ND Filters and motion blur

colonelcamp

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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
 
Look at it this way:

Your camera needs a slower shutter speed when there isn't enough light. The slower the shutter speed, the more light makes it into the camera. Inversly, the faster the shutter speed, theres that much less getting in.

Now when you're looking to freeze motion, you want a FAST shutter speed. When looking to make fluid motion, i.e make a waterfall look dreamy and smooth, you're looking to slow your shutter speed down.

On a bright day, no matter what your camera has ample light. Using the ND filters adds a darkening effect to the overall picture. This means that even on a bright sunny day, you can limit the light making it into the camera, effictively slowing down the shutter speed to the desired length.

The filters don't directly add motion blur. They help decrease the light coming into the camera to give you more control of shutter speeds.

You mentioned in your post you want to freeze the water? If that's the case you won't use a ND filter but rather a Gradual ND filter. You won't get the blown out skies (dark area of the filter over the sky, clear area over the water), but motion of the water will be frozen.
 
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?

Using the exact same settings -1/500sec @ f/9- you will still freeze motion whether or not you are using an ND filter. The only difference is that the photograph will be underexposed (uniformly dark), because the ND filter is preventing sufficient light from entering the camera in such a short amount of time.

I'm not really sure if your goal is to blur motion or to freeze motion. If your goal is to blur the water (I presume that's why you would bother with an ND filter, at all), then you need to lengthen your shutter speed in order to allow the camera to gather enough light to make a proper exposure. Incidentally, this means that motion will be blurred, since your camera's shutter will be opened for a longer amount of time while motion is occurring.

ND filters are just dark filters that hold back light. So, for example, if you were using aperture-priority mode, you might see 1/500 sec @ f/9 for a given scene. When you put the ND filter in front of the lens, you might see it drop to, say, 1/50 sec @ f/9. It'll take a longer amount of time to gather the same amount of light.

The blurring is, to some extent, the whole point of using the ND filter... but it only has that effect because it reduces the light entering a camera, thus requiring that the shutter stay open longer while motion occurs.

**** EDIT

Or will it just darken the parts of the scene the filter is covering

Ohhhh... I missed this... If you're positioning the filter to "cover parts of the scene", then you're using a Grad ND, not an ND. Changes everything, really.

In the case that you're using a Grad ND to cover the sky and leaving the water to come through the clear portion of the Grad filter, then leaving your manual exposure at 1/500 @ f/9 will freeze motion just the same as in the normal exposure, but darken the sky.
 
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Thanks for the responses guys. I got it now. A friend was telling me that the filter itself was causing the blur.

Thanks again for clearing this up.

Danny
 

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