In Cam (DSLR) settings for better shots

Dark frame subtraction basically gets a "dark" (closed shutter) shot of equivalent exposure time AFTER your mains shot is taken.

Then the camera would compare the hot pixels on the dark frame and on the main shot - if the same "not dark" pixel is present on both the main exposure and on the dark frame, the camera will know it's noise, and not part of the picture, and remove it.

For example, on a night landscape, if you have a red blinking light on the top of a skyscraper, the camera won't know if this is a hot pixel turned red, or a a red light.

It will take the "dark" frame, and compare - if this red light is not present on the dark frame but is present on the main picture - the camera will know it's part of the picture, not an artifact.

It works pretty well most of the time.

Unless something has changed in the menus since the E-510 - NR is dark frame subtraction, and it will help for long exposure shots, be it night or day with neutral filter.

It will not kick in on short exposures anyway, usually it kicks in only on exposures of more than a few seconds.