Evaluation of gray scale steps on Scheimpflug photographs

A method for the calibration of gray scale density in a Scheimpflug photograph was established. Based on the result of statistical analysis, it was found that variability in the film emulsion was insignificant if the measuring area > or = 0.01 mm2. Each lens image should be calibrated with the reference gray scale in the same photograph. Photometric readings of the lens image was converted to gray scale levels. This conversion can be obtained from fitting photometric readings of the gray scale as a function of gray scale steps with a third order polynomial. Adopting this calibration procedure, 95% of estimated nuclear densities are within +/- 3-5% of the real value.