This question can only be answered by considering both the degree of longitudinal chromatic aberration and the amount of MP. As discussed in the Introduction, Reading and Weale
7 calculated the blur circle resulting from chromatic aberration for typical sunlight. They then derived the spectral transmittance of an ideal filter that would reduce the bluish penumbra to threshold while minimizing light loss. Such a filter closely resembles the average OD and the spectral absorption of MP, albeit without the characteristic carotenoid cusps. Bradley,
39 however, modeled the effects of chromatic aberration on the modulation transfer function (MTF) and concluded that image degradation is small. The MTF for broadband light is similar to that for monochromatic light, with approximately 0.15-D defocus. His conclusion suggests that the filtering effect of MP would have an insignificant effect on spatial discrimination tasks. We performed an analysis similar to that of Bradley
39 by evaluating the effects of different levels of MP on the spectral energy distribution of our white stimulus at the receptor level. First, we modeled photopic spectral efficiency functions for hypothetical subjects with zero, average, and high levels of MP by simply correcting the standard V
λ function, with MP levels corresponding to maximal spectral absorbance values of 0.0, 0.50, and 1.0, respectively. (No correction was made for lens absorption, thus implicitly assuming an average value.) We then multiplied, wavelength by wavelength, our hypothetical V
λ functions with the measured spectral energy of our white light
(Fig. 7B) . The resultant three product curves indicating relative sensation luminance to wavelength are shown in
Figure 7C . (
Relative luminance would be appropriate only for the 0.50-MPOD condition because the underlying luminance efficiency function is, in fact, the standard V
λ. The other two curves correspond to the 0.0- and 1.0-MPOD conditions and, therefore, are not identical to V
λ. Kaiser
40 introduced the term
sensation luminance to refer to situations in which an individual observer’s spectral sensitivity is used instead of V
λ, which refers only to the CIE standard observer.) Even a casual inspection of
Figures 7B and 7Cindicates that the SW lobe of the white light is drastically reduced compared with the long-wave lobe when correcting the energy distribution to sensation luminance (e.g., from 90% to 13% at the 460 peak for the zero MP condition). Thus, apart from MP absorption, lens absorption and receptor spectral sensitivity alone reduce the effectiveness of the corneal, SW component by a factor of almost seven. The addition of MP reduces the SW lobe even further (
Fig. 7C , curve 0.5 and 0.0 MPOD), but a critical question remains: does the MP reduce the SW component enough to improve acuity or hyperacuity? Closer examination of the relevant factors suggests that, in agreement with Bradley,
39 little improvement would be expected.