The Lee 9ND 4x4" Standard Neutral Density Resin Filter reduces exposure by 3 stops without affecting colour balance.
ND standards can be classed as essential in modern photography. These filters absorb light evenly across the visible spectrum, therefore reducing the light entering the lens without affecting the image colour. Traditionally used on film for extending shutter speeds in bright conditions, they are now becoming essential to digital photographers for reducing the sensitivity of sensor chips on cameras which have relatively high minimum ISO settings.
ND Standards extend exposure times allowing for movement in an image, be it people, water, foliage or clouds. If used with care the effects can be impressive.
This is the construction of a type of filter. The type of process used to create a filter can affect its price. Filters can be constructed out of regular glass that sandwiches a coloured gel in between or in high-end filters, raw elements are added to the molten optical glass so there is no risk of uneven colour or fading.
When placed in the optical path, many filters block a certain amount of light from reaching the lens. Filter factor relates to how much exposure compensation is required in order to adjust for this. Most cameras have TTL metering and will be able to do this automatically.