Fig. 3. Retinoscopic photographs (A,B) and comparisons of longitudinal spherical aberration (LSA) curves with schlieren photographs for individual lamprey lenses (C–F). (C) Comparisons between the LSA curve and a schlieren photograph of a representative L. fluviatilis lens. The grey line is the mean LSA curve of 21 lenses from Astatotilapia burtoni, the species in which multifocal lenses were discovered (Kröger et al., 1999). The A. burtoni curve was scaled for easier comparison and shows less variation because averaging smoothes the data somewhat. The graph (LSA) shows the axial distance between the centre of the lens and where the beam intercepts the optical axis (back centre distance, BCD) as a function of the lateral distance between the optical axis and the undeflected entrance beam (beam entrance position; BEP) in units of lens radius (R). The curves were terminated at 0.95R because most of the energy incident on a fish lens at higher BEPs is reflected (Sroczynski, 1977). Close to the optical axis the laser-scanning method has low accuracy (Malkki and Kröger, 2005). (D) Comparisons as in C for P. marinus. (E) Comparisons as in C for M. praecox. (F) Comparisons as in C for G. australis. Note that the BCDs are short for BEPs corresponding to radial positions in the lens with reddish areas in the schlieren photographs. By contrast, the BCDs peak at positions corresponding to bluish areas in the schlieren photographs. The results obtained with both methods therefore consistently indicate that lamprey lenses are multifocal and compensate for LCA. Zones that are dark on the schlieren photographs may focus either infrared (short BCDs) or ultraviolet (long BCDs) light. Note that the scale of the y axis is different for G. australis. Scale bars, 1 mm.