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Neutral density filters are used to control exposure with mirror-lenses catadioptric optics, since the use of a traditional iris diaphragm increases the ratio of the central obstruction found in those systems leading to poor performance.
This slim ND1000 filter can effectively weaken the light. Its ultra thin frame is to help avoid the vignetting on super wide angle lens.
It can be used to get following exact functions:
Blurring water motion (e.g. waterfalls, rivers, oceans).
Reducing depth of field in very bright light (i.e. daylight).
When using a flash on a camera with a focal-plane shutter exposure time is limited to the maximum speed -often 1/250th of a second, at best- at which the entire film or sensor is exposed to light at one instant. Without an ND filter this can result in the need to use f8 or higher.
Using a wider aperture to stay below the diffraction limit.
Reduce the visibility of moving objects.
Fit SLR camera / digital camera with lens of 77mm filter diameter.