What is a Neutral Density Filter?
A neutral density filter (ND filter) is simply a filter that’s cuts the amount of light that enters your camera’s lens. They are most commonly used in landscape photography when a photographer wants to create a longer exposures than what would normally be possible using just the camera’s internal settings. In this article you will learn how to use a neutral density filter and how to calculate long exposures.
Imagine that you want to crate a 30 second exposure in bright sunlight. Even with the camera set to ISO 100 with an aperture of f/22 the shutter speed will still be relatively fast. The only way to get a longer exposure in this situation is to place a neutral density filter in front of the lens.
Types of Neutral Density Filters.
There are three types of ND filters. Solid ND, graduated ND, and variable ND. Graduated filters are useful when you only want to darken a portion of the image such as when the sky is brighter than the foreground. While these filters can be quite useful, a whole article could be written about their application so I will save that for a future post.
Solid and variable ND filters darken the entire image which allows the photographer to blur anything that is moving. I highly recommend purchasing solid ND filters over variable ND filters since variable filter are often more trouble than what their worth. A variable filter allows you to change the density of the filter simply by turning it. Sound too good to be true? That’s because it is. You will spend a little more money purchasing multiple solid ND filters but the quality of a solid neutral density filter will be much higher than a variable filter. Variable ND filters will often cause artifacts on wide angle lenses and have thick frames which can cause pretty terrible vignetting even without stacking filters.
Square VS. Circular ND Filters
When choosing a solid ND filter you can either choose a screw on filter or invest in a filter holder system. If you’re new to neutral density filters, you may want to start off with a screw on filter or two. In fact, I love my circular ND filters by Breakthrough Photography due to their natural color, ease of setup and low profile. A filter system can take a bit longer to setup and when the lighting is good I want to get the shot, not wrestle with my gear. Square filters also have more surface area which means more area for moisture to collect when shooting near the ocean, waterfalls, or in the rain. With that being said, good filter systems do have their advantages. They allow photographers to stack solid and graduated ND filters with little to no vignetting. Again, I prefer Breakthrough Photography.
When to use a Neutral Density Filter.
Landscape photographers use ND filters when they want to create silky smooth water. This effect works wonderfully on waterfalls, creeks, lakes and oceans. Long exposures can also be used to create dreamy streaks in a cloudy sky and can even be used to remove moving objects from a scene. If your shutter is long enough and the objects are moving fast enough, they will not even show up in the shot!
Portrait photographers will sometimes use low density ND filters when working with flashes. Since portrait photographers generally like to use a very shallow depth of field (open aperture), it may not be desirable to stop down the exposure using the aperture and if the ISO is already maxed out the next best option is to use a one or two stop neutral density filter.
How many stops should a Neutral Density Filter be?
ND filters come in a wide variety of densities that are described in stops. A light ND filter may only be one or two stops which is perfect for portrait photographers using a flash while a very dense ND might be 15 or 20 stops. So how many stops should a ND filter be? That depends on the amount of light that you’re working with and the desired results that you’re going after.
For most practical landscape uses, three, six and ten stop filters are the most common. Three stop filters are great for when you need to push the exposure just a little longer. I find three stop filters to be really useful on the far edges of day when the light is already quite low but the there’s still too much light to create the length of exposure I’m looking for.
Six stop filters are perhaps the most flexible of the solid ND filters since they are dark but not welding glass dark. Most modern cameras will still be able to focus through a six stop ND filter in decent lighting conditions without a problem. If I were to pick just one neutral density filter to carry with me, it would be a six stop filter.
Ten stop ND filters are very dark and are capable of achieving 30 second to two minute exposures in the middle of the day. These filters are useful for completely blurring moving water so it looks like a soft haze. Ten and fifteen stop filters are also very good for removing moving objects from an image as if they were never there in the first place.
Calculating Long Exposures
When working with dense filters such as ten stop NDs it may be necessary to calculate the exposure manually because the light meter may not be able to give a proper reading through the dark glass. Luckily, calculating long exposures is not as hard as it may seem. Any time the shutter speed is doubled or halved you adjust the exposure of the image by one stop. Similarly, the exposure is adjusted by one stop any time you double or halve the ISO.
For example, if I take a photo at ISO 200, for one second I can get the exact same exposure by lowering the ISO to 100 and increasing the exposure time to two seconds while leaving the aperture where is was. In this example I took away one stop in the ISO but I also added a stop by doubling the exposure time so the shot is effectively the same exposure.
Calculating aperture requires a lot more math than anyone in their right mind is ever going to do in the field. Luckily, it is still very easy to calculate the aperture’s effect on exposure because we know that all modern cameras operate in 1/3 and 1/2 stop increments. Most cameras will adjust exposure in 1/3 increments by default but many can be programed to shoot in 1/2 stop increments as well.
Check your camera’s menu to see if you’re shooting in 1/3 or 1/2 stop increments. Hint: If you have never changed the increment settings in your camera, you’re probably shooting in 1/3 stop increments. If your camera is set to 1/3 stop increments, adjusting the aperture dial by three clicks will adjust the exposure by one full stop. If you camera is set to 1/2 stop increments every two clicks will adjust the exposure by one stop.
Real World 10 stop ND Example
Now let’s look at a real world situation using a ten stop ND filter. As mentioned above, your light meter might not be able to calculate the proper exposure due to the large amount of light that is being cut by the filter. For this reason is is best to set your camera’s mode to manual. If there is a lot of light you might be able to meter through the filter but if you’re shooting on the edge of day you may have to calculate the exposure manually.
So let’s walk through how to shoot with a 10 stop ND filter in low light. First we’ll take a shot without the ND filter. For this example let’s assume that our settings without a filter are ISO 100, f/16, 1 second.
After adding the ten stop filter we have to find a way to add ten stops of light to our exposure. We can do this by adjusting the aperture, shutter speed, ISO or a combination of the three which is often the route that I end up taking. If we’re adding a ten stop filter, we probably going after a very long exposure so we’ll start off by adjusting our shutter speed.
Remember that we were shooting at one second before we added the filter. To add one stop of light we’ll adjust the exposure from one second to two seconds. Now we just have nine more stops to go! We can double the time again to four seconds and then double that to eight seconds which leaves us with seven more stops to go. Our exposure is still relatively short so let’s double it again to sixteen and then to thirty seconds. We’re half way there! (I know I shaved off two seconds but I’m going to assume that most people reading this are like me and not math wizards. It’s best to keep things as simple as possible when in the field.)
At this point we need to switch our camera to bulb mode in oder to achieve an exposure over thirty seconds. Make sure to pay attention to the aperture settings when switching from manual to bulb mode. The aperture is often set independently between manual and bulb modes. After making sure our aperture value remained at f16 we can program in our exposure using an intervalometer timer.
Let’s continue to double our exposure to one minute, then to two minutes and finally to four minutes. We have gained eight stops of exposure but we still have two more to go. We could continue to double our exposure but we’d end up at 16 minutes!!! Rather than shoot for 16 minutes, let’s add a stop by increasing the ISO form 100 to 200. Finally, let’s add our final stop by opening up the aperture from f/16 to f/11.
Apps for Calculating Exposures
If the above example has your head spinning, don’t worry. There’s are many different apps that can be used to help you calculate the proper exposure. LE Calculator is a free app that will calculate long exposures and even comes with a built in timer in case you do not have an intervalometer. PhotoPills is a paid app that has a long exposure calculator amongst many other photography planning tools. It’s a fantastic app and I highly recommend it for any landscape photographer.
Dealing with Long Exposure Noise
Long exposure times equate to added noise even at low ISO settings. As long as your exposure does not exceeded five minutes, you should have relatively low amounts of noise. Noise is created when the sensor heats up during a long exposure. If it’s cold outside, you may be able to get away with longer exposures before noticing much noise. Full frame cameras will handle ultra long exposures better than crop sensor cameras. While shooting for longer than five minutes can add noise to a shot, sometimes the results can be worth the added noise. Experiment with different exposure times and see how they effect the image. If needed, you can apply some noise reduction during post.
Dealing with changing light
When shooting on the edge of day it’s important to remember that the earth moves faster than you think. The amount of light falling on a scene can change dramatically in the matter of a few seconds, let alone a few minutes. Changing light throws one more variable in our exposure equation that can be very hard to predict. When shooting during sunrise, you will need to subtract a stop or two to compensate for the increase in light that will happen as the sun continues to add light to the scene throughout the exposure. The opposite of this is true at sunset. During sunset you will have to add a stop or two in order to compensate for the fading light.
ISO and aperture can sometimes be more effective at controlling exposure when the light is rapidly changing because they do not add or subtract any time to the exposure. Imagine that the sun is setting and your exposure is already 5 minutes. If you add a stop of light using the shutter, your shot will then be ten minutes long. The amount of light that disappears in those added five minutes will more than like be greater than the stop that you added. Therefore, a better option might be to boost the ISO or open up the aperture.
Even during the middle of the day, moving clouds can cause each long exposure shot to look quite different. It’s a good idea to try multiple shots if there are a lot of clouds in the sky. The difference a few minutes makes can be amazing.
I will often take a few shots without a filter if the light is looking good. This way I can blend the shots if the light changes or at the very least I have a few solid exposures that were taken with a fast shutter.
Watch out for Unwanted Movement.
Pay close attention to elements within the scene that could cause unwanted motion blur. Tree branches and plants rarely stay still for very long. Sometimes the blurry edges of plants can be an interesting look. Other times you may want the vegetation to stay still. If your exposure is only a few seconds long you may be able to time your exposure so it happens when the wind is not blowing. If you are attempting a very long exposure there might not be much you can do other than embrace the soft edges from the wind. Photographers who have strong Photoshop skills might be able to composite an image using one fast shot that was taken without the ND filter and one that was taken with the ND filter. This technique can prove to be quite tedious though so I will not cover it in this article.
Don’t let light leaks ruin your shots
Light is a photographer’s best friend but it can also be our worst enemy. Light leaks will surely ruin a good long exposure. The telltale sign of a light leak is a magenta cast that appears in only a portion of the image. It is always a good idea for DSLR shooters to cover the camera’s eye piece especially if the sun is behind the camera. If you fail to cover the eyepiece, light can enter the camera from the rear and cause a terrible color cast.
High endNikon DSLR’s have a a convenient eyepiece curtain that should be used whenever performing long exposures. If your camera does not have a built in eyepiece cover you may want to carry some gaffers tap that can be used to temporarily block light while you are performing a long exposure. Mirrorless shooters do not have to worry about rear light leaks since there are the view finder is electronic.
Light leaks can also occur when using filter folders. Sometime light will creep into the filter and bounce around during the shot. For this reason, I prefer screw on filters in most cases. If you notice a light leak when using a filter system, try to use gaffers tape to stop the leak.
Neutral Density Filters and Color Cast Problems
Most ND filters have at a little bit of a color cast. Some are worse than others. Breakthrough Filters tend to be the most color neutral filters that I have found and they are backed by a twenty year warranty.
Dark Polarizer Filters
In recent years, filter companies have started to make dark PL filters. A dark PL is a combination of a neutral density filter and polarizer all in one. While most all in one products are terrible, dark PL filters work amazingly well! They come in various densities and remove of the need to stack a ND and PL filter together which reduces the chance of vignetting at wider focal lengths.
Neutral Density filters are a great way to add atmosphere to a shot with moving elements. I recommend experimenting with various density filters and exposure times to gain a greater understand of how ND filters work. Notice the differences in a half second exposure and a two second exposure. Then compare those results with a 30 second exposure and a five minute exposure. Practice gauging exposures under changing lighting conditions and see if you can accurately predict the correct exposure. Watch your histogram and try to bring it to the right side of the graph.