Neutral density filters limit the amount of light entering a camera lens in order to intentionally increase exposure length or to reduce light that is too bight to be controlled by reducing the aperture. This may, at first, seem strange since from the beginning of most people’s journey into photography, the emphasis is on getting the fastest shutter speed possible to help eliminate blurry photos. So why then would you ever want to intentionally extend the amount of time the shutter remains open?
The answer is to intentionally blur photos in a controlled way. One element of this control is use of a tripod. The images I’ll be talking about involve some type of movement in one element and no movement in others. For example, clouds or water or both.
If you visit photo sites, you probably see photos of creamy white waterfalls or rivers with the same type of effect. You may see skies with soft clouds that seem blurry but not the type of blur that comes from camera shake. Photographers intentionally blur the water and/or clouds to make them look that way. The blur is visually appealing because it is blurred in one direction and everything else is sharp because the camera was on a tripod.
Notice the blur in the clouds created by their movement, while the hills in the background are still sharp. This image was created using a 10-stop neutral density filter on the front of the lens. The settings were f-stop f/20: ISO 200; exposure time 30 seconds. Thirty seconds of motion were captured in this image. The clouds were moving toward me which created a very dynamic feeling to the image. If the clouds were blurred in the horizontal direction also, they would probably just look like bigger clouds or blurry clouds. In this photo the clouds show movement.
The image above is Toketee Falls in Oregon. F-stop f/11: Exposure 60 seconds: ISO 100. If a faster shutter speed, say 1/60 sec, had been used, the falls would look like water, but this long exposure of 60 seconds blurred the water into a smooth, creamy, white texture that is much more appealing. The camera was on a tripod and a remote trigger used to eliminate as much vibration as possible so that only the water which had movement appears blurred and in only one direction.
Want to try this for yourself? (Hopefully that’s a rhetorical question)
What you need to get started:
DSLR or other camera with a lens that can accept filters and is able to take long exposure shots. Smart phones and point and shoot cameras are generally not well suited for this type of photography since they may not accept filters and almost certainly cannot take very long exposures. If you’re still using film, you can take long exposures, but I certainly prefer using my DSLR for this – well I really prefer it for all my photography.
You need a good tripod unless you are counting on being lucky and finding a place to set your camera; you definitely will not be able to get good shots if you try to hand-hold your camera.
And, most importantly, you will need at least one neutral density filter. The more you have the better your odds of having the one you will need for the opportunities that arise. Whether you decide to use round, screw on filters or the square (rectangular) type of filter is up to you, but you should carefully study and think about both types before you invest unless you have an unlimited budget. I prefer the rectangular filters that slide into a holder which is mounted to the end of my lens and held in place by a screw like the one pictured below. Filter holders on the left, lens mounting side on the right. You need a filter adapter ring that screws into your lens to mount the filter holder. The brass screw shown on the left is used to tighten the filter holder to the filter adapter.
With the Hitech holder that is shown, I can stack up to three filters with no concern about vignetting. If you choose to use this type of filter system, be sure to buy a filter holder that is big enough for the biggest thread sized lens you have or even one or two sizes bigger to prepare for future additions to your lens collection. The adapter ring will allow you to mount a filter holder that works on a 77mm or bigger lens to smaller lenses with no image quality issues. Filters (and holders) basically come in 85mm or 100mm sizes. I suggest buying the 100mm because it will fit just about any lens you might buy, but that’s a preference not an absolute rule.
When you use neutral density filters, you are going to have to do some math to get close to a good exposure and then likely have to adjust your settings after the first shot. (This is so much easier in the digital age than it was with film). The formula for deriving a shutter speed is somewhat complex and many of us aren’t fast enough with math to do that type of thing in the field. If you have any type of computer (tablet, smart phone, laptop….), you can find plenty of free apps that will do the math for you. The process is simple; get a reading from your camera without the filter in place and then use that to compute the exposure you will need with the filter in place. For example, if I’m shooting with a .9 ndf (10 stop) and my camera, set to aperture priority mode, tells me I need a 1/125 shutter speed, I can use the app on my phone and see that I’ll need a shutter speed of 13 seconds. Or if my camera is telling me I need 1/30 second, my app tells me with the .9 ndf I will need 50 seconds of exposure.
This may seem a bit complex when you are first trying to understand this, but after you try it a few times, it’s not really all that difficult (especially if you use your digital tools and some free software). The process I have just described is also the big reason that I prefer the square filters over circular filters. When I’m preparing to shoot a scene, I leave the filter off, compose my shot and focus the lens where I want it. Then I set my lens to manual focus and mount the filter holder to my lens being very careful not to shake or move my camera by bumping into it. This process is quite difficult using the round, screw on filters. With a .9 (10 stop) filter on the camera most lenses will not be able to auto focus, so everything needs to be set up, ready to shoot before mounting the filter holder with the filter(s) inserted.
In addition to the solid filters, you can use graduated neutral density filters which are solid at one end and gradually become completely transparent at the other end. This type of filter is great when the sky is really bright and the scene you are trying to capture is shaded, and you want the sky in your image. Circular, screw in filters are less desirable for this situation because you have to compose in a way that fits the filter. With the square filters in a holder, you can slide the filter up and down to get the coverage you want.
I also encourage you to look at other sources (never get your info from only one opinion).
A couple of good resources are: Cambridge in Colour and Matt Kloskowski (this is a great tutorial on youtube). One thing I will mention about Matt Klowsoski that I do differently. When removing the filter, I remove the entire assembly – filter holder and filter. Matt pulls his filter out while leaving the holder on his lens. I just find removing the whole assembly makes it easier to put it back on without moving the camera. (Personal preference.)
I hope that you find long exposure shooting another valuable tool in your photographic toolbox.