If you are interested in long exposure photography then you really need to understand what a neutral density filter is and how it can be used to your advantage when creating images. As a long exposure photographer myself, I find that neutral density filters are integral to my workflow. The ability to condense longer periods of time into a single image and record what the eye cannot see can yield magical results. So how do we do this? – this post should cover all you need to know about neutral density filters.
What is a Neutral Density Filter?
A neutral density (ND) filter is a darkened piece of glass that cuts back on the amount of light that can pass through the lens onto the sensor. The filter is not supposed to affect anything else such as the colour or hue of the light passing through it.
They come in different strengths depending on the amount of light that you want to block from the camera. These different strengths are measured in stops of light. The most common filters are 2 stop, 6 stops, 10stops and 16 stops. Please see below for more explanation of these measurements and how they affect shutter speeds.
Filters also come as circular or square/rectangular shapes. There are pros and cons for each:
- Simply screw onto the thread of your lens so easy to apply
- Small and easy to carry
- Less chance of light leaking onto the lens from poorly fitting filters
- Each filter is a fixed size for a specific lens so if your lenses are all different diameters then you will require different filters (or step-up/down rings) to be able to use them on other lenses.
- Stacking multiple filters can result in vignetting where the corners of your images are darkened.
- Difficult to focus with the filter attached and it is possible to affect pre-focused lens when attaching them
- Quick and easy to attach to the front of the lens
- Easy to focus before attaching the filter
- As they are one size (usually 100mm) they will fit over most lenses so multiple filters will not be required.
- Stacking filters will not lead to vignetting
- They require a filter holder and different attachment rings for each lens diameter.
- They are bulky and heavier to carry around
- The filters can be more prone to scratching or breaking due to their size
- More prone to light leak if not set correctly
How to Calculate shutter speeds with ND filters
Once you have metered your scene and determined the correct exposure, it is necessary to make a calculation to factor in the strength of the filter you will attach. Please check my earlier tutorials to read how to correctly establish exposure.
Whilst it is easy to use one of the many available long exposure apps that can calculate these for you, it is good to know how to do this just in case you do not have access to them at a particular location.
Quite simply, a two stop ND filter will reduce the amount of light by two times. So what does this actually mean? Each time the light is reduced by a stop, it is halved. If the shutter speed required without the filter is 1/250thsecond then one stop light reduction would result in a speed of 1/125th– namely half of the 1/250. A second stop of light (as we are using a 2-stop ND filter in this example) would result in a shutter speed of 1/60thof a second (half of the 1/125). So 1/60thwith a 2 stop ND filter is the same as 1/250thwithout the filter.
Depending on the strength of the filter you are using, you simply continue halving the shutter speed; a 10 stop filter requires you to do the calculations 10 times to find out the correct shutter speed.
What are Graduated ND Filters?
Just a quick mention here about graduated ND filters (ND Grads). This is a square/rectangular filter where half of it is clear and gets progressively darker through the other half. These are used to help darken down skies and balance out an exposure where the sky will be brighter than the remainder of the scene.
There are soft and hard edge ND Grads – as the names suggest – the difference between the clear and darkened sections are gradual or with a harder linear edge.
The idea is that they can be moved in a filter holder to the exact place in the frame where the light is brighter so for example a hard edge ND grad could be placed on the horizon when taking a seascape.
Potential problems using ND Filters
It goes without saying that long shutter times require the camera remains fixed firmly in position and a strong stable tripod is essential.
It is very difficult for the camera to focus automatically through an ND filter. It is unlikely you will be able to see clearly through the viewfinder either when the filter is attached.
The best way to deal with this is to focus before attaching the filter to the lens. Just remember to turn off auto-focus if you have used this before applying the filter. Alternatively, it can be possible to use live-view mode to manually focus the camera when the filter is attached. This depends on the brightness of the scene and the strength of the filter.
Long Shutter Times
Bear in mind that your camera will only allow for a 30 second exposure in manual mode. If your resulting shutter speed is over this, then you will need to use bulb mode and a remote shutter release to control the shutter to the required time beyond 30 seconds.
Change in Exposure Values
Bear in mind that once you have metered your scene, the light levels can change and so try to act efficiently when you are taking your image so that you do not end up with an over or under exposed image. This can be an issue when taking images at sunrise/set as the light can dramatically vary at these times.
Light leak is where some ambient light can enter into the camera and affect the final captured image. This can be where the filter is not attached to lens correctly (more prevalent when using square filters) or through the viewfinder. I recommend covering the viewfinder when taking long exposures.
ND filters as their name suggests are supposed to be neutral and not affect the colour or hue of the light. However you will sometimes find that depending on the brand and strength of the filters used that the resulting image will have a strong magenta or green hue.
Shooting in RAW can enable this to be fixed, although this can be difficult and frustrating when using high quality lenses and for longer shutter durations. However, I personally recommend using the excellent Hoya PROND filters which are definitely the best from my experience of filters in keeping colour cast under control.
Neutral density filters can be used to achieve a wide array of inspirational effects. They can take an average mundane scene and turn it into something captivating and spectacular.
Although some scenes will not be improved by using longer exposures, in the right settings the filters can make such a dramatic improvement to an image. Choosing the right filter and determining the right conditions can open up a whole new element of creativity where it comes to your photographs.
I hope that you find this article useful. For more detailed information on ND filters and all other aspects of long exposure photography, please do check out my new book – Mastering Long Exposure Photography – A Definitive Guide, published by Ammonite Press and available at all good bookstores and online. https://www.ammonitepress.com/photography/photography-techniques/mastering-long-exposure/
I also have an earlier tutorial on long exposure photography that you can read here.
Naturally please also feel free to contact me if you have any questions or comments relating to this topic or any other photography question. Please do pass this post onto any photographer friends that you think may find this useful