This plots illustrates the dependance of sensitivity on the seeing. A doubling in the seeing represents a 0.75 magnitude loss in sensitivity.

Sensitivity as a function of sky surface brightness. For sky one magnitude brighter than normal, exposures have to be increased by a factor of 2.5 to reach the same limiting magnitude. Where the seeing is constant for a filter, i.e. U and i, the slope is 1. By contrast, for z, g and r the seeing is dominating.