Blur is an intrinsic feature of retinal images that varies widely across images and observers, yet the world still typically appears 'in focus'. Here we examine the putative role of neural adaptation in the human perception of image focus by measuring how blur judgments depended on the state of adaptation. Exposure to unfocused images has previously been shown to influence acuity and contrast sensitivity, and here we show that adaptation can also profoundly affect the actual perception of image focus.