Filters come in two basic forms: the traditional round type that screw directly to the front of your lens and the square system variety that slot into a special holder.
In terms of flexibility, the square systems win hands down. All you need is one holder and a selection of inexpensive adaptor rings to use the whole filter system on lens with different screw thread sizes. You can also use three or four filters in the holder at once to combine effects.
Round filters are made to fit lenses with a specific filter thread. They're fine if all your lenses have the same thread, but to use them on larger or smaller lenses you'll need step-up or step-down rings. These rings are available in small jumps, such as 52-49mm, but if you have lenses with threads from, say, 49mm-67mm, you'll need to buy the same filter in different sizes. The depth of the filter mount itself also means if you use two or more together there's a risk of vignetting.
Many filters reduce the amount of light entering your lens, so the exposure compensation is needed. The amount of compensation required is shown by a filter factor which indicates how many times the initial exposure needs to be multiplied by -a factor of x2 indicates an exposure increase one stop., x4 two stops and so on. If your camera has TTL metering, meter with the filter in place and the light loss will be taken into account. However, if you meter without the filter in place, you must increase the exposure accordingly.
This table shows the filter factors for a selection of filters.
|Filter||Filter factor||Exposure Increase|
|81A warm-up||x1.3||+1/3 stop|
|Yellow/Green||x3||+1 1/2 stop|
|Blue 80A||x4||+2 stops|
|Blue 80B||x3||+1 1/2 stops|
|Orange 85A||x4||+2 stops|
|Orange 85B||x3||+1 1/2 stops|