distance with focal length adjusted).
- Exposure/aperture remains the same in terms of light per square mm received on the sensor, but since the sensor is a quarter the area of FF there are 4x less photons gathered. What happens in effect - and this is the point in question - is that the sensor is working at two stops higher ISO to compensate, eg at ISO1600 rather than ISO400 on FF. Hence the extra noise, less dynamic range, reduced low light performance etc.