The optical device thought to have influenced the soft focus in Girl with a Pearl Earring is the camera obscura.
A camera obscura is a dark chamber or box in which light enters through a small opening or lens and projects an image onto an interior surface. Artists could study that projection as a guide for composition, light, color, and perspective. It was an optical aid, not a camera that recorded a permanent photograph.
Vermeer’s paintings contain clues often associated with projected images: unusually convincing perspective, glowing highlights, subtle transitions between sharp and blurred forms, and small circular “facets” of light on reflective objects. In Girl with a Pearl Earring, the soft treatment of the face and the luminous earring are frequently cited in this debate.
The wording “thought to have been used” matters. No camera obscura belonging to Vermeer survives, and scholars disagree about whether he used one directly, used it only as inspiration, or achieved the effects through observation and painting technique. Anamorphosis is distorted perspective, pointillism is a later dot-based method, and sfumato describes smoky tonal blending.