Fig. 4. Mean longitudinal spherical aberration (LSA) curves with 90% confidence intervals for the four species of lamprey and the mean curve of A. burtoni (grey line). Averaging the results from several lenses (for numbers see Table 1) leads to some smoothing of the curves (compare with curves in Fig. 3). Note that the confidence intervals do not overlap in some regions (arrowheads), which means that the curves are different at the P<0.05 level. The laser-scanning method has low accuracy for small beam entrance positions (BEPs; close to the optical axis) because of technical reasons (Malkki and Kröger, 2005). This leads to very large error margins. All curves have been terminated at 0.95R (see Fig. 3 legend). The A. burtoni curve has been scaled for easier comparison. Note that the LSA curves of the lampreys have at least as much variation in back centre distance (BCD) as the curve of A. burtoni lenses (grey line) which are known to be multifocal. (1) The LSA curve of L. fluviatilis is different from all other curves. (2) The M. praecox curve is different from all other curves. (3) G. australis lenses had longer normalized focal lengths than the lenses of all other species studied, which is indicated by the vertical shift of the LSA curve of G. australis.