Effect of defocus on blur thresholds and on thresholds of perceived change in blur: comparison of source and observer methods.

The defocus levels required for normal observers to notice the first perceptible blur of a clear test target (blur threshold) and the least perceptible change in the degree of blurriness of an already blurry target (threshold of perceived change in blur) were measured using both the source and observer methods. In the source method observers viewed defocused stimuli presented on a projection screen, whereas in the observer method focused stimuli were presented to observers who were defocused using lenses placed in the spectacle plane. Blur thresholds were found to be dependent on target size and when the Landolt ring targets were near threshold acuity size blur thresholds were as small as 0.10 D. For larger target sizes (0.6 log min arc or more above threshold acuity size) the blur thresholds remained relatively unchanged and were about 0.18 D. Thresholds of perceived change in blur were found to be independent of the initial defocus level. Measurements of the threshold of perceived change in blur were found to be 0.05 to 0.07 D, which is much smaller than the blur threshold values. Comparison of results from the two methods of producing defocus indicate that the source and observer methods can be used interchangeably. However, for the same angular blur disc diameter, the blur thresholds found with the source method were significantly lower than those found with the observer method.