# Thread: neutral density filters to remove moving objects

1. ## neutral density filters to remove moving objects

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?

2. ## Re: neutral density filters to remove moving objects

The right filter is what leads to an exposure long enough.
That is if you want some "ghosts" in the picture.
An alternative method is to take several pictures as people are milling around, then layer them and mask out those you don't want in photoshop.

The math goes like this if you want to lengthen exposure:
Each stop is the equivalent of doubling the exposure.
Many filters are rated by optical density which is calculated as the logarithm of the ratio of light passing through the filter.
density = - Log(in/out) so a filter that passes half the incoming light (one stop) has a density of 0.3, a two stop filter is 0.6 and so on. Each incremental 0.3 in density cuts the light an additional stop.

So if your daylight exposure were, for example 100/sec at f/16 and a 60 second exposure were desired, then it would be necessary to reduce the light by a factor of 6000, so the necessary filter density would be -log(1/6000) or 3.79. You then just add up the filter densities in the stack until you reach 3.79. Of course, you might want to use the minimum number of filters to preserve optical quality, so consider a combination of an ND 3 and an ND .7 ( or .8).
But now lets say you only have an ND 3 which reduces the light by 1/1000
so then your exposure might be 1/100 times 1000 or 10 seconds.
hope this helps.
-bob

3. ## Re: neutral density filters to remove moving objects

You will need 3 filters to cover full sun to indoor mall lighting range.
I found that a ( 0.9, 1.8, 3.0 aka 3-stops, 6-stops, 10-stops ), set will cover about everything.
The need to stack 2 of them will sometimes be required to get the correct exposure time.
This set gives you 3 stops to 16 stops with 3-stop jumps and 2 filters stacked.

Note: To stack filters they must have outer threads, most brands have filters that do
and a thinner set that do not. The trade off is size and vignetting with some lens.
The 0.9 density filter can be a thin-design and always the last one on the stack.

To stack filters and not produce vignetting you need to get a filter size that are larger than your largest lens and use adapters on each lens to that filter size. Stacking two filters is an excellent compromise to buying 6 or 7 filters to cover the same range. There is one filter company that makes a variable density 2.5 stop to 8 stop, however it is expensive and you still need to stack another filter with it to get a range that extends into full sun use. Some photographers use a polarizing filter as a replacement for the ND(0.9). That filter can serve 2 purposes or you might already have one. It is thicker and has more glass but will save you buying 3 ND filters.

4. ## Re: neutral density filters to remove moving objects

Stacking filters may lead to reflexes and ghost images (esp. of sharp contrast steps within an image, black - whaite transitions) due to double reflections between filters, so beware of that.

5. ## Re: neutral density filters to remove moving objects

With 3 ND filters, the need to stack them is