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

Abstract

Due to the scarcity and high cost of conventional film-based hemispherical photographic systems, some forest scientists are now using multi-purpose, consumer-grade digital cameras for the analysis of forest canopy structure and gap light transmission. Although the low cost of digital cameras and direct capture of digital images appear to offer significant advantages over film camera systems, relatively little is known about their technical differences from an applications perspective. In this study, we compared the performance 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 different stand structures and sky conditions. Our findings show that the Nikon Coolpix 950 digital camera produced hemispherical canopy photos with substantial color blurring towards the periphery of the exposure. We believe that chromatic aberration associated with the camera’s lens optics may be the source of this phenomenon; however, other factors may have also contributed to the diminished image sharpness. Color blur influenced (i) the size, shape, and distribution of canopy gaps; (ii) the accuracy of edge detection and the binary division of pixels into sky and canopy elements, and (iii) the magnitude, range, and replication of canopy openness, leaf area, and transmitted global radiation results. The Nikon Coolpix 950 produced canopy openness measures that were 1.4 times greater than film estimates in 22 of the 36 photo pairs. Cloud cover and sky brightness also influenced the spectral characteristics of the lateral chromatic aberration (halos), and thus had an added and unpredictable effect on canopy openness. Setting the Nikon Coolpix 950 to record in black and white, and shooting only under uniformly overcast skies will help to minimize the unpredictable effects of chromatic aberration. Nevertheless, we recommend a cautious approach when undertaking canopy measurements with the Nikon Coolpix 950, particularly when stands are dense and canopy openness falls below 10%. High-quality (1:4) JPEG compression had no significant influence on mean canopy openness; however, lower XGA and VGA image resolutions combined with 1:4 JPEG compression produced mean canopy openness results that were significantly lower than openness data extracted from uncompressed, full-resolution TIFF photos.