Thumb's rule tested: visual angle of thumb's width is about 2 deg

Area of Science:

• Human anatomy
• Visual perception
• Anthropometry

Background:

• Understanding the visual angles of body parts is crucial for fields like ergonomics and art.
• Previous estimations lacked precise empirical validation.

Purpose of the Study:

• To empirically validate a simple rule of thumb for estimating the visual angle of hand parts.
• To investigate the relationship between hand part size and its visual angle at arm's length.

Main Methods:

• Measurements of hand part dimensions and viewing distances were taken from 118 undergraduate students.
• Visual angles were calculated for the thumb width, thumbnail width, and index fingernail width at arm's length.

Main Results:

• The visual angle for thumb width at arm's length was confirmed to be approximately 2 degrees.
• Thumbnail and index fingernail widths subtended visual angles of approximately 1.5 and 1 degree, respectively.
• A significant linear relationship was found between hand part size and its visual angle.

Conclusions:

• The established rule of thumb for visual angles of hand parts is empirically supported.
• Hand part size directly influences its visual angle, a finding relevant for visual estimations and design.