A Perspective on Perspectives: Methods to Reduce Variation in Shape Analysis of Digital Images

Abstract

We present digital imaging methods for geometric morphometric analysis of shape, and we describe issues associated with improper image acquisition by using lake trout Salvelinus namaycush as an example. The choice of imaging equipment, the configuration of that equipment, and the orientation of the specimens with respect to the camera lens can lead to inaccurate imaging and ultimately to error in landmark placement during morphometric analysis. Lake trout that were imaged at 15‐mm focal length and 0.5‐m focal distance (treatment 1) were distorted in comparison with fish that were imaged at 50‐mm focal length and 2‐m focal distance (treatment 2). Deformation grids showed dramatic variation in the horizontal plane along the length of the fish, especially midbody, suggesting that barrel distortion was occurring at the 15‐mm focal length. Partial warp scores resulting from geometric analysis of body shape differed for all fish on all 18 warps as a result of the different focal length and distance treatments for image capture. To minimize perspective (orientation) and distortion (equipment) errors, we recommend using a digital single‐lens reflex camera (>5 megapixels) with a lens that has a focal length exceeding 35 mm, a horizontal tripod to position the lens directly over the specimen, a mesh cradle to create a planar imaging surface, and dissection pins to display the fish in a standard orientation. The method presented herein will aid in reducing measurement error associated with landmark homology and will promote comparability of geometric shape data among studies.

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