ND - Neutral Density. For the beginning landscape photographer this term could just as well stand for Niggling Disorientation. The ND filter is possibly one of the more misunderstood accessories for photographers. Looking at one for the first time, whether a split-graduated type or a solid one, many may vaguely understand what its purpose is, but few instinctively know how to use it and under what circumstances. Yet, the ND is one of the two filters I consider a “must have” in my photo bag, right there next to my circular polarizer. While this article might not be for the experienced outdoor photographer, I offer this information for those who are starting out in this field and need some solid and basic information on the description, use, and purpose of this very important filter.
WHAT ARE ND FILTERS?
A neutral density filter is designed to do one thing – reduce the transmission of light to the photo sensor. Think of it as a sunglass for camera lenses. The better quality filters will filter the intensity of the light without introducing color shifts or imposing a color cast – just like good quality sunglasses do versus cheap ones. The better ones are also carefully manufactured to minimize flaring and ghosting. By reducing the amount of light hitting the camera sensor the photographer is able to make changes to exposure values. This is done to achieve certain effects - such as to take advantage of reduced depth of field, by allowing wider aperture settings, or to photograph subjects in such a way as to project movement or fluidity through longer exposure times. A variation on the solid ND filter is the graduated ND filter, or ND grads. These serve a unique purpose and so are a little more specialized. They allow you to capture the full dynamic range of an outdoor scene within one single exposure. Since they have a clear section and an opposing filtered (darkened) section, they can compress the exposure values of the light areas and dark areas present so that the camera can record both highlights and shadows with a reasonable amount of image detail. A bright sky against a darker foreground is a good example. Neutral density filters come in several forms and configurations. I’ll group them into three basic types.
The Solid ND
These filters come in two shapes, the more common circular, screw-on type (just like your polarizer), and the slide-in square/rectangular shape. Both accomplish the same thing; they are just designed to attach to the front of your lens differently. Is one better than the other? The end result is the same, but as I’ll point out later, I believe each is better suited based on one’s shooting preferences and budget.
The solid ND also comes in varying strengths. The strength of an ND is typically measured in f-stops. The strength of these filters usually range from 1 to 15 f-stops of filtration.
The Variable ND
This filter is only made in the circular design and screws on to the front of your lens. It has a rotating bezel much like a circular polarizer. As the name implies, these ND’s are designed to give you a varying range of density from about 2 to 8 f-stops of filtration in one package. A great concept, especially if you have limited space in which to carry your gear. While very popular, they do have some limitations, and they also tend to be more expensive. These are not your usual ND filters – they do not use density dyes to darken the filter material. They use opposing polarizing filters, that when rotated on the same axis increase or decrease the light stopping capacity of the filter.
The Graduated (or Split) ND
The graduated ND filter also comes in both circular screw-on and rectangular types. I’m going to stop here and make a rare, absolute statement. Circular screw-on graduated neutral density filters are absolute rubbish. Do not buy one. If you own one, sell it to your worst enemy. Get that pointless thing, designed by evil elves from somewhere south of the North Pole, out of your bag once and for all. Okay, I feel much better. ND grads, as they are sometimes called, also come in varying strengths - from 1 to 5 f-stops. Some makers even offer what is called a reverse ND grad. This unique filter is used when shooting into the sun at sunrise/sunset. The filter has the usual density transition except that it is darker at the center and then fades slightly toward the top. This will give you more filtration where the sun is hanging over the horizon without making the upper sky too dark.
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