The level of agreement and the degree of linear correlation between nephelometer and photographic determinations of light extinction were examined from data obtained as part of the Eastern Fine Particle and Visibility Network (EFPVN). Information about the relative equivalence of the two methods was used (along with site-specific topographic features and meteorological factors affecting slide codes and visual range determinations) to assess the significance of various sources of inaccuracy affecting each method. For the EFPVN measurements, significant seasonal and inter-site variation were found in both the level of agreement between seasonal averages and the degree of correlation between short-term (hour to daily) nephelometer and photographic measurements. These differences are examined in terms of seasonal changes, intersite differences, and coding and treatment of photographic measurements for meteorological conditions.