The visual fidelity (fidelity) of a design diagram affects perception and design performance. Hand-drawn diagrams are more effective working documents for early design tasks such as user interface designs than the equivalent computer-prepared formal representation. However people prefer more formal representations because they feel that hand-drawn diagrams look unprofessional. Sketch-based design tools make it possible to present partially tidied designs. We have postulated intermediary levels of visual fidelity in a systematic manner and implemented these levels into a sketch tool to evaluate the effect of computerization and fidelity on perception and design performance. Our findings show that: performance decreased systematically with increased fidelity; that computer presented designs decreases performance and that performance was decreased by computerization of the hand-drawn diagrams. In contrast, user satisfaction was higher with increasing levels of fidelity. These results pose challenges to the sketch tools community and further questions for effective computer support for early design.