How A Neutral Density Filter Can Effect Your Images
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How A Neutral Density Filter Can Effect Your Images

 
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Neutral Density filters come in two basic varieties: solid and graduated. Both function by simply cutting down the light that is transmitted to the film, just like someone turned off the lights. The key difference is between the two types is, of course, the graduation.

Solid neutral density filters typically come as traditional screw-on filters in a variety of sizes and strengths. Their main purpose is to allow longer exposures when you can't or don't want to stop the lens down any further. This is usually to afford longer exposure times when photographing moving water to get that silky effect, but can also be used as another means to control depth-of-field. Owners of digital cameras may find these filters especially handy if their slowest film speed is no better than ISO 200.
 Neutral Density is my favorite category of filter. As I've said before, I'm not a big filter user, by my ND filter go everywhere with me. The idea with Neutral Density is that one end is clear and the other is neutral density. In between the two ends, the filter transitions through either a soft-edged or a harder edged graduation which allows you minimize the likelihood that the transition will be visible within the picture frame. The aim is to position the filter within the image by sliding it up and down in order to equalize the brightness across the frame. The trick to getting the graduation in the right place is practice. Practice and the depth-of-field preview button that is. By stopping the lens down to your shooting aperture and then moving the filter around, it becomes much easier to see the edge of the graduation. Bracketing your compositions slightly with respect to how the filter is positioned in order to make sure you've gotten it in the right place can help if you are unsure. Don't pay any attention to where the graduation appears to be when you look at the filter sitting there on the end of your lens. It may seem like it covers a third of it, but if the lens is stopped down at all, it may not even cover any of the actual image; it is only what you can see through the stopped-down diaphragm that counts, not the entire lens diameter.

A neutral density (ND) filter is an optical filter to reduce the brightness of the camera’s view without affecting colour or saturation. It exists as a conventional screw-in filter that you attach to the end of a lens, as a drop-in filter for some lenses where it’s not possible or convenient to use conventional screw-in filters and it can also be built in to the camera and selectable from a camera control or even automatically. But in a time where camera manufacturers and photographers are seeking the ultimate in low light performance, why would you want to use a neutral density filter to darken things?



Neutral density filter – Maximum lens aperture
There are times when, for creative purposes, you will want to use a wide or even a maximum lens aperture. This could be, for example, to limit the depth of field and achieve a great de-focused background which we would describe as nice ‘bokeh’. In bright conditions this would require a correspondingly fast shutter speed in order to avoid over-exposure. While some cameras can boast shutter speeds of up to around 1/8000th second, many can’t.
Mt. Rainier from Paradise Valley (with a graduated ND filter of course)
An ND filter can reduce the brightness and so help you to avoid running out of shutter speed. From another perspective you may want to deliberately reduce the shutter speed to achieve long time exposures and capture the dreamy effects this technique offers. Stopping down your lens may not be enough. In recent times the so-called ‘Big Stopper’ ND filter has come to the fore. While typical ND filters reduce light transmission by 1, 2 or even 3 stops and more (respectively 1/2, 1/4, and 1/8 light transmission and correspondingly called ND2, ND4 and ND8 types, etc.) a Big Stopper reduces the light by 9 or 10 stops (ND512 and ND1024, or commonly referred to as ND1000). Don’t confuse a neutral density filter with a polarising filter, which can be used in a similar way but can introduce both desired and undesirable side effects, like uneven density of blue skies.
filters come in strengths of one, two, or three stops with transitions either soft-edged or hard-edged. The most useful are the two-stop soft-edged for general use, and the three-stop hard-edged for the extreme brightness differences when the sun is near the horizon. On occasion, I've been known to use more than one ND at the same time in situations with even wider exposure differences between foreground and background. 
A useful variation on the Neutral Density is the "Daryl Benson" Reverse  ND marketed by SONEON. This filter starts off clear at one end, then has a hard edged two or three-stop in the middle that then tapers off to only one or two stops at the opposite edge. For shots right at sunrise or sunset, this can be just the ticket.
Metering with ND can be somewhat tricky. The easiest way I know of goes something like this: spot meter various portions of the frame in order to determine if you need a filter at all and if so how many stops it should block. If you need to cut down the brighter end of the frame (typically the sky and background) by two stops to get it in line with the darker end (typically the foreground), then choose a two-stop ND, and so forth. Adjust your exposure for the portion that will be covered by the clear end and would thus be unaffected. Before adding the filter, the brighter areas would therefore be overexposed. Finally, slide the filter into place (holding down the depth-of-field preview button of course) and you should be set.
When buying neutral density filters, quality matters. What you want is true neutral color, but many cheaper brands will impart a magenta or brown color to your image.

 

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