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Old Sep 27, 2011, 11:58 AM   #1
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Default Can an ND filter affect DOF?

This might sound like a really stupid question, but here goes.

I just picked up a Tiffen .6 ND filter, mostly to play around with blur-motion photography, waterfalls and the like. I've read that ND filters can also be used to achieve shallow DOF by reducing the amount of light entering the camera and allowing wider aperture openings, but I don't see how that's possible. In aperture-priority mode you can open the aperture as wide as it'll go with or without an ND filter and the camera will simply select a matching shutter speed to avoid over-exposure. Since shutter speed has no effect whatever on DOF, I don't see how this would work. Can someone end my confusion?
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Old Sep 27, 2011, 1:08 PM   #2
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It's not that the filter affects DOF directly. What it does, is allow you to use a wider aperture and get the same shutter speed. Assume that a scene is 'properly' exposed at f4 ISO 100 and 1/4000.

If you use a 1 stop ND filter, you can use f2.8 instead of f4 and keep ISO at 100 and shutter at 1/4000. The filter itself did not cause the shallower DOF, but it allowed a wider aperture and that wider aperture resulted in shallower DOF.

Now, if your lens is only capable of f4 - the ND filter won't allow you to open up to f2.8. In that instance, you would have to change shutter speed or ISO to compensate for the loss of light.
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Old Sep 27, 2011, 8:47 PM   #3
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Another way to look at it, is when you want to get the softening effect of a longer shutter opening, you are letting a lot more light in, which would require a small aperture for proper exposure. The ND filter will let you use a wider aperture in those situations where there is lots of light, thereby providing a shallower DOF.

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Old Oct 6, 2011, 12:07 AM   #4
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Thanks, I get it now...I think...
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Old Oct 7, 2011, 7:40 PM   #5
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Just think of it this way.......the ND filter only globally allows less light to reach the sensor. Thats all.
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