A comparison of digital and film fisheye photography for analysis of forest canopy structure and gap light transmission

Due to the scarcity and high cost of conventional film-based hemispherical photographic systems, some forest scientists are now using multi-purpose, c onsumer-grade digital cameras for the analysis of forest canopy structure a nd gap light transmission. Although the low cost of digital cameras and dir ect capture of digital images appear to offer significant advantages over f ilm camera systems, relatively little is known about their technical differ ences from an applications perspective. In this study, we compared the perf ormance of a popular digital camera (Nikon Coolpix 950 with FC-E8 fisheye) with a conventional film camera (Nikon F with Nikkor 8 mm fisheye) under di fferent stand structures and sky conditions. Our findings show that the Nik on Coolpix 950 digital camera produced hemispherical canopy photos with sub stantial color blurring towards the periphery of the exposure. We believe t hat chromatic aberration associated with the camera's lens optics may be th e source of this phenomenon; however, other factors may have also contribut ed to the diminished image sharpness. Color blur influenced (i) the size, s hape, and distribution of canopy gaps; (ii) the accuracy of edge detection and the binary division of pixels into sky and canopy elements, and (iii) t he magnitude, range, and replication of canopy openness, leaf area, and tra nsmitted global radiation results. The Nikon Coolpix 950 produced canopy op enness measures that were 1.4 times greater than film estimates in 22 of th e 36 photo pairs. Cloud cover and sky brightness also influenced the spectr al characteristics of the lateral chromatic aberration (halos), and thus ha d an added and unpredictable effect on canopy openness. Setting the Nikon C oolpix. 950 to record in black and white, and shooting only under uniformly overcast skies will help to minimize the unpredictable effects of chromati c aberration. Nevertheless, we recommend a cautious approach when undertaki ng canopy measurements with the Nikon Coolpix 950, particularly when stands are dense and canopy openness falls below 10%. High-quality (1:4) JPEG com pression had no significant influence on mean canopy openness; however, low er XGA and VGA image resolutions combined with 1:4 JPEG compression produce d mean canopy openness results that were significantly lower than openness data extracted from uncompressed, full-resolution TIFF photos. (C) 2001 Els evier Science B.V. All rights reserved.