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Sheila Rae, Heather Shorrock, Ian van der Linde, Shahina Pardhan; Role Of Target Spatial Content And Size In Perceptual Detectability Of Blur. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4770. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Depth of focus gives an indication of the response of the eye to blurring of the retinal image. Perception of blur has been shown to be dependent on a variety of extrinsic and intrinsic factors including target type, spatial content, size and contrast as well as optical properties of the eye. The variance of perceptual detectability of blur as a function of target size and spatial content was investigated in this study.
Limits of perception of blur were determined for 17 subjects (mean SE refraction -2.16 ± 2.5) following cycloplegia for targets presented via a Badal optometer. Targets were blocks of text in Times Roman font in two sizes (N10 and N20). Targets were filtered via a log30 bandpass filter using Matlab. Band widths were one octave wide with peak frequencies at 2.5, 4.9, 9.9, 19.7 and 39.5 cdeg-1 .
Perceptual blur sensitivity was significantly worse for the larger text size in the unfiltered condition (paired t test: t= -6.216; p<.001). There was a significant effect of target bandpass filter for the larger text size (repeated measures ANOVA: F=8.036; p = .012) with poorer sensitivity for the 4.9 and 9.9 cdeg-1 filters (means: 0.240 and 0.250D) and higher sensitivity for the 2.5 and 19.7 cdeg-1 filters (means: 0.189 and 0.196D). Comparing the filters on each text size containing the peak object frequency (9.9 filter for N10 and 4.9 filter for N20) to the unfiltered condition, the unfiltered target gave significantly poorer blur sensitivity than the filter containing the peak object frequency for the larger text size (t= 4.645; p< .001) and was also poorer but not significantly so for the smaller text size (p = .08).
As might be predicted, perception of blur is more effective with higher spatial frequency detail, however, when high frequency detail is present along with a mix of other frequencies, it does not appear to be used to judge blur. The poorer blur sensitivity with the targets containing a broad range of spatial frequencies would allow for greater focusing errors to be perceptually tolerated. Blur sensitivity for complex targets such as printed text appears to result from a combination of factors including overall text size, spatial detail and familiarity with the stimulus.
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