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The variable polarizer filter is composed of two filters that can rotate one relative to each other. In this way you can select the amount of light that pass through the filter: it is therefore particularly useful in observing the Moon where it is necessary to change the transmissivity of the filter as a function of magnification with which you look at the Moon. In fact, with increasing magnification, the light that reaches the eye or the recording camera decreases and then the transmissivity of the filter must be increased.
