Neutral density filters (ND filters) can be an extremely useful tool for achieving proper exposure under challenging lighting conditions. In this post, I'm going to introduce ND filters and explain what they're for and how to use them.
What is a neutral density (ND) filter?
Essentially, an ND filter allows less light to pass through the lens in a given amount of time. Ideally, this should be accomplished in a uniform way, without affecting color. One or more ND filters can be placed on, or in front of, a lens. To understand how an ND filter can be useful, we have to first understand how the light "pipeline" works when we make an exposure. I'll break it down into steps:
1) The aperture setting determines how much light passes through the lens in a given unit of time. Aperture also affects depth of field (DOF).
2) The shutter speed setting determines the duration of the exposure; the amount of time light coming in through the lens is allowed to reach the film or sensor. This can also affect the appearance of motion or blur.
3) A film's speed/sensitivity (ISO rating) or a digital camera's ISO setting determines how quickly the light reaching the film or sensor is recorded.
Reducing the amount of light coming into the camera before the beginning of this process is what an ND filter does. It's just one more tool to control the amount of light reaching and affecting the film or image sensor in a given unit of time. Another way to think about what an ND filter does: it's like lowering the intensity off all light coming in through the lens; like turning down or dimming the lights. It effectively makes ambient light lower, and reduces the power of flash/strobes.
ND Filter Numbers and Stops
There are several densities available and their numbers correlate to a reduction in light measured in stops. I have a kit of Tiffen ND filters that fit three of my lenses (58mm filter size):
- ND .6 = 2 stops light reduction
- ND .9 = 3 stops light reduction
- ND 1.2 = 4 stops light reduction
These filters can be stacked (screwed on) for a combined effect. For example, you can combine the .6 and the .9 filters to reduce the light by 5 stops. Use all three and you get 9 stops of reduction. Some ND filters are variable. Instead of stacking filters, you can simply turn a ring to increase or decrease density. The reviews on variable filters are mixed, so I prefer straight filters.
Why would you use an ND filter?
You would want to use an ND filter when your camera settings, lens aperture selection, and/or shooting situation limit what you can do because there is too much light to work with to achieve the desired effect. For example:
- ND Filters for Shallow DOF in Ambient Light. For ambient light situations, if you are using a DSLR, you'll likely have no use for an ND filter to help achieve shallow DOF. This is because DSLRs are generally capable of very fast shutters speeds. Combined with a slower ISO, even at wide apertures like f/2.8, an adjustment to a fast enough shutter speed is all that's needed to reduce the amount of light reaching the sensor (see flower image below). Of course, some film cameras have limitations that might make an ND filter necessary to achieve shallow DOF in bright ambient light, especially if a camera is loaded with fast film and isn't capable of very fast shutter speeds.
- ND Filters for Shallow DOF with Studio Lighting. In studio setups where flash is used, aperture and ISO are your tools for controlling the amount of light affecting the sensor. Since shutter speed isn't a light reduction tool where flash is concerned, ND filters can come in handy when you are looking for a very shallow DOF. For example, let's say that you have your lighting setup a certain way to create the effect you want for a portrait, but even at your lighting's lowest power settings, the shots will be overexposed at the desired f-stop (i.e. f/2.8) even when using your camera's lowest/slowest ISO setting. What do you do? Slap one or more ND filters over the lens to effectively reduce the light's output (or camera's ISO, depending on how you want to think about it) in line with the number of stops you'd otherwise be overexposing by. If your setup will result in a 3-stop overexposure, just put an ND .9 filter on the lens. Problem solved. See the portrait image of Cat below and the description of how I applied this technique in the studio.
- ND Filters for Longer Exposures in Ambient Light. Want to capture motion or get that smooth, fluffy water effect, you need long exposures (see sequence below). The problem is, that long exposures, when there is too much ambient light can result in over-exposures. Sure, you can stop down your aperture to its lowest setting (i.e. f/22) and reduce the ISO to its lowest setting, but that might still not be enough to make up for the duration of the exposure (shutter speed) you need to use to get the effect you want. If your desired shutter speed of say, 60 seconds, will result in an overexposure by 9 stops, just use an ND 2.7 filter (9 stop reduction), or stack the right combination of filters in your kit to get the same reduction amount. In my case, stacking the three filters I have will give me the 9 stops reduction needed in this case.
As you can see from the example shots sequence (waterfall). As the density increases, it might cause the camera's internal meter to give inaccurate results. This is especially evident in the 9-stop reduction which was underexposed. After that, I switched to manual mode and continued increasing the duration of the shutter speed until I got the result I wanted which didn't happen until I approached a 60-second exposure. Some of the other images also showed unexpected exposure results, so I would suggest bracketing and/or testing until you get the exposure you're looking for.
ND filters are a great tool if you need a uniform reduction in light coming through the lens. If you haven't tried them, you might want to check the filter sizes of your favorite lenses and pick up a set of ND filters to fit. If you do decide to invest in one or more, make sure not to skimp on quality. This is glass that sits between a good lens and your subjects so there's no reason to use something that will noticeably degrade your images.