His artistic approach was defined by technical precision and a refusal to embellish. He used the burin to create fine lines that replicated the textures of starched lace and polished armour. Unlike artists who focused on dramatic lighting, Delff maintained a steady and clear style. He often included the exact age of the sitter in the print, as seen in his portraits of Jacob Cats and Cornelis Liens. This factual detail adds a sense of documentation to the work.

These engravings appeal to modern eyes because of their stark, monochrome aesthetic. The lack of colour allows the viewer to focus on the character of the sitter and the skill of the line work. They fit well in modern rooms that require a sense of history without the clutter of ornate frames or busy compositions. Each print is a piece of 17th-century history that remains visually quiet and balanced.