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Can't I Just Reverse a Regular Graduated ND Filter?

The use of graduated neutral density filters is an excellent way to help create more dramatic landscape images. I use them regularly and have written about them here before. One question that comes up often though is what the difference between a regular graduated ND and a reverse GND. Can't you just flip a regular one end for end? Or something?

Let's start with what a regular graduated neutral density filter is and how you use it. It's awfully bright in places, but not so much elsewhere. When trying to photograph a scene encompassing both extremes, neither digital nor film can adequately handle it in a single shot without help. This is where a graduated neutral density filter comes in.

The filter clear at one end and gray ("neutral density" in photographic techno-speak), with a smooth transition from clear to gray across the middle. Across the transition zone, the density increases going towards one and decreases towards the opposite end. Going in either direction, once the transition is complete, the density stays the same the rest of the way to that end. By positioning the darker end of the filter over the brighter portion of the scene, you can help to even out the extremes of brightness to better fit it within the range that photographic mediums can handle.

A GND filter is rectangular. To use it, you generally slide the filter into a holder that screws onto the front of your lens, positioning it in the holder such that the transition zone falls between the light and dark portions of the scene. By stealthily positioning the transition you can often hide it within the scene so that both ends of the filter have their desired effect without the transition itself showing.

But it's not always this easy. There are a couple of reasons why this sometimes doesn't always work. Sometimes there's no relatively straight area within the scene to hide the transition and for this you are better off with various digital blending techniques. Sometimes though you can line the transition up along a level horizon without any problems, but the filter still doesn't produce the results you wish it would. With the sun low on a level horizon, the brightest area will be right along that transition, with the sky getting progressively darker as you go up from there. When you position a strong enough regular graduated filter at this transition, it darkens the already dark sky so much that it often turns black. The problem is that its density increases as you go from the middle of the filter towards the darker end when what you really need in this situation is a filter that is the darkest in the middle, progressively getting less dark as you go towards the edge.

You can put a regular graduated ND filter in the holder either way around, with the dark area at the top or the bottom (or of course to either left or right sides or any other direction you so choose if the scene calls for it). No matter which direction you turn it though, it still gets progressively darker towards the dark end and progressively lighter towards the other. There is no way you can turn it or flip it such that it will be darker in the middle than at either end.

If you are clever, it may have occurred to you that it would be possible to slide the filter in only half way such that its dark end lie in the middle of the frame where the transition zone should fall, with no filter at all over the other end of the frame. As you proceeded towards the end of the frame covered by the filter, the density would actually decrease as called for since the middle of the filter would lie at that edge, with the clear end of the filter sticking up beyond that, outside the frame completely. The only problem is though that you would be shooting through the edge of the filter, something sure to be problematic since it was never designed for this. The filter edge would distort the image. It would be unavoidable.

But what if that weren't the edge of the filter? What if it had a clear part that extended beyond this point? The filter density would still do what we needed it to do for the scene with the sun low in the sky over a level horizon, but there would be no filter edge getting in the way of the optical path through the lens.

This is where a reverse graduated neutral density filter comes in. Imagine you cut off the original clear end of the regular filter that ended up uselessly hanging out in space beyond the edge of the frame and instead added a clear section on the opposite end to extend it out so you could avoid looking through the filter edge. That's what a reverse grad ND is. And its effect can't be duplicated with a regular grad ND no matter which way you turn it. Perhaps a better name for it would have been an "inside out" graduated neutral density filter since it basically consists of what you would get if you cut a regular GND in half and fastened the pieces together the other way around. What was the two outside edges now becomes the middle, and what was the cut across the middle becomes the new edges.

A regular grduated ND filter - dark at one end and light at the other
A regular grduated ND filter - dark at one end and light at the other
A regular GND flipped the other way around - that way of reversing things doesn't work, does it?
A regular GND flipped the other way around - that way of reversing things doesn't work, does it?
What you would see with a regular grduated ND inserted only half way - but now you have to shoot through the edge of the filter
What you would see with a regular grduated ND inserted only half way - but now you have to shoot through the edge of the filter
A true reverse grad ND - a reasonably abrupt transition in the middle, tapering off in density toward the edge
A true reverse grad ND - a reasonably abrupt transition in the middle, tapering off in density toward the edge

Thankfully, Daryl Benson convinced Singh Ray to make such a thing so you don't have to take a hack saw to a regular filter. Good graduated filters such as those from Singh-Ray aren't cheap, but they are worth it. The reverse grad ND is one of my favorites.

Update 07/24/2007 - I've read in various posts online that it is possible to use a two-stop and three-stop regular grad ND together to simulate the effect of a reverse grad. The theory is that by sliding one into the holder from the top and the other in from the bottom, they would overlap in the middle to create a reverse grad effect. Unfortunately, this isn't true. Even if the overlapping portion could be made darker than what was above it and below it (not easy since the regular grad filters become more dense as you go from their center toward the one edge), you would still have the awkward transition edge where it didn't belong. In practice, overlapping two filters in this way can be a useful technique for certain circumstances, but it more closely matches the effect of a strip filter rather than a reverse grad.


Date posted: July 8, 2007 (updated July 24, 2007)

 

Copyright © 2007 Bob Johnson, Earthbound Light - all rights reserved.
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Related articles:
Essential Filters: Neutral Density and Graduated ND
Cokin P-sized versus the Larger Lee-sized Graduated ND Filters
 

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