Photographic filters are colored gel or glass, used in front of a camera lens to either alter the quality and color temperature of the light or to give some special effects. Apart from colored filters, there are some other special effect filters and attachments, which can be used for manipulating an image.
These filters are to be mounted in front of the lens. There are two basic mechanism to mount a filter. One is a circular type with threads on the ring, which can be fixed on the front of the lens barrel. In this case, if we change the lens on our SLR camera, we will need a converter ring to match the diameter of the filter with other lens barrel. (Converter rings are either step-up or step-down. Avoid a step-down ring as it may cause vignette on the corners of an image.)
Another mechanism is a filter holder, which has different threaded plates to match with the diameter of different lenses. There is a holder, which is to be fixed on the threaded plate and the holder can take filters, which are square in shape.
Always go for the best quality of filters. Any attachment in front of the lens is going to add its own faults in a photograph. We pay for the good quality of lens and then contaminate the result with poor quality of filter.
Whenever a colored filter is introduced in path of the light, it absorbs some light, depending on the density (opacity) of the color of the filter. We have to compensate this loss by over exposing the exposure. This compensation is known as filter factor.
Filter factor is termed as 1X, 2X, 3X, 4X and so on. 1X requires no compensation. 2X requires 1 f-stop, 3X requires 1 1/2 f-stop and 4X requires 2 f-stops of over exposing.
A DSLR, having through-the-lens (TTL) metering system will take care of this filter factor - but for red and orange color filters. The TTL meter is not so sensitive to orange color and not at all sensitive to red color. So in this case, first take the readings of light without the filter and then open-up by suggested filter factor given in the filter manual.
Read more about different kinds of photographic filters used in photography
Filters for black and white photography
Color correction filters
Neutral density filters
Soft focus filter
Cross star filter
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