Existing methodologies that use ellipsoidal objects for the analysis of geological strain typically assume that these objects acted passively during deformation. This assumption, when not valid, can lead to significant underestimates of strain in rocks deformed under low-grade conditions in orogenic forelands. This is especially true when clastic sedimentary rocks are utilized to measure strain: the competency contrast between clasts and matrix possibly leading to marked 'non-passive' behaviour. The systematic nature of this finite strain underestimation may allow for a correction of strain estimates when this type of behaviour is evident. (C) 2008 Elsevier Ltd. All rights reserved.