ND Filters
We're moving along in our series dedicated to helping you buy the right filters. In this article we're going to take a good look at ND filters. They are one of the most important tools for landscape photographers, but there are plenty of other uses for them in other types of photography.
What are ND filters and what are they used for? First of all, ND stands for Neutral Density.
The basic function of a ND filter is to reduce or modify colors of light while keeping the hue of color rendition. To put it in simpler terms, the ND filter reduces exposure by limiting the amount of light that goes into the lens. It is an altogether different way of adjusting exposure and we're going to look at its advantages.
For example, if you want to photograph something using a long exposure, in broad day light, you will find that it can be very frustrating. Let's say you're on the beach trying to shoot some waves in a way that will blur the motion and make the scene look smooth and silky. What you're first going to do is lower the ISO to the lowest value allowed by the camera. Then you're going to close the aperture way down to f16 for starters. After that you start experimenting with shutter speeds. You'll most likely find that you need at least a two second exposure to get that nice, continuous flow of the waves. Now, to get a two second exposure in mid day is almost impossible, even if you close the aperture all the way down to f22 or more.
That's where ND filters come in. They reduce the exposure enough to allow you to expose for two seconds or even more. Video shooters who regularly use speeds of 1/30th/s or 1/50th/s depend on ND filters for proper exposure in daylight.
Like I said in the beginning, there are other uses as well. For portrait photographers who shoot outdoors with flash and want to enjoy a nice, shallow depth of field, things can get out of control due to some technical limitations. If you want that nice, soft bokeh behind the subject, you're going to have to use a wide aperture. If you also want to combine ambient light with flash, it's going to get tricky. A wide aperture demands a fast shutter speed, however most flashes sync at 1/200th or 1/250th, which is often too long to get the exposure right. Again, the ND saves the day by reducing exposure with a few stops and making it possible to shoot with flash and a wide aperture.
There are multiple types of ND filters. The easiest to use is the Standard ND. You screw mount it on the front of your lens and that's that.
The variable ND filter on the other hand is also circular, but just like a circular polarizer, it has a ring that you turn to achieve a specific level of darkening.
Because of this "doubled effect "`, variable NDs are considerably more expensive than regular ones. Also, you need to be careful when using them with ultra-wide angle lenses, as they can cause serious ingesting around corners.
Finally, we have GNDs or Graduated Neutral Density filters. They are widely used by nature and landscape photographers because they make a smooth transition between dark and light areas of the frame. Each time you see a landscape photo that has a beautiful sky as well as a correctly exposed terrain, it's very likely the photographer used a GND. They are different from the other ND filters. They are usually square or rectangular in shape and use a special mount that is attached to the front part of the lens. You can adjust its position in the mount depending on the look you want to achieve. They usually provide between 1 and 4 stops of light blocking.
Now that you know what NDs are best for and what the options are, it's up to you to figure out which one is best for your style of shooting.