In the present study, in-focus criteria and depth of field in image processing of spray particles within the size range 10-300 mum have been examined. As the indices of the in-focus criteria, the critical normalized value of contrast (VCcritical) proposed earlier was carefully reviewed, and the concept of the critical gradient indicator (GI(critical)) was newly introduced. The criterion with VCcritical was confirmed to be appropriate only for the small particle range (less than or equal to 30 mum). In contrast, the criterion with GI(critical) was determined to be appropriate for large particles greater than or equal to 30 mum). The dependence of the depth of field on the values of VCcritical and GI(critical) was examined. The depth of field decreases with higher values of VCcritical and GI(critical) and varies linearly with particle size in both VC- and GI-applicable ranges. To keep the normalized standard deviation of the measurements below 0.1 while maintaining the continuity of the depth-of-field variation at the borderline of 30 mum, the adjustable constants C and Delta GI for VCcritical and GI(critical) should be set to 0.85 and 0.22, respectively for the present optical setup.