Nikon Owner Issue 15
The Facts Of Filters - Part II
Simon Stafford continues his series on the art of good filtration.
In Part One of this series I examined the purpose of filters, the various systems that are available, some practical considerations in respect of choosing which type to use, and concluded by discussing probably the most useful filter of all - the polariser. In this, the second part, I am going to take a look at neutral density filters, both continuous and graduated types, which, in addition to the polariser, I consider as essential when shooting on film or digitally, as well as colour correction filters.
Neutral Density
As photographers, we rarely complain about having too much light, especially when we want to use slow film, or low sensitivity settings with our digital cameras to maximise image quality. However, there are occasions when reducing the amount of light passing through the lens is desirable. In these instances a continuous neutral density (ND) filter is the solution. Most filter manufacturers offer ND filters in a variety of strengths (density), often in increments of a whole stop value.
I often like to experiment with a slow shutter speed in an attempt to emphasise motion, but in bright conditions this can be difficult, especially with digital cameras such as the D70 and D2Hs that have a base-sensitivity setting equivalent to ISO200 film. Consider a situation where you wanted to show motion blur in a person running. Typically, to achieve the desired amount of blur will require a shutter speed in the region of 1/30 second. If you apply the 'sunny f/16' rule of thumb, which suggests that with your lens set to f/16 the shutter speed required for 'correct' exposure of a subject in direct sunlight under a clear sky is the reciprocal of the ISO value, you would need to shoot at an aperture of f/128 (1/200sec f/16 is equivalent to 1/25sec f/128)! Put a 3-stop (x8) ND filter on the lens and the exposure is reduced to 1/25 at f/16, which provides a usable shutter speed and an aperture value that is attainable on most lenses.
Using fill-in flash in daylight is a popular technique to help reduce excessive contrast, and another occasion when ND filters can be useful. The high flash sync speed of the Nikon D70 (1/500second) offers great flexibility in bright conditions but with a camera such as the D100 that has a maximum flash sync speed of 1/180 second, and a base sensitivity setting equivalent to ISO200, things can be more difficult. If you apply the 'sunny f/16' rule of thumb again to a D100 used in these conditions it would require an exposure of 1/200 second at f/16; the shutter speed is still in excess of the maximum flash synchronisation speed. Putting a 2-stop (4x) ND filter on the lens allows a combination of shutter speed and aperture that is within a usable range (e.g. 1/100 second at f/11).
Neutral Density Graduated
A graduated filter is 'clear' on one side and introduces an increasing filter effect across its width. In the case of a ND graduated filter, the density of the filter changes from no effect to the full quoted effect on the opposite side of the filter.
Why do we need ND graduated filters? In many scenes, particularly when shooting outdoors, it is not uncommon to encounter very high levels of contrast (the range of brightness between the darkest shadows and lightest highlights); a typical situation is a bright sky above a dark foreground, where the difference in light levels can be as much as 10 to 12-stops. Film and digital sensors have far less tolerance to such an extreme range of brightness than our eyes, which are capable of adjusting to various intensities of light very quickly. The dynamic range of a film, or digital sensor is a measure of its ability to retain discernible detail in shadow and highlight areas. Transparency film has the narrowest dynamic range, generally no more than 5-stops; most digital sensors have a dynamic range of around 7-stops, while negative film (colour and monochrome) can record information over a range of about 9-stops.