Anyone who is interested in daytime long exposure photography must know about Neutral Density Filters.
Neutral Density filters commonly called as ND filters reduce the amount of light entering camera to extend the exposure time resulting in images with sliky smooth water and cloud streaks in the sky. ND Filters allow photographers to be more creative and are a “Must Have Accessory” for landscape photographers.
Its one of the most under-rated piece of gadgets in photography and the numbers that come printed on the pack are confusing. Let me try and share whatever I can to help you understand ND filters better.
Though an ND filter may look like another piece of dark glass there is lot of precision science involved in making them. They stop specific amount of light from reaching the sensor and that is what make them so unique and special. They also need to ensure that this blocking of light is uniform all across to keep the image sharp all across the frame and does not alter the color of the image; this is why they are called “Neutral”.
ND Filter Types
- Screw-in type ND Filters
- Screw-in type Variable ND Filters
- Slot-in type square ND Filters
- Slot-in type rectangular Graduated ND filters
Screw-in types can be screwed on in front of the lens just like a UV filter while Slot-in types have a filter holder system where 2-3 filters can be stacked.
What are Graduated ND Filters?
GND as they are called, are rectangular shaped with one half clear and 2nd half dark. Dark half usually stop the light by 2-4 Stops. GND are also come in two types:
- Soft GND, and
- Hard GND
Soft GND have the gradual smooth transition from clear to dark while Hard GND have abrupt change from clear to dark side.
There is another type of GND which are “Reverse Graduated ND Filter”. In this the darkest part of the filter is right in the middle of the filter and then gets lighter towards top. This is used for shooting Sunsets when the Sun is right on the horizon and help in reducing the amount of light coming in from the horizon where the Sun is strong while letting more light in from the other parts of the scene.
Nomenclature and Numbers
The most confusing aspect in purchasing ND filter is trying to solve the mathematics behind some numbers mentioned on the label. It took me some time to figure that out and let me share that for your better understanding.
Different manufacturers use different terminologies and that makes things even more difficult. Some manufacturers label their filters using Optical Density of the filter, whereas some of them use what’s known as the Filter Factor.
Most commonly used terminologies are as under:
- Filter Factor, and
- Optical Density
What is Filter Factor?
This is a simple representation of the factor by which the ND filter reduces the incoming light into the lens. E.g, an ND filter that reduces light by one stop (one stop reduction of light is always half the light) has a filter factor of 2. When the manufacturer uses Filter Factor on the label it is usually mentioned as ND2, ND4 etc. Now this many a times ends up confusing buyers. Remember the numerical part in ND2 is not the Stop of light it reduces but just a Filter Factor. To further clarify let me take an example of ND4. In this 4 is a filter Factor that by its definition I explained above is the factor by which it reduces incoming light reaching the lens. Since 2 stop light reduction means 4 times reduction, ND4 represents a 2 Stop filter. Pls. check the chart below for better understanding.
To understand this you must have a better understanding of Stops of Light.
This is the most commonly used terminology by most manufacturers. Some use both this and Filter Factor. It represents the optical density of the glass, which further defines the amount of light it cuts. There is a complicated mathematics behind this. Just refer to the chart below for your reference.
I hope this article clears up some doubts you might have had about ND Filters. Do ask your questions if you have any.