Originally posted by PentaxScott Its kind of hidden. Page 238 indicates that the metering range is "+2 EV using the ND filter" indicating a 2-stop benefit for the filter. Ive found this to be the case in actual use too.
Its nice that Pentax added the ND filter to the non-toy lenses. I suspect though that it was done simply to add more flexibility to exposure control (since f-stop control becomes futile after f/5.6 due to diffraction) than anything else.
I believe it was included because of the limited use of aperture as you post, plus the fact that the max leaf shutter speed is only 1/2000.
If you prefer to use the leaf shutter only, and not enable the electronic shutter to take over at 1/2000 to give you additional shutter speeds up to 1/8000, then the Q becomes significantly handicapped for use outdoors in bright sunlight. The added 2 stops, allows essentially the same exposure latitude as enabling the electronic shutter using wider apertures in these conditions.
Personally, I don't have much problem with using the electronic shutter at high shutter speeds, but do have a problem with remembering to disable the ND filter when I go back inside, so my Q's internal ND filter has not been used much.
Scott