To obtain a natural motion blur, that is to reproduce the movements so that they are not too fluid or jerky, in cinematography it is normal to use a shutter speed of 1/48 or 1/50 of a second.
To keep these settings at the desired aperture even when the light on stage would cause overexposure of the images, we use Neutral Density filters, more or less dark gray filters whose purpose is to reduce the amount of light that reaches the sensor, without at the same time altering the colors of the scanned images. Let’s assume we have the correct exposure at 1/50 “- f / 8 – 800 ISO. Wanting to use the f / 2 aperture without changing the shutter speed or ISO sensitivity, we can use a 4-stop ND filter. In this way we can use the desired aperture, while continuing to expose the images correctly.
Despite the neutral designation, each ND filter somehow alters the colors of the images because no manufacturer can make perfectly neutral glass or optical resins. Generally, the color drift is more visible the higher the density of the filter, i.e. the darker the filter. The best ND filters are able to produce images with no obvious color casts up to an optical density of 1.8 or 2.1, equivalent to 6 or 7 stops of light reduction.
The density of an ND filter, also called gradation, can work in various ways depending on whether we refer to its optical density, its ability to block light or its influence on exposure parameters.
The gradations of the ND filters
|Reduction:||1 Stop||2 Stop||3 Stop||4 Stop||5 Stop||6 Stop||7 Stop|
|Initials:||ND 2||ND 4||ND 8||ND 16||ND 32||ND 64||ND 128|
|ND 0.3||ND 0.6||ND 0.9||ND 1.2||ND 1.5||ND 1.8||ND 2.1|
|ND 2x||ND 4x||ND 8x||ND 16x||ND 32x||ND 64x||ND 128x|
In addition to a higher level of chromatic neutrality, the best NDs work by the greater adherence between the declared and actual density, by the more homogeneous density over the entire surface of the filter and by the overall superior optical quality; the latter is a particularly important factor when using long focal lengths. With cheaper NDs, it is common for a drop in sharpness, geometric distortions and optical vignetting to be seen in addition to the obvious color casts. Only the best NDs are usable in combination with focal lengths greater than 200mm, as the others make the sharpness fall below the threshold of acceptability.
Not all NDs, regardless of price, are equipped with anti-reflections. Some manufacturers, to favor the achromy of the glass, prefer to avoid this type of treatment.
ND filters can be made of glass or optical resin. Those in glass are much more expensive and more fragile, but also more resistant to scratches and above all much better from an optical point of view. Resin filters scratch very easily and rarely offer acceptable performance, especially once the 0.9 density has been exceeded. Resin systems often have very different chromatic dominants between one filter and another; this can compromise the visual continuity of a shot until it is practically irrecoverable. When using cheap ND filters it is therefore advisable to use only one for each scene and keep it mounted for all shots of that scene.
Using multiple ND filters together results in a density equal to the sum of the individual densities of the filters used. The combination of ND 0.3 with ND 0.9 therefore produces a density equal to that of ND 1.2. Obviously, the simultaneous use of multiple filters negatively affects image quality. If the goal is to save, however, rather than buying 7 resin filters, it is advisable to buy 2-3 glass filters and work in stacking, even up to using 3 filters together.
Only the best ND filters offer infrared cutoff. Unless you are working on film, where infrared is not recorded, the ability to block this part of the light spectrum becomes crucial in filters with a density greater than 0.9. A common ND 1.2 will almost certainly produce images characterized by infrared pollution. That is without blacks and with visibly altered colors. A non-IR-Cut 2.1 ND will certainly be unusable in most cases.
ND filters exist in both solid and graduated versions. Graduated NDs, whose acronym is GND, are characterized by a gradient density that decreases from the declared value up to 0, at which point the glass or resin appear completely transparent. The extent of this gradient is determined by the type of GND, which can be Soft Edge or Hard Edge. In the first case the transition is wide and soft, in the second it is almost clear. Graduated Neutral Density filters were created to allow photographers to darken the skies without lowering the overall brightness of the panoramas. GNDs can also be Reverse GNDs. In this case the filter has a darker part in the center that degrades towards one edge, while in the opposite direction the glass or resin almost immediately becomes transparent.
At one time CND filters were also quite common of the GND Soft. And the dark part was a central circle. The purpose of these filters was to compensate for the difference in exposure between the center and edges of the frame should a cinema lens exhibit excessive vignetting. With the advent of digital and modern color correction possibilities, these filters have almost disappeared from the market.