ND Filters and Motion Blur

colonelcamp · Aug 17, 2010

Hi Everyone,

I hope the experts in this forum can clear this up for me.

Assuming AE mode:

Do ND Filters, of any stop, directly cause motion blur?

or

Do ND Filters let in less light which causes the shutter to automatically (again, in AE mode) slow down producing motion blur?

Let me pose the question a different way:

Hypothetically, If I am shooting in manual mode and I have the shutter set to 1/500 and the aperture set to F9, again hypothetically, and I take a picture of waves crashing on to the beach, I will effectively freeze the motion of the water.

Using the same exact settings, if I add an ND Filter, of any stop, will this give me any motion blur at all? Or will it just darken the parts of the scene the filter is covering but still freeze the water?

My understanding is that ND filters themselves don't directly cause motion blur, but when used (in AE mode) do cause it indirectly since the amount of light is lessened causing shutter to slow down to let in more light and the motion to be blurred.

Any help would be greatly appreciated.

Thanks!
Danny

timlair · Aug 17, 2010

The ND filter allows you to slow down the shutter speed from 1/500 to *insert what you want here* hypothetically. As long as there still isnt too much light. An ND Filter restricts the amount of light let into the lens. Its almost like an aid to the Aperture ultimately allowing for slower shutter speeds in daylight. Which will result in motion blur. Thats my understanding anyways.

So to answer your question, it will allow for motion blur if used correctly. With a longer shutter speed the more light allowed in will counteract with the ND filter and you will get your blur but also have the brighter light.

OrionsByte · Aug 17, 2010

Motion blur is a function of shutter speed, period.

Basically, if anything has moved between the time the shutter opened (front curtain) and the time the shutter closed (rear curtain), it's going to appear blurred. Fast shutter speeds freeze the action because it doesn't give the subject the opportunity to move much between curtains; show shutter speeds produce motion blur because the subject can move substantially between curtains.

ND filters simply reduce the amount of light entering the lens. If you're using an ND8 filter, for example, it reduces the light by 3 stops. That means that a shot at f/8 for 1/200 with no ND filter will give you the exact same exposure as a shot at f/8 for 1/25 with an ND filter. The difference in shutter speed, however, is going to increase the chances and strength of motion blur.