focal plane

The plane occupied by the film at the focal point of the lens. It can be positive or negative; it is commonly referred to as positive and negative planes, respectively.

fog

i. The meteorological condition in which the horizontal visibility at the earth’s surface is less than 1000 m because of suspended small droplets or ice crystals in the air. Relative humidity in this case is generally 100% but not less than 95%. Fog is formed by the cooling of the air by contact and mixing or, occasionally, through saturation of the air by increasing water content. Fogs are classified according to their effect on visibility: in dense fog, visibility is less than 50 m; in thick fog, visibility is between 50 and 200 m; in fog, visibility is between 200 and 500 m; in moderate fog, visibility is between 500 m and 1000 m; and in thin fog, visibility is between 1000 and 1500 m.

ii. As it relates to aerial photography, a darkening of negatives or prints because of a deposit of silver that does not form a part of the image. Fog tends to increase density and decrease the contrast. It can be caused by exposure to unwanted light, to air during development, to impure chemicals, etc.