His work marks a transition from the stylised forms of Mannerism toward the naturalism of the Dutch Golden Age. Ketel was a pioneer of the large-scale civic guard portrait, a genre later made famous by artists like Frans Hals. In pieces such as "The Company of Captain Dirck Jacobsz Rosecrans", he arranged thirty-eight figures with a sense of individual character. He also incorporated allegorical symbols into his portraits, such as bubbles to represent the brevity of life in his portrait of Adam Wachendorff.

Modern collectors value Ketel for his ability to capture the specific textures of sixteenth-century textiles and armour. His portraits offer a direct connection to the personalities of the Dutch Republic. The X-radiograph studies of his work reveal the structural layers of his compositions and his specific painting process. These prints bring a sense of historical weight and intellectual curiosity to a contemporary interior.