This short post will take you through what an ND filter is and how to use a neutral density filter to achieve some amazing results with your own photography.
What does a neutral density filter do?
Graduated ND filters only reduce light across a certain amount of the frame, which can be adjusted by sliding the filter up or down in your filter holder, so if you have a really bright sky, you can adjust the ND filter to cover just the sky and expose correctly for your foreground. While this is possible to bracket your exposure and combine them later in photoshop, a graduated ND filter allows you to capture the entire scene in a single shot and save yourself the time in photoshop.
It is also possible to purchase variable neutral density filters, which – as the name suggests – can vary the amount of light stopped, usually over a 1-10 stop range. They have the convenience of only having to carry around one filter with you, but are generally more expensive and can produce a noticeable colour shift. This is because they use two polarising filters which, when turned, reduce the amount of light entering the lens.
There are many different companies out there selling neutral density filters and, as you can probably guess, they all use a slightly different scale to determine how they grade their filters and how much light they prevent from hitting your sensor. Hopefully, this handy table will help you make sense of it all.
|F-Stop Reduction||Optical Density||Filter Factor||% transmittance|
|10||3.0||1024 (sometimes called ND1000)||0.09765625|
How to use a neutral density filter for your photography
For example, say you are shooting an image of a river and your settings, ignoring your iso as it should usually always be at 100, are f11 with a shutter speed of 1/125. Obviously you’re not going to get much movement in the water at that shutter speed and the image will look boring. However, if you choose a 10 stop ND filter you can use exactly the same aperture but set your shutter speed to 8 seconds and get the same exposure, but the movement from the water will create an amazing fog-like effect as if recorded over time.
There are a number of great apps out there which can help you do this out in the field, such as this one.
As well as being great for landscapes, there are a number of other uses for a neutral density filter, especially when taking portraits in bright sunlight and when using flash.
For example, if you want to take a portrait in the sunshine and your settings are f8 at 1/2000 of a second but you want to use a wider aperture to blur the background more, the simplest way to do this is to use an ND filter which reduces the amount of light hitting your sensor by 4 stops. This will allow you to keep the same shutter speed but use a much wider aperture of f1.8.
This also works great with flash where you are often limited to a shutter sync speed of 1/250. Assuming you don’t want to use a HSS flash system, then you can simply attach an ND filter to your camera to reduce the exposure to match your shutter speed of 1/250. This way, you can expose for your background and then use your flash to make the subject pop.
Here is a quick example below, the original exposure was iso 100, f8 and 1/250th of a second. By putting on a 3 stop ND filter the background becomes nicely blurred by being able to use an aperture of f2.8, and the subject has clearly been made to pop from the background by using an off camera flash.