We consider that to objectively measure immersion, one needs to assess how each sensory quality is reproduced in a virtual environment. In this perspective, we introduce the concept of functional threshold which corresponds to the value at which a sensory quality can be degraded without being noticed by the user of a virtual environment. We suggest that the perceived realism of a virtual experience can potentially be evoked for sensory qualities values ranging from the perceptual threshold to the functional threshold. Thus, the identification of functional thresholds values allows us to constrain immersion. To lay the foundation for the identification of functional thresholds, we applied a modified version of the method of limits. We measured the value at which 30 participants were able to identify the degradation of their field of view (FOV), visual acuity, and contrast sensitivity while executing a multidirectional selection test. This enabled us to identify functional perceptual thresholds of 96.6 degrees for FOV, 12.2 arcmin for visual acuity, and 25.6% for contrast sensitivity.