We used a monochrome camera (1920 × 1080 pixels resolution) to measure spatial frequency spectra for unfiltered, defocused, and low-pass filtered movies. Sample pictures for the three conditions were grabbed from 2 m distance with the camera with similar optical parameters as the human eye (16 mm focal length, f/# 2.6; human eye 16.7 mm focal length, f/# 2.6 for a 6.5 mm pupil). The camera's peak sensitivity was around 500 nm. Because the camera lens (Pentax B5014A, f/1.4, F = 50 mm C-mount objective) was corrected for longitudinal chromatic aberration, its effect on focus settings was negligible. The output of the camera was linearized by adjusting the camera driver settings to generate an almost linear relationship between pixel output and target luminance (DMK 33UX174, Imaging Source, Germany; driver settings: bright 0, gain 88, exp -9, gamma 100), as measured with a Minolta luminance meter (LS-100; Minolta Camera Co., LTD, Tokyo, Japan). A squared area of 1080 × 1080 pixels in the center of the frames was selected for Fourier analysis, which was done with publicly available software (ImageJ, National Institutes of Health [NIH],
https://imagej.nih.gov/ij/), and its FFT function. Software was written in C++ for rotational averaging of the two-dimensional spatial frequency spectra to convert them into one-dimensional plots, as described by Flitcroft et al.
24 Average pixel values obtained from rotational averaging were normalized so that the maximum brightness value was 1 and the lowest 0 (
Fig. 3A). In
Figure 3B, values were logarithmized as done by Flitcroft et al.
24 To illustrate that our calculated low-pass filter generated a spatial frequency spectrum similar to the targeted defocus (+2.5 D), data of two further levels of defocus are also shown (+1.5 D and +4.5 D; see
Fig. 3, gray lines). The linearity and correctness of the spatial frequency scale on the x-axis in
Figure 3 was verified by measuring various sine wave gratings with known fundamental spatial frequencies. Different from descriptions by Flitcroft et al.,
24 the resulting spatial frequency amplitudes showed no linear decline with increasing spatial frequencies between 1 and 10 cyc/deg (see
Fig. 3B), possibly because of nonlinearities in the contrast distributions in the movies and/or nonlinearities in the screen showing the movies.