Long Exposures – Using ND Filters

Overview

With the exception of only a few genres in photography, most photographers at some stage will purchase a Neutral Density Filter (ND). There’s probably not one Landscape Photographer on the planet that doesn’t own one if not several of these filters. As they are mostly used in conjunction with a CPL and Graduated ND filter for landscapes, photographers will usually invest in the Square Filter system. This filter certainly should be near the top of your list of “must-have” creative filters. They are available in a variety of densities in both circular and square formats. The square ND filters will require a filter holder to use them.

So, what is a Neutral Density Filter?

Neutral Density Filters (ND) reduce light from reaching your sensor/film. In essence, it is the same as putting a dark piece of glass in front of the lens. The filters can range anywhere from 1 stop to 20 stops reduction in light. Probably the most common strengths are the 6-stop and 10-stop filters but many photographers also favour the flexibility of a circular Variable ND Filter which will allow multiple strengths in one filter. The filters can be referred to in 3 different ways. The list below refers to a 10-stop filter –

1. By the amount of light reduction in stops eg 10 stops
2. By the optical density of the filter eg 3.0
3. By the filter factor eg. ND1000

Besides the amount of light they reduce in stops, these filters also can be referred to by the optical density of the filter eg. ND 3.0. This would reduce about 10 stops of light. To calculate the reduction of light of any Neutral Density Filter simply multiply the optical density by 10 and then divide by 3 to obtain the f-stop reduction in light. For example, a 3.0 filter is 3.0 x 10 = 30 / 3 = 10 stops and a 0.9 filter is 0.9 x 10 = 9 /3 = 3 stops.

Most photographers use these filters to create a smooth water effect or to blur moving clouds in an image to give it mood and emotion although it also has other benefits.

In the middle of the day, you will sometimes find that the light levels when shooting are too bright to allow you to shoot at slow shutter speeds even when stopped down to the smallest aperture (usually F22) and the ISO is set to its lowest (usually 50 or 100). Although the shutter speed will be reduced dramatically it usually will not be enough to get the effect you want especially when shooting the following –

• Waterfalls – where you want that dreamy blur look of the water cascading over the waterfall.
• Seascapes – to create the misty look and soft flat look in seascape shots.
• Clouds – for images that you want blurred/streaky clouds.
• Reduce the visibility of moving objects – For example, removing people from shots in busy thoroughfares.

In many cases, the use of an ND filter will extend your exposure beyond what you can handhold so you obviously will need to use a tripod and a shutter remote. Also, most cameras only have minimum shutter speeds of 30sec which means you will need to shoot in BULB mode. One of the easiest ways to calculate the exposure beyond your camera’s minimum shutter speed is to simply take a light reading before applying the ND Filter and calculate the exposure using a phone app (search for ND Calculator in the app store). You will need to only enter the strength of the ND Filter you are using and it will give you the exposure in sec/min. Alternatively, you can always try to calculate manually in your head by the strength of the filter eg if your shutter speed without the filter is 1/60th then 10 stops would be 1/30th, 1/15th, 1/8th, 1/4, 1/2, 1sec, 2sec, 4sec, 8sec, 16sec. Your exposure after mounting the ND Filter would be 16sec.  After your first shot, you could also check the in-camera histogram to make sure you have a good exposure and adjust your shutter speed based on the result.

There are quite a few brands on the market but beware as you can get some weird effects with some brands therefore we recommend that you stick with the well-known brands such as Hoya, B+W, Tiffen, Formatt-HiTech, Singh Ray, NiSi, *URTH , and Lee. If you live in Australia you can buy NiSi filters here.

*Use the coupon code ‘widescenes‘ to receive a 15% discount on all URTH products

The filters are also generally quite expensive so make sure that you keep them clean and scratch-free. Make sure you keep it in its original plastic container or in a soft filter holder. Also note that with the increased exposure time, every spec of dust on your sensor is amplified so you may need to do a bit of cleaning of the image in your post-processing.

Above: Taken in the Bay of Fires Conservation Area in Tasmania. As soon as we saw this image we knew it would be a great shot using the URTH ND1000 filter (10 stop). We took several images as the water wasn’t always extending as far into the foreground as we wanted. In Photoshop we simply opened the images we wanted to use as layers and using masks we painted through the water in the foreground to give it more blur and make it a better image.

If you love extremely long exposures for special effects then try a filter with 15 or even 20 stops reduction in light. URTH Filters produce both Square & Circular ND filters up to 10 stops that are suitable for long exposures and produce fantastic results. Visit their site here and add ‘widescenes‘ at checkout to receive 15% off all their products.

We have released a free E-book which includes a step-by-step guide to using these filters for long exposures and a handy printable pocket guide to using the filters.

If you have any questions or comments we would love to hear from you.

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