Long exposures are a simple way to get creative with your photography. As I’ve discussed before, a good long exposure requires three elements:

• Something moving in the frame (to be blurred)
• Something stationary in the frame (to anchor the shot)
• Slow shutter speeds

The traditional ways of slowing shutter speed are to (1) use a low ISO setting on your camera and (2) to stop down the lens aperture (f/16 or smaller). However, if you want to use a long exposure in daylight conditions, those two settings won’t help you much. Consider the standard “sunny 16” exposure: 1/100s @f/16 @ISO 100. Hmm. Stop down to f/22 and you get 1/50s. Not very slow. Maybe your camera can be set to a lower ISO, say ISO 50. That will get you to 1/25s. Again, that’s slow enough to blur cars or fast-moving water slightly, but it’s a big constraint. Moreover, some DSLRs have a base ISO of 200, meaning that you are even more limited in getting a slow shutter speed.

Solid ND Filters

The solution to slowing down your shutter speed in daylight is to block light from entering the camera lens. To do this, you need a solid neutral-density filter. These filters are simple: they block light. The darker the filter, the less light enters the camera, and the longer your shutter speed will need to be for a proper exposure. There are lots of different options for neutral density filters, so let’s take a look at each kind and view their pros and cons.

Traditional Solid ND filters and Mount Types

The basic ND filter is simply a piece of dark glass or resin that mounts to your lens. These filters vary in the number of “stops” of filtration. A 2-stop ND filter reduces light by two EV, or “stops.” Your 1/100s base exposure at f/16 becomes 1/25s.

You can get solid ND filters in strengths up to 10 stops in both ring-mount (circular) and rectangular/square mounts. Ring-mount filters are easy to pack, and less likely to break in the field. They also do a good job of preventing light leaks, as they screw onto your lens. With a screw-in filter, you can usually use your lens hood. However with thick filters (or stacked filters), you risk vignetting (seeing the filter ring in the corners of the image) with wider focal lengths.

Square/rectangular filters require a holder to use. A big advantage of these filters is that you can easily stack multiple filters, including more solid NDs or graduated NDs, without vignetting. It’s also fairly easy to remove these filters in the field (important for focusing and metering) by simply removing the holder from your lens without having to unscrew anything.

Variable ND filters

Another type of solid ND filter is something called a variable ND filter. These filters are essentially two polarizing filters stacked together. The front part of the filter rotates, and you can use them to dial in a range of filtration between 2-8 stops, depending on the filter. These filters are fairly expensive, and some add in other features, like a polarizer (great for removing glare from water shots) or a color enhancer. Variable ND filters are ring-mounted and fairly thick, meaning you’ll need to be careful using them at wide focal lengths or risk vignetting. I can safely use my Singh-Ray Vari-ND and Vari-N-Duo filters at a 28mm before I see any vignetting. You need to be very careful using variable ND filters with skies. At the strongest settings, it is possible to see strange polarization artifacts in your images.

Which Filters to Choose?

As with all things in photography, both standard and variable ND filters have practical uses in the field. Variable ND filters are perfect for situations where you just need to slow the exposure down enough to get the desired motion effect. An example of this is on a stream, where you want to blur water with an exposure between 1-30 seconds. In most cases, a variable ND offers enough filtration to do the job, and if conditions change, you can adjust the filter without having to switch it for a different strength one. Another benefit of the variable filter is that you don’t have to remove it to focus your shot; you simply turn it to the weakest setting and focus before setting your final exposure.

Solid-ND filters are ideal when you really want to go slow, and you need maximum filtration. A ten-stop ND filter will turn your 1/100s exposure into a 10-second one. Stack a 5-stop filter on top of that, and you now get a 327s exposure… that’s over five minutes! Because standard ND filters have a set exposure value (stops), it makes it possible to calculate exposure times longer than 30 seconds. Simply multiply your original shutter speed by 2^y, where y is the number of stops of filtration.

Example: Base exposure is 1/100s (0.01s)

15 stops of filtration: 2^15 = 32,768

32,769 x 0.01s = 327.68s

Of course, this calculation is a pain to do in the field unless you’ve got a calculator handy, so I use the ND Timer App for iPhone in the field to make the necessary exposure calculations quickly and easily.

How many stops is enough?

Having extra filtration provides an additional degree of freedom when shooting. For example, my example above was assuming a normal “sunny 16” exposure as the baseline. I don’t have to use f/16 in my exposure, though. I could use f/8 and get a 81s exposure. The extra filtration gives me the freedom to use a range of f-stops (and control depth of field) and ISO settings as needed to get the exposure duration that works best for a particular scene.

Filter Neutrality and Other Considerations

Neutral-density filters are supposed to be color-neutral. However, once you start using very dark filters, you’ll usually start to see some color casts in your images. My Lee Big-Stopper, for example, produces a very strong blue cast that must be removed in RAW or with a custom WB setting. My Singh-Ray filters, on the other hand, produce a mild warm cast that sometimes looks fine as-is, depending on the scene.

Price is another factor. 10-stop filters are fairly expensive, especially the Singh-Ray ones. Variable ND filters are also fairly expensive, so they’re not something the casual shooter will just want to purchase on a lark. Price is another factor. 10-stop filters are fairly expensive, especially the Singh-Ray ones. Variable ND filters are also fairly expensive, so they’re not something the casual shooter will just want to purchase on a lark.

Quick Filter Comparison Chart

My Current ND Filters

• Singh-Ray Vari-ND (variable ND filter)
  I use this filter when I want to blur motion but don’t need to polarize the scene
• Singh-Ray Vari-N-Duo (variable ND filter with polarizer)
  This is my go-to filter for shooting streams and water, where I want to slow the shutter speed and reduce glare from the water and foliage.
• Singh-Ray Mor-Slo 10-stop square ND
  I use this filter for getting long exposures in daylight conditions. The square filter is ideal for wide-angle lenses and I prefer the mild warm color cast, as it’s very easy to correct.
• Singh-Ray Mor-Slo 5-stop square ND
  I use this filter in combination with my 10-stop Mor-Slo to get really long exposures (several minutes) in the field.