Ralph A. DeJur, retired president and chairman of the DeJur-Amsco Corporation, which made photographic equipment and office copiers, died on Wednesday at his home in Manhattan. He was 93 years old. Mr. DeJur, who was born in Manhattan, founded the company in the late 1920's with his brother, Harry. The company, in Long Island City, Queens, originally made photographic equipment and later expanded to include office copiers. Mr. DeJur retired in 1974 and the business was sold.