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This is the third installment of my mini-series on filters.  In case you missed it, Part I introduced the series and covered protective filters; Part II discussed polarizing filters.  In this article, I’m going to cover neutral density filters—both the solid and graduated varieties.

What Is Neutral Density?

A neutral density filter—a high quality one, anyway—is a colorless filter that holds back all wavelengths of visible light equally.  Solid neutral density filters screw on to the end of your lens and, if you look through the viewfinder after attaching one, you’ll note that the entire scene appears darker.  Exactly how much darker depends on the strength of the filter (I’ll discuss some of the options below).

Solid Neutral Density Filters

Why would anyone want to use a solid neutral density filter?  They can be very useful if you want to slow down your shutter speed for long exposures.  It may seem counterintuitive, but there are times when you want to use longer exposures than are possible in given lighting conditions.  There are times when you are at the smallest aperture that your lens allows and have the lowest ISO setting that your camera can handle and you still can’t slow the shutter speed to the level that you want.  Common examples of such circumstances include waterfalls and cascade photography.  Many people also like to photograph extremely long exposures at the shore, to render surf with a “misty” look.  There are certainly other occasions when a long shutter speed is desirable.

The measurable ability of a neutral density filter is expressed in exposure values (EVs) and just about any strength is available, from 1/3 EV to 10 EVs or more, and many increments in between.  It’s common for an EV to be expressed in decimal terms:

0.3 = 1 EV

0.6 = 2 EV

0.9 = 3 EV

1.2 = 4 EV

1.5 = 5 EV

…and so on.

The filters can be stacked (as always with stacking filters, watch for vignetting, particularly on wider angle lenses); if you have, for instance, a 0.6 and a 0.9 filter, you have the ability to slow your shutter speed by 2 EVs (0.6 alone), 3 EVs (0.9 alone) and 5 EVs (the 0.6 and 0.9 stacked together).

Tip:  A polarizing filter, when fully implemented, can function much as a 2 EV (0.6) neutral density filter would.

Variable Neutral Density Filters

A wrinkle to the fixed solid neutral density filter is a variable neutral density filter.  These filters are adjustable (they can twisted, much as one would a polarizing filter) and are often advertised as allowing anywhere from 2 EVs to 8 EVs of neutral density to be dialed in with a single filter.  This, of course, sounds phenomenally attractive, and in many respects it is.  But there is a downside:  the price.  Singh-Ray makes the most highly regarded variable neutral density filter on the market and it costs approximately $340 US ($390 for the thin variety).  That’s a very large amount of money for a single filter (77mm thread).  Tiffen and Fader make significantly cheaper versions of the same product ($200 US, or thereabouts) and some people swear by them.

Issues to Pay Attention to With Solid Neutral Density Filters

  • If you add enough neutral density, it can be difficult—bordering on impossible—to see through the viewfinder to focus (and autofocus will be out of the question).  The way to deal with this is to pre-focus; set focus before putting the filter on the lens.
  • Keep in mind that color shifting can occur with filters that provide very large amounts of neutral density.  It’s recommended that you read reviews of such filters (usually in excess of 6 EVs).
  • As with all filters, watch out for vignetting.
  • It may be easier to meter the scene without the filter and then compensate manually for adding the relevant amount of neutral density.

Graduated Neutral Density Filters

Simply put, a graduated neutral density filter (GND) has the neutral density effect applied to only part of the filter.  GNDs are used when a distinct part of the image frame (the sky for instance) is significantly brighter than the rest of the frame—when the dynamic range of the scene exceeds the capability of the camera’s image sensor.  The filter, then, is placed in front of the lens in such a manner that the treated part of the filter approximately covers the bright area of the scene, thereby balancing the exposure.  GNDs come in several forms:  soft, hard and reverse.  GNDs are square or rectangular filters that fit in a filter holder that attaches to the end of your lens.  The filter slides up and down in the filter holder and the holder itself can be rotated clockwise or counterclockwise.  The filters can, in the case of an experienced user, be handheld as well.

Hard GNDs

A hard GND has an abrupt transition between the neutral density part of the filter and the untreated part.  This kind of filter is best used, naturally enough, when the transition area in the scene is sharp and dramatic.  An example would be photographing a bright sky at the seaside.

Soft GNDs

A soft GND has a gradual transition from the neutral density treated part of the filter and the untreated part, so the area near the transition line isn’t as dark as the area near the top of the filter.  The linked picture demonstrates the effect.  Soft GNDs are best used for images with predictable, but somewhat uneven, transition lines in the image, such as a sky above a mountain range.

Reverse GNDs

The reverse GND has a very specialized use.  It is designed to be used to photograph sunrises and sunsets when the sun is in the sky to deal with the fact that the area right at the horizon is significantly brighter than the sky above and the ground below it.  As a result, the largest amount of neutral density is near the middle of the filter.  The area below the middle is untreated and the area above the middle gradually transitions to a lesser amount of neutral density.

Alternatives to GNDs

One of the negatives to all kinds of GNDs is that they don’t deal very well with irregular areas of brighter and darker regions.  Consider the example of a bright sky pierced by tree branches.  Any attempt to use a GND to deal with such a situation will likely result in a visible filter line on the final image.  With digital photography, using manual blends of multiple images, processed in Photoshop (or another image processing program), it is possible to simulate or improve upon the use of graduated filters.  Some photographers prefer to use filters, others prefer to blend multiple images or use high dynamic range (HDR) imaging to deal with scenes that exceed their camera’s dynamic range.

Thursday Tips is written by Kerry Mark Leibowitz, a guest blogger on 1001 Scribbles, and appears every other Thursday.  To read more of his thoughts on photography, please visit his blog:  Lightscapes Nature Photography.