Just testing my ND filter in my garden, focusing between the sky and the top of some buildings. Without the filter and at F11 and ISO 100, manual mode, I set it so that the exposure compensation was dead centre at M.M 0.0 and it gave me 1/250th sec shutter speed. I autofocused and then switched focus to manual and screwed the filter on. Apparently for 1/250th sec starting exposure it's meant to be 4 seconds when the filter is on. But when I put the filter on and dialled in my exposure, the exposure compensation once at M.M 0.0 was only giving me 2 seconds. How come?