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This Note reports the results of measurements of the modulation transfer function (MTF) of artificial fog. Fog was produced in a large room through which hot water vapour was blown at high pressure. First, the optical properties of the fog were investigated and proved similar to those of natural fog. Next, the luminance contrast transmission in fog was measured as a function of spatial frequency, using black and white bar patterns. From the contrast transmission, the MTF of fog was estimated. Using the MTF of 5% transrza~ttauce fogs as a filter for image processing, and reducing the contrast at specific spatial frequency, the effect of fog is made visible.