Neutral density filters help a photographer to use a longer exposure. They are used to block the amount of light that is coming through the lens and reaching the sensor of the camera. Think of them as sunglasses for your DSLR. They are known as neutral density because they stop all colors of light irrespective of their wavelengths equally and have no effect whatsoever on the white balance of the picture. They appear grey or very dark gray or even opaque and come in different strengths of stopping power. A photographic store show you ND filters in 2, 3 10 or even 20 f/stops of light stopping power. These denote their ability to block light. Hoya’s NDx2 filters for example have an effect of providing 1 stop shutter speed reduction. The NDx4 has a 2 stops of shutter speed decrease and the NDx8 has 3 stops. The brighter the light available the more is the stopping power required if all other conditions are the same. Photographers shooting with a single neutral density filter in variable light conditions can often compensate for the lack of (or excess) stopping power by adjusting the aperture and the shutter speed in manual mode.
Applications of ND filters and graduated ND filters: creating motion blur, water trails and using long exposures for creative photography
Neutral density filters are also available in Graduated & Variable variants. Graduated means the density of filter or the amount of light that they stop decreases gradually from the edge of the filter towards the center. They are used to partially block out light from a frame. A small example will make this easier to understand. Say you’re shooting a landscape but without a ND filter on. If the sun is up you’re bound to see the sky blow out. If you expose for the landscape the washed out sky will render the picture unusable. At the same time if you expose for the sky the landscape will look dark. This is a classic situation where you can use a graduated Neutral Density filter to block out the bright sky and take a perfect picture.
Again when you want to capture water trail or motion blur in broad daylight, longer exposure time is warranted which also runs the risk of complete washing out your images. A neutral density filter will allow you to substantially increase the exposure time capturing water trails or motion blur without overexposing the shot.
Neutral density filters are mostly used to capture landscape shots or increase the shutter speed, but one use of them is to increase the aperture to capture bokeh in broad daylight. Usually if your camera metering systems says that the light is too bright (giving you a shutter speed of 1/2000th or even 1/4000th of a second) you will need a narrow aperture of something like f/5.6 or even f/8 to capture a shot. The result is most of the frame appears in sharp focus. It is nearly impossible to capture bokeh for a wide angle shot as such without risking blowing away the frame. The answer is yet again using a neutral density filter. Screw it on and let it compensate for the amount of light coming in. Set it to a wider aperture and then re-take the shot. You will have achieved the desired soft bokeh.
View through the viewfinder
When using a ND filter, photographers will notice that the view through the viewfinder looks darker than usual. This is because the image bounced off the mirror is actually coming through the filter. The intensity of the light is thus substantially diminished. T may be an issue so if you find the view too dark for your comfort, take the filter off, compose the shot and then put the filter back on before taking the picture. Modern DSLR camera systems have a metering system that can automatically adjust for the stopping power of a ND filter mounted on the lens.
Buying a neutral density filter: things to know
There are mainly two types of ND density filters, one that you can screw on to the front of the lens via the thread provided in both and the second one is a square or somewhat rectangular filter which one can mount using a filter holder. While the first one may sound a bit easy to operate there are noticeable problems of vignetting when the round corners of the ND filter obstruct the light. Comparatively the “cokin” type square or rectangular filter is suitable for 35mm bodies which normally suffer from a problem of vignetting and thus want to avoid additional issues by using a screw in filter. However such filters may be more difficult to set up and use and also take up more space in the kitbag because of the holder ring.
Camera body and effect of vignetting
Vignetting is a problem that is going to effect once you screw on a ND filter on the top of your lens; i.e.; if you use a 35mm sensor. The corners of the image are going to have significant vignetting. If your lens is something that is not designed with the best quality optics this problem can be aggravated. As such it is advised to use a filter that is not too thick.
Filter thread specification allows you to correctly match a lens with its filter. Look for a sign at the front of the lens like this Φ and then followed by a number. This is the size of the filter that you need for your lens. Even if you don’t have a filter of the same specification as your lens, step-up/down rings can help you to attach a bigger or smaller filter on to your lens. You will need to screw on the step-up/down ring at the front of the lens and then screw the filter on top of the ring. But this is not recommended as there can be degradation of the image quality when the filters are placed further away from the lens.
Further reading Variable Neutral Density Filters