Ever if you are a beginning photographer you must be aware of what filters are and their various uses in photography. But for those who do not know, filters are camera accessories made up of glass that are used to get various coloured, black and white and focus effects, such as split focus, diffusion, colour correction and contrast reduction, in photography. The common held belief is that in zoom lenses the intensity of light reaching the focal plane reduces on account of the abrasion caused by the glass lens, since the light must pass through multiple layers of glass in a zoom lens, rather than just a single piece of glass like with prime lenses. Then why, when we add so many glass filters to the front of the lens, does the light intensity not decrease? Well the answer is that the light intensity does decrease with every additional filter and this characteristic of filters is called the filter factor.

Every filter has a filter factor which is the proportion of light that it allows to transmit through its glass surface. Theoretically some filters are sensitive only to a particular kind of wavelength but that is not the case practically. In reality every single filter, even polarising filters like the Cokin P 164, will have some impact on the light intensity. These filters (except close up lenses and diffusion filters that are not actually filters but a kind of lens) cut down a certain intensity of light also along with blocking light of particular wavelengths.

Each filter has a filter factor marked on its rim. Each filter factor corresponds to a particular f stop value. The number stands for the amount of f stops by which the light intensity is reduced through the use of the filter. For example, a filter, with a filter factor of 8, only transmits 1/8 of the light (12.5%), which is equivalent to 3 f stops. Likewise, a filter, with a filter factor of 1, typically transmits around 95-100% of the light and has zero f stops. Hence to get proper exposure you need to take the filter factor into consideration and increase the exposure according to the corresponding f stop reading by either opening the aperture or reducing the shutter speed.