Typically, neutral density filters reduce the exposure by one to three f-stops (ND2, ND4 and ND8, respectively), but can reduce the exposure by as many as 13 f-stops (ND8192). Neutral density filters are not cheap, especially the darker ones that reduce the exposure by eight f-stops or more. Some cost several hundred dollars.
Why would you want to use a neutral density filter? To reduce the light entering the lens so that you can use a longer exposure. You can blur motion in daylight.
The neutral density filter that I'm demonstrating here will reduce the exposure by 10 to 15 f-stops. The main ingredient is welding glass, which comes in 16 different shades. The one I used is a #10, which reduces the exposure by about 13 f-stops.
Welding glass isn't expensive, typically ranging from $3 to $25. If you know a welder you might be able to get some welding glass for free simply by asking--that is what I did.
You will need welding glass, a UV filter (which, if you don't have one already, can be found for just a few dollars), some electrical tape, and scissors. Make sure the welding glass and UV lens are clean.
Secure the UV filter to the welding glass using small pieces of electrical tape. You want to use electrical tape to prevent light-leaks (unless you want light-leaks). Make sure the threaded side of the UV filter is up and take care that the tape doesn't cover the threads.
Screw the filter onto the lens. In the photograph above the home-made neutral density filter is attached to the lens on my Samsung NX200.
|Snowing - Tehachapi, California|
In the above photograph, a neutral density filter was NOT used, and you can clearly see the falling snow. If I did not wish for the falling snow to be visible, I could have used the home-made neutral density filter. With a long exposure, the falling snow would look like a barely visible light fog.
|Winter Train Streak - Tehachapi, California|
Shutter 35 seconds, f9, ISO 200, Samsung NX200, 40mm, ND filter
I used the home-made neutral density filter to capture the above photograph. Without the neutral density filter, the train would have appeared stationary and the falling snow would have looked like the image above this one.
You will want to mount the camera on a tripod because you will be dealing with long exposures. It will be tough to focus the lens after you screw on the home-made neutral density filter, so you will want to do that before hand. It's helpful to use manual focus instead of auto focus.
While your camera might be able to figure out the exposure in auto mode, it is better to use manual mode and do it yourself. I metered the scene using the camera's meter before placing the filter on the lens and subtracted 13 stops from the shutter speed. You might find a hand-held light meter useful.
Another issue is white balance. The tint color will be different depending on what welding glass you use. Take a few test shots to figure out what the white balance should be, or save the image in RAW format and figure out the white balance later in post-processing. If the photograph will be black-and-white then white balance doesn't matter.
|Rock And River - Kernville, California|
I used a store bought 2-stop neutral density filter to capture the above photograph. With the home-made neutral density filter, the water would look more like mist or fog than water.