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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.
A camera lens accessory that fits onto the filter thread of a lens to allow a filter system holder to be attached. Other types are available that step-up or step-down from the actual lens filter diameter to allow other size filters to be used.
A flashgun that has a circular electronic tube that is positioned around the camera lens or on the filter thread. It’s used in macro photography to produce an even distribution of light while fashion and still-life photographers create interesting halo shadows around the subject.