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Most SLR cameras, some digital and a few compacts have a threaded ring at the front end of the lens. This is used to attach coloured and special effect filters as well as other accessories to alter the light that reaches the film. The diameter of the lens' filter thread is measured in millimetres. Stepping rings can be bought to allow one size filter to fit on a different size thread. It's better to step up (use a filter with a larger thread than the lens) to avoid corners being cut off resulting in dark edges to your pictures. Buying advice: If you're considering buying a camera for use with a particular lens accessory, make sure you can either fit the accessory direct to the camera lens or at least adapt it to fit.
Used to balance the light of a scene with overly bright highlights. There are physical filters to use in front of a camera lens, and digital filters for use in imaging software. A physical graduated neutral density filter, for instance, has one clear edge and then gradually increases in density towards the other edge. A so-called "hard grad" has a graduated section that reaches the middle of the filter, whereas a "soft grad" has a graduation all from one side to the opposite side. Neutral density graduated filters are the most common types, but there are also coloured versions available - gradient sunset filters, for instance, or tobacco-coloured ones. Physical filters are usually square or rectangular, are used in a filter holder that attaches to an adapter ring, which screws into the front of the camera lens. Sizes depend on the diameter of the front of the lens. Gradient filters can also be applied digitally in photo editing programs. Even when this is not a standard option in the program, often a so-called "plug-in" can be used for use in the program.
A filter which is used in front of the lens in order to filter out UV light that can cause a blueish haze. UV filters have much the same purpose, but a skylight filter is also coloured slightly pink (or yellow) in order to give pictures a warmer appearance. In the digital age these filters are used less than before, since colour temperature can be influenced in-camera and in processing, especially when using the RAW file format.
An image editing device used to cover part of a digital image that can then be left untouched while you apply a filter or effect to the rest of the picture.
The density of a material such as a film or filter. In digital terms, opacity can be adjusted to make it look less prominent or allow a lower layer to show through.