In 1997 a summer student (Nicholas Ross) investigated the relationship between magnitude and the length of diffraction spikes appearing on sky limited (survey) UK Schmidt plates.
The spike length was measured using a handlens (an average of three measurements) and spans the full width of the spike/star. Several plates were used in each waveband.
The resualtant plots are presented below together with an empirical best fit line. Different symbols show different plates
J-plot (blue survey, typically 60 minute, GG385/395 filter, IIIaJ emulsion)
R-plot (red survey, typically 60 minute, OG590 filter, IIIaF emulsion)
I-plot (near infrared survey, typically 90 minute, RG715 filter, IVN emulsion)
J-plot (as J-plot above but measured on the digitized sky survey)