Filters come in two forms, viz. mounted glass discs which get screwed in front of the lens and plastic or glass squares that fit into filter frames and thus get attached in front of the lens. With the aid of adapter rings (also called step-down or step-up rings) filters of one diameter can be used on lenses with different diameters. There are specialized companies making a whole range of photographic filters.
[...] Types of photo filters Introduction Without a doubt, filters can be a great tool for a photographer, in as much as they can add to the photos, with color and special effects. They are an invaluable aid in livening up a visual. One can use filters in both color and black-and-white photography. Filters come in two forms, viz. mounted glass discs which get screwed in front of the lens and plastic or glass squares that fit into filter frames and thus get attached in front of the lens. [...]
[...] Neutral density filters Neutral density filters reduce the amount of light entering the lens without affecting the colors. They are useful when longer exposure times or wider lens apertures are needed. They are designed in filter factors, i.e. 4X, 8X, etc. they have the same color appearance as polarizer but are meant for a different purpose altogether. Diffusion or soft focus filters Diffusion or soft focus filters produce soft images by diffracting (bending) some of the light rays. Some light rays go straight through the filter, producing a sharp image, while other rays are bent by the filter, not producing sharp secondary images overlying the sharp primary one. [...]
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