Most often you might have seen YouTubers and DSLR filmmakers using an extra piece of glass on their lens! When I was starting out with learning DSLR videomaking, it's fascinating how I had imagined them to be just 'sunglasses' for the lens for protection purpose. They serve such an important purpose that I can only be surprised thinking how wrong I was :)
With the advent of DSLR Filmmaking, many complementary camera accessories have cropped up. One such indispensable equipment for the camera lens is the ND Filter.
What are ND Filters?
Any additional glass to put upon the lens of the camera is called a ‘Filter’. It is analogous to ‘Sunglasses for a lens’. There are many types of filters to put upon a lens. Commonly UV Filters – which are made up of UV protective glass – are used to protect camera lens surface.
ND Filter stands for Neutral- Density Filter. This simply means that the filter is made up of glass which has a tint of black over it. Thus we allow much lesser light to enter inside our camera. This is necessary when we are shooting in broad daylight/afternoon sun when the light outside is too bright and our images are overexposed.
The ND Filters come in two types – Variable and Fixed exposure.
Variable ND Filters : In variable ND filter, amount of light entering into the sensor can be controlled.
That is, the darkness of the filter can be increased or decreased.
Fixed ND Filters : In fixed ND filter, , amount of light entering into the sensor cannot be controlled. That is, every filter comes with a certain amount of dark tint.
Thus, exposure can be effectively brought under limits using ND Filter.
How does ND Filter help in achieving shallow depth of field?
Depth of field is primarily controlled by the Aperture of the lens. The wider is the aperture, the shallower is the depth of field and vice versa. The equation grows as follows:
[Graphic]
The problem is, in brightly lit conditions (such as outdoors in noon), the aperture has to be narrower; otherwise the image becomes overexposed.
In such conditions, it becomes very tough to achieve a shallow depth of field. This problem can only be solved by controlling the amount of light entering on the camera sensor. So naturally, the ND filter comes to rescue. When a ND filter is put on the lens, it allows lesser amount of light to enter on the camera lens, this makes it reasonable for the camera-person to increase the Aperture a tad-bit more, resulting in a shallower depth of field. If this sounds science to you, the following pictures will make it all clear:
[Image 1 : Overexposed Image in afternoon]
[Image 2 : properly exposed image with filter on]
[Image 3 : image with shallow depth of field ]
