Have you seen the photos on the internet where water or clouds looks silky smooth? If you are into photography, you probably know that it’s long exposure photography. You tried to do it a few times but couldn’t get the same results as you saw on the internet. Well, you need a special piece of equipment called Neutral Density Filter or in short ND filter.
What is ND Filter?
ND filter is a piece of dark glass that we put on the front of the lens to cut the amount of light entering into the camera. It’s like sunglasses for your camera. These sunglasses don’t change the colors of the picture and that’s why we called it neutral density filters. If you are interested in long exposure photography, this is a must-have tool in your camera bag.
Why we need an ND filter?
We need ND filters in two cases. First when we want a low shutter speed in bright daylight and second when we want a shallow depth of field again in bright daylight. Let’s say you want to shoot a landscape with flowing water and you want to show the flow of water. For that, you need at least ten seconds of exposure. You set the camera at 100 ISO which is the lowest and aperture to f/16 but still, you are getting 1/50 shutter speed. What would you do?
Or let’s say you are shooting a model in bright daylight, you have a fast lens like 50 mm f/1.4 and you want a shallow depth of field. So you set ISO to 100, shutter speed to 1/8000 but still, you are getting f/4 aperture. In such cases, you need an ND filter.
Types of ND Filters:
There are two types of ND filters available in the market. One is round screw-on filters, we can screw it on the front of the lens. Second is a square filter that we need to insert into a special holder that is fixed in the front of the lens.
Each filter has pros and cons like you don’t need to buy a separate holder for round filters and they are also available in variable density from 2 stops to 9 stops. On the other hands, if you have an ultra-wide lens, these filters can cause vignette in photos.
Square filters can be stacked on the holder and allow you to get a longer exposure. These filters are also available in graduated neutral density (GND). These GND filters provide a gradual transition for a smooth blending effect and can balance the bright sky and darker foreground. So they are good for landscape photographers. But the con is that it’s very costly compared to round one.
How to calculate exposure time with a neutral density filter:
Neutral density filters come in different intensity. It mentioned in either number like ND2, ND4 ND8, or ND16 or numbers like ND 0.3, ND 0.6, ND 0.9, ND 1.2 and so on. Each increased number reduces one stop of light. Like ND 0.3 will reduce one stop of light, ND 0.6 will reduce 2 stops of light, ND 1.2 will reduce four stops of light. See the following image to know how much light each filter reduces.
To calculate exposure, do the following process. First, fix the camera on the tripod and set the frame. If you put 9 stops ND filter on your lens, you can’t see through the viewfinder, you can’t focus manually and even autofocus won’t work. So focus manually before putting up the filter and switch your camera to manual focus mode.
Now set the camera to aperture priority mode, set ISO to 100, aperture to F/11 or F/16 (for deeper depth of field) and take a shot. By doing this you’ll know the required shutter speed. Next, change the camera to manual mode, and set ISO, aperture and shutter speed to the same as last shot.
Now it’s time to calculate shutter speed required with ND filter. If you have ND 1.8 or ND64 filter, as per the above image it reduces the light by 6 stops so we need to give 6 stops more light for proper exposure. Let say your shutter speed was 1/60 seconds before putting the filter.
1 stop 1/30 second
2 stops 1/15 second
3 stops 1/8 second
4 stops 1/4 second
5 stops 1/2 second
6 stops 1 second
You need to set the one-second shutter speed. In the same example, if your ND filter is ND 3.0 or ND1024 which reduce 10 stops of light, your shutter speed would be 16 seconds.
Things to remember:
1. If you have multiple lenses, always buy bigger size filter. Like your 2 lenses have 77mm filter thread and one lens has 72mm, buy a 77mm filter and use a 72mm to 77 mm step-up ring to use with 72mm filter thread. If your 72mm filter thread lens is wide angle, you need to buy separate filter otherwise step-up ring will create a vignette.
2. Always buy good quality filters. There are cheap filters available on the marker that is made of plastic or lower quality glass. Never buy them as it’ll reduce image quality because you are putting an extra glass or plastic in front of your lens.
3. When you put the ND filter on the lens, be careful and not to rotate focus ring accidentally.
4. Always use tripod and shutter release cable because you want to avoid any camera movement during the long exposure.
5. Always shoot in RAW and on manual mode. RAW will give you more details in editing later on. Your white balance setting should be fixed too. Don’t shoot with auto white balance setting as with filter, the camera will have difficulty getting a proper white balance.
6. If you are taking a long exposure, always cover your viewfinder with something. Otherwise, the light that is entering through the viewfinder can ruin your shot.
Related Article: The Go-Getter’s Guide to Long Exposure Photography
About the Author
Ramakant Sharda is an author, iOS App publisher, passionate photographer and a MIOPS Ambassador based in the beautiful “Pink City” of India, known as Jaipur. His work has been published in various magazines, newspapers, and blogs. He has published three Coffee Table Books, he writes about photography and also teaches photography in his workshops. Check out his website http://ClickManic.com to see the masterpieces created by him or download his free app for iPhone and iPad “30 Days to an Ace Photographer“.