Designed so that absolutely no color from the entire visible spectrum prevails, the neutral density filters can be used in many different contexts, depending on which type is used : uniform shading (square) or graduated shading (rectangular). Uniform ND filters reduce the quantity of light that reaches the sensor – or the film – increasing the exposure time. These filters have 3 main practical applications: emphasizing the flow of movement, reducing the depth of field, avoiding overexposure.
Neutral density filter example:
Graduated Neutral Density (GND)
Graduated ND filters are used to reduce the contrast difference of a composition. They allow for a well-balanced image; they are the filters most used by landscape photographers to yield both harmonious skies and detailed foregrounds at once. With these filters, images which are impossible to obtain in digital post-processing can be created.
Graduated Neutral Density filter example:
How to use a Gradual Neutral Grey filter ?
• To decide which filter density to use, you just need to measure – spot measuring with your camera or with a separate spot-meter – the clear zone where you wish to keep the detail and the zone that will be used for the final exposure. Then count the number of stops difference – at constant speed. For a 2 stops difference you will need a density of 0.6 (ND4).
• Take care to modulate your effect depending on the subject; for example, a reflection must be less bright than its source. Finally, depending on the way that the zone separating light and shadow presents itself, you will choose either a short or long transition zone filter.
• You must then adjust the filter – close the diaphragm as much as possible by pressing the depth of field preview button to better see the transition zone in the viewfinder while adjusting the filter vertically until its transition zone corresponds perfectly with the light intensity line of your framing. The effect of the filter depends both on the lens and on the diaphragm setting. The more the aperture is reduced, the more the effect of the graduated shading will be noticeable. Note that the capture settings has an impact on what the filter can do.
• Expose for the foreground. With experience, you will determine at a glance the filter you need to use and it will only take you a few seconds to adjust it efficiently with precision. The best practice would be to always carry the three densities to fit almost any lighting condition.