His portraits are known for their objective clarity. He used a cool palette and sharp lighting to define the anatomy of his subjects. Key avoided the soft blending common in Italianate styles. Instead, he preferred a precise technique that recorded every line and feature. This approach created a sense of physical weight in works like his 'Portret van Willem I, prins van Oranje'. His religious scenes, such as 'David Playing Before Saul', share this focus on clear, human observation.

Modern collectors often select these images for their stark honesty. The dark, neutral backgrounds allow the character of the sitter to be the primary subject. These prints suit minimalist spaces where a single, strong image is needed. Technical views, such as the X-radiograph of his 'Portrait of a Doctor', offer a unique look at his process. His focus on individual identity makes sitters like Grietje Pietersdr Codde feel like people one might meet today.