Whenever I want to impress somebody, I bring up the topic of neutral density filters. It sounds highly technical and makes me look smart. Truth be told, neutral density filters (a.k.a. ND filters) are just shaded pieces of glass or plastic that go over your camera lens. The fancy name is so that the suppliers can charge ridiculously high prices for them.
Although there are a variety of types, here are the most commonly used ND filters:
- Solid ND filter This is a piece of glass or plastic with the same level of shading throughout. Using this filter is like putting sunglasses on your camera, but without a polarizing effect. The basic idea is to just reduce the light getting to the sensor. These are typically available in 1-3 stop varieties. A 2-stop ND filter will reduce the light by 2 stops. Now, why would you want to reduce the light reaching your camera’s sensor? One reason is to slow your shutter speed during the day in order to blur running water, as I did in the following waterfall picture.
- Extreme ND filter (a.k.a. “black glass”) These are solid ND filters on steroids. They reduce the light by around 10 stops, allowing for very slow shutter speeds. I really like these filters. You can get a night exposure quality during the day. This can result in cool effects, especially when a body of water is in the picture. The following picture of the Portland Headlight was taken in the afternoon (in the pouring rain, actually) using black glass.
- Graduated ND filter. The graduated ND filter has a transition line in the middle, with half the filter clear and half shaded. These are available with both hard and soft transitions. The hard transitions go very quickly from clear to fully shaded, whereas the soft variety blends the transition more slowly. The most common use is when you are shooting a landscape shot which includes the sky. The sky is typically brighter than the land, so without the filter you might either blow out the sky or the land goes dark. You simply set the filter so the sky is behind the shaded part of the glass and it reduces the contrast between the sky and the land. Generally, if you only buy one of these, get a 2-stop soft graduated ND filter. If you can buy two, get a 2-stop soft and a 3-stop hard.
All of the above filters come in two main varieties:
- Filters that screw onto the lens. So, if you have lenses with different diameters, you would likely need one for each diameter. Screw-in graduated ND filters make no sense to me. That’s because the transition line is fixed right at the center, so they won’t work if you want the transition anywhere but the center of the frame.
- Flat pieces of glass or plastic (mostly plastic) which go into a holder. The holder either screws onto your lens or gets pushed onto the outside of the lens, depending upon the holder. You can then position the filter however you like. These come in different sizes. I use 4×6 inch filters, which is the most common size used for digital SLR cameras.
As for brands, the highest end (and most expensive) are the Singh-Rays. If you don’t like spending $160 on a 4×6 inch piece of plastic, but like the idea of spending $80 or $90 for a piece of plastic, then Lees are also known to be excellent. There are other cheaper brands available.
So, why not just make the adjustments in Photoshop, or blend exposures using HDR or Photoshop layers? A few issues here:
- Depending upon your camera and software, it is difficult to handle extreme contrast in a single shot. You either blow out the highlights are get digital noise in the shadows.
- Blending two or more pictures with different exposures using layers adds to your post processing work.
- HDR is cool, but it doesn’t always look natural and generally requires multiple shots.
- Conventional wisdom says to get the picture as right as you can when you press the shutter, rather than fixing it later.
There’s a lot more to say on the topic, but I hope this brief intro helps explain the basics.