neutral density filters to remove moving objects

[empkae]

I would like to understand how to use a very dark neutral density filter to remove moving objects. Example: A picture of a building where there are people milling about. If the filter is dark enough, and the people move enough, and the exposure is long enough, then most of the moving people will be reduced to phantom blurs, or gone entirely, leaving the building nice and sharp.

My problem is figuring out the math to get the right filter to do this.

And, is it possible to stack a couple of filters, one 1/3 as dark as needed above, and one 2/3 as dark as needed above, and get the same effect? Reason, to have some means to accommodate more levels of ambient light, from cloudy to bright sun. If this can be done, what are the "right" filters to use?

[Mad Aussie]

You'll find the right ND filters to use will depend entirely on the scenes brightness and dynamic range. This would vary quite a bit I would think.
You can stack the ND filters but depending on your focal length you will have to be away of vignetting from the filters.
I've never heard of any particular formula/math to tell you what/how many ND filters to use though.

[AntZ]

I'm guessing here, but I would think you work out what time you need to get the desired result to blur the moving objects, then what your desired DOF and therefore apperture needs to be. You can then adjust the filter darkness and ISO to get the desired result.

I have shot of my bother chasing a ball taken late one afternoon in which he is almost invisible. I had been playing with a CPL on to slow the water. It was less than 1sec exposure, but my brother was moving quite fast.

[ArtTwisted]

to completely remove all people ? Im guessing go as dark as you can and see what happens, stack if you have too. Havent tried it though.

[erwin.zeez]

to completely remove all people ? Im guessing go as dark as you can and see what happens, stack if you have too. Havent tried it though.

have no idea

[Greg_Nuspel]

Biggest problem with stacking is the introduction of more layers of glass/plastic in front of your lens. Each surface is a source of possible reflections, flare or in the case of low quality filters distortion.